Add an interactive constraint solver test

This is an example described in the original
Cassowary paper, reimplemented from scratch.
It exposes some instability and crashiness
in our solver.

demos/gtk-demo/constraints.c

@@ -0,0 +1,280 @@
+/* Constraints/Simple
+ *
+ * GtkConstraintLayout provides a layout manager that uses relations
+ * between widgets (also known as "constraints") to compute the position
+ * and size of each child.
+ */
+
+#include <glib/gi18n.h>
+#include <gtk/gtk.h>
+
+G_DECLARE_FINAL_TYPE (SimpleGrid, simple_grid, SIMPLE, GRID, GtkWidget)
+
+struct _SimpleGrid
+{
+  GtkWidget parent_instance;
+
+  GtkWidget *button1, *button2;
+  GtkWidget *button3;
+};
+
+G_DEFINE_TYPE (SimpleGrid, simple_grid, GTK_TYPE_WIDGET)
+
+static void
+simple_grid_destroy (GtkWidget *widget)
+{
+  SimpleGrid *self = SIMPLE_GRID (widget);
+
+  g_clear_pointer (&self->button1, gtk_widget_destroy);
+  g_clear_pointer (&self->button2, gtk_widget_destroy);
+  g_clear_pointer (&self->button3, gtk_widget_destroy);
+
+  GTK_WIDGET_CLASS (simple_grid_parent_class)->destroy (widget);
+}
+
+static void
+simple_grid_class_init (SimpleGridClass *klass)
+{
+  GtkWidgetClass *widget_class = GTK_WIDGET_CLASS (klass);
+
+  widget_class->destroy = simple_grid_destroy;
+
+  gtk_widget_class_set_layout_manager_type (widget_class, GTK_TYPE_CONSTRAINT_LAYOUT);
+}
+
+/* Layout:
+ *
+ *   +-----------------------------+
+ *   | +-----------+ +-----------+ |
+ *   | |  Child 1  | |  Child 2  | |
+ *   | +-----------+ +-----------+ |
+ *   | +-------------------------+ |
+ *   | |         Child 3         | |
+ *   | +-------------------------+ |
+ *   +-----------------------------+
+ *
+ * Constraints:
+ *
+ *   super.start = child1.start - 8
+ *   child1.width = child2.width
+ *   child1.end = child2.start - 12
+ *   child2.end = super.end - 8
+ *   super.start = child3.start - 8
+ *   child3.end = super.end - 8
+ *   super.top = child1.top - 8
+ *   super.top = child2.top - 8
+ *   child1.bottom = child3.top - 12
+ *   child2.bottom = child3.top - 12
+ *   child3.height = child1.height
+ *   child3.height = child2.height
+ *   child3.bottom = super.bottom - 8
+ *
+ */
+static void
+build_constraints (SimpleGrid          *self,
+                   GtkConstraintLayout *manager)
+{
+  GtkConstraintGuide *guide;
+
+  guide = gtk_constraint_guide_new ();
+  gtk_constraint_guide_set_min_size (guide, 10, 10);
+  gtk_constraint_guide_set_nat_size (guide, 100, 10);
+  gtk_constraint_guide_set_max_size (guide, 200, 20);
+  gtk_constraint_layout_add_guide (manager, guide);
+
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new_constant (GTK_CONSTRAINT_TARGET (self->button1),
+                        GTK_CONSTRAINT_ATTRIBUTE_WIDTH,
+                        GTK_CONSTRAINT_RELATION_LE,
+                        200.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (NULL,
+                        GTK_CONSTRAINT_ATTRIBUTE_START,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        self->button1,
+                        GTK_CONSTRAINT_ATTRIBUTE_START,
+                        1.0,
+                        -8.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (self->button1,
+                        GTK_CONSTRAINT_ATTRIBUTE_WIDTH,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        self->button2,
+                        GTK_CONSTRAINT_ATTRIBUTE_WIDTH,
+                        1.0,
+                        0.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (self->button1,
+                        GTK_CONSTRAINT_ATTRIBUTE_END,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        guide,
+                        GTK_CONSTRAINT_ATTRIBUTE_START,
+                        1.0,
+                        0.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (guide,
+                        GTK_CONSTRAINT_ATTRIBUTE_END,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        self->button2,
+                        GTK_CONSTRAINT_ATTRIBUTE_START,
+                        1.0,
+                        0.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (self->button2,
+                        GTK_CONSTRAINT_ATTRIBUTE_END,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        NULL,
+                        GTK_CONSTRAINT_ATTRIBUTE_END,
+                        1.0,
+                        -8.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (NULL,
+                        GTK_CONSTRAINT_ATTRIBUTE_START,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        self->button3,
+                        GTK_CONSTRAINT_ATTRIBUTE_START,
+                        1.0,
+                        -8.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (self->button3,
+                        GTK_CONSTRAINT_ATTRIBUTE_END,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        NULL,
+                        GTK_CONSTRAINT_ATTRIBUTE_END,
+                        1.0,
+                        -8.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (NULL,
+                        GTK_CONSTRAINT_ATTRIBUTE_TOP,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        self->button1,
+                        GTK_CONSTRAINT_ATTRIBUTE_TOP,
+                        1.0,
+                        -8.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (NULL,
+                        GTK_CONSTRAINT_ATTRIBUTE_TOP,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        self->button2,
+                        GTK_CONSTRAINT_ATTRIBUTE_TOP,
+                        1.0,
+                        -8.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (self->button1,
+                        GTK_CONSTRAINT_ATTRIBUTE_BOTTOM,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        self->button3,
+                        GTK_CONSTRAINT_ATTRIBUTE_TOP,
+                        1.0,
+                        -12.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (self->button2,
+                        GTK_CONSTRAINT_ATTRIBUTE_BOTTOM,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        self->button3,
+                        GTK_CONSTRAINT_ATTRIBUTE_TOP,
+                        1.0,
+                        -12.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (self->button3,
+                        GTK_CONSTRAINT_ATTRIBUTE_HEIGHT,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        self->button1,
+                        GTK_CONSTRAINT_ATTRIBUTE_HEIGHT,
+                        1.0,
+                        0.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (self->button3,
+                        GTK_CONSTRAINT_ATTRIBUTE_HEIGHT,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        self->button2,
+                        GTK_CONSTRAINT_ATTRIBUTE_HEIGHT,
+                        1.0,
+                        0.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (self->button3,
+                        GTK_CONSTRAINT_ATTRIBUTE_BOTTOM,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        NULL,
+                        GTK_CONSTRAINT_ATTRIBUTE_BOTTOM,
+                        1.0,
+                        -8.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+}
+
+static void
+simple_grid_init (SimpleGrid *self)
+{
+  GtkWidget *widget = GTK_WIDGET (self);
+
+  self->button1 = gtk_button_new_with_label ("Child 1");
+  gtk_widget_set_parent (self->button1, widget);
+  gtk_widget_set_name (self->button1, "button1");
+
+  self->button2 = gtk_button_new_with_label ("Child 2");
+  gtk_widget_set_parent (self->button2, widget);
+  gtk_widget_set_name (self->button2, "button2");
+
+  self->button3 = gtk_button_new_with_label ("Child 3");
+  gtk_widget_set_parent (self->button3, widget);
+  gtk_widget_set_name (self->button3, "button3");
+
+  GtkLayoutManager *manager = gtk_widget_get_layout_manager (GTK_WIDGET (self));
+  build_constraints (self, GTK_CONSTRAINT_LAYOUT (manager));
+}
+
+GtkWidget *
+do_constraints (GtkWidget *do_widget)
+{
+ static GtkWidget *window;
+
+ if (!window)
+   {
+     GtkWidget *header, *box, *grid, *button;
+
+     window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
+     gtk_window_set_display (GTK_WINDOW (window), gtk_widget_get_display (do_widget));
+
+     header = gtk_header_bar_new ();
+     gtk_header_bar_set_title (GTK_HEADER_BAR (header), "Constraints");
+     gtk_header_bar_set_show_title_buttons (GTK_HEADER_BAR (header), FALSE);
+     gtk_window_set_titlebar (GTK_WINDOW (window), header);
+     g_signal_connect (window, "destroy",
+                       G_CALLBACK (gtk_widget_destroyed), &window);
+
+     box = gtk_box_new (GTK_ORIENTATION_VERTICAL, 12);
+     gtk_container_add (GTK_CONTAINER (window), box);
+
+     grid = g_object_new (simple_grid_get_type (), NULL);
+     gtk_widget_set_hexpand (grid, TRUE);
+     gtk_widget_set_vexpand (grid, TRUE);
+     gtk_container_add (GTK_CONTAINER (box), grid);
+
+     button = gtk_button_new_with_label ("Close");
+     gtk_container_add (GTK_CONTAINER (box), button);
+     gtk_widget_set_hexpand (grid, TRUE);
+     g_signal_connect_swapped (button, "clicked",
+                               G_CALLBACK (gtk_widget_destroy), window);
+   }
+
+ if (!gtk_widget_get_visible (window))
+   gtk_widget_show (window);
+ else
+   gtk_widget_destroy (window);
+
+ return window;
+}

demos/gtk-demo/constraints2.c

@@ -0,0 +1,245 @@
+/* Constraints/Interactive
+ *
+ * Demonstrate how constraints can be updates during
+ * user interaction.
+ */
+
+#include <glib/gi18n.h>
+#include <gtk/gtk.h>
+
+G_DECLARE_FINAL_TYPE (InteractiveGrid, interactive_grid, INTERACTIVE, GRID, GtkWidget)
+
+struct _InteractiveGrid
+{
+  GtkWidget parent_instance;
+
+  GtkWidget *button1, *button2;
+  GtkWidget *button3;
+  GtkConstraintGuide *guide;
+  GtkConstraint *constraint;
+};
+
+G_DEFINE_TYPE (InteractiveGrid, interactive_grid, GTK_TYPE_WIDGET)
+
+static void
+interactive_grid_destroy (GtkWidget *widget)
+{
+  InteractiveGrid *self = INTERACTIVE_GRID (widget);
+
+  g_clear_pointer (&self->button1, gtk_widget_destroy);
+  g_clear_pointer (&self->button2, gtk_widget_destroy);
+  g_clear_pointer (&self->button3, gtk_widget_destroy);
+
+  GTK_WIDGET_CLASS (interactive_grid_parent_class)->destroy (widget);
+}
+
+static void
+interactive_grid_class_init (InteractiveGridClass *klass)
+{
+  GtkWidgetClass *widget_class = GTK_WIDGET_CLASS (klass);
+
+  widget_class->destroy = interactive_grid_destroy;
+
+  gtk_widget_class_set_layout_manager_type (widget_class, GTK_TYPE_CONSTRAINT_LAYOUT);
+}
+
+static void
+build_constraints (InteractiveGrid          *self,
+                   GtkConstraintLayout *manager)
+{
+  self->guide = g_object_new (GTK_TYPE_CONSTRAINT_GUIDE, NULL);
+  gtk_constraint_layout_add_guide (manager, self->guide);
+
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new_constant (GTK_CONSTRAINT_TARGET (self->guide),
+                        GTK_CONSTRAINT_ATTRIBUTE_WIDTH,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        0.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (NULL,
+                        GTK_CONSTRAINT_ATTRIBUTE_START,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        GTK_CONSTRAINT_TARGET (self->button1),
+                        GTK_CONSTRAINT_ATTRIBUTE_START,
+                        1.0,
+                        -8.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (GTK_CONSTRAINT_TARGET (self->button1),
+                        GTK_CONSTRAINT_ATTRIBUTE_END,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        GTK_CONSTRAINT_TARGET (self->guide),
+                        GTK_CONSTRAINT_ATTRIBUTE_START,
+                        1.0,
+                        0.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (GTK_CONSTRAINT_TARGET (self->button2),
+                        GTK_CONSTRAINT_ATTRIBUTE_START,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        GTK_CONSTRAINT_TARGET (self->guide),
+                        GTK_CONSTRAINT_ATTRIBUTE_END,
+                        1.0,
+                        0.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (GTK_CONSTRAINT_TARGET (self->button2),
+                        GTK_CONSTRAINT_ATTRIBUTE_END,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        NULL,
+                        GTK_CONSTRAINT_ATTRIBUTE_END,
+                        1.0,
+                        -8.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (NULL,
+                        GTK_CONSTRAINT_ATTRIBUTE_START,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        GTK_CONSTRAINT_TARGET (self->button3),
+                        GTK_CONSTRAINT_ATTRIBUTE_START,
+                        1.0,
+                        -8.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (GTK_CONSTRAINT_TARGET (self->button3),
+                        GTK_CONSTRAINT_ATTRIBUTE_END,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        GTK_CONSTRAINT_TARGET (self->guide),
+                        GTK_CONSTRAINT_ATTRIBUTE_START,
+                        1.0,
+                        0.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (NULL,
+                        GTK_CONSTRAINT_ATTRIBUTE_TOP,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        GTK_CONSTRAINT_TARGET (self->button1),
+                        GTK_CONSTRAINT_ATTRIBUTE_TOP,
+                        1.0,
+                        -8.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (GTK_CONSTRAINT_TARGET (self->button2),
+                        GTK_CONSTRAINT_ATTRIBUTE_TOP,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        GTK_CONSTRAINT_TARGET (self->button1),
+                        GTK_CONSTRAINT_ATTRIBUTE_BOTTOM,
+                        1.0,
+                        0.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (GTK_CONSTRAINT_TARGET (self->button3),
+                        GTK_CONSTRAINT_ATTRIBUTE_TOP,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        GTK_CONSTRAINT_TARGET (self->button2),
+                        GTK_CONSTRAINT_ATTRIBUTE_BOTTOM,
+                        1.0,
+                        0.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+  gtk_constraint_layout_add_constraint (manager,
+    gtk_constraint_new (GTK_CONSTRAINT_TARGET (self->button3),
+                        GTK_CONSTRAINT_ATTRIBUTE_BOTTOM,
+                        GTK_CONSTRAINT_RELATION_EQ,
+                        NULL,
+                        GTK_CONSTRAINT_ATTRIBUTE_BOTTOM,
+                        1.0,
+                        -8.0,
+                        GTK_CONSTRAINT_STRENGTH_REQUIRED));
+}
+
+static void
+drag_cb (GtkGestureDrag  *drag,
+         double           offset_x,
+         double           offset_y,
+         InteractiveGrid *self)
+{
+  GtkConstraintLayout *layout = GTK_CONSTRAINT_LAYOUT (gtk_widget_get_layout_manager (GTK_WIDGET (self)));
+  double x, y;
+
+  if (self->constraint)
+    {
+      gtk_constraint_layout_remove_constraint (layout, self->constraint);
+      g_clear_object (&self->constraint);
+    }
+
+  gtk_gesture_drag_get_start_point (drag, &x, &y);
+  self->constraint = gtk_constraint_new_constant (GTK_CONSTRAINT_TARGET (self->guide),
+                                                  GTK_CONSTRAINT_ATTRIBUTE_LEFT,
+                                                  GTK_CONSTRAINT_RELATION_EQ,
+                                                  x + offset_x,
+                                                  GTK_CONSTRAINT_STRENGTH_REQUIRED);
+  gtk_constraint_layout_add_constraint (layout, g_object_ref (self->constraint));
+  gtk_widget_queue_allocate (GTK_WIDGET (self));
+}
+
+static void
+interactive_grid_init (InteractiveGrid *self)
+{
+  GtkWidget *widget = GTK_WIDGET (self);
+  GtkGesture *drag;
+
+  self->button1 = gtk_button_new_with_label ("Child 1");
+  gtk_widget_set_parent (self->button1, widget);
+  gtk_widget_set_name (self->button1, "button1");
+
+  self->button2 = gtk_button_new_with_label ("Child 2");
+  gtk_widget_set_parent (self->button2, widget);
+  gtk_widget_set_name (self->button2, "button2");
+
+  self->button3 = gtk_button_new_with_label ("Child 3");
+  gtk_widget_set_parent (self->button3, widget);
+  gtk_widget_set_name (self->button3, "button3");
+
+  GtkLayoutManager *manager = gtk_widget_get_layout_manager (GTK_WIDGET (self));
+  build_constraints (self, GTK_CONSTRAINT_LAYOUT (manager));
+
+  drag = gtk_gesture_drag_new ();
+  g_signal_connect (drag, "drag-update", G_CALLBACK (drag_cb), self);
+  gtk_widget_add_controller (GTK_WIDGET (self), GTK_EVENT_CONTROLLER (drag));
+}
+
+GtkWidget *
+do_constraints2 (GtkWidget *do_widget)
+{
+ static GtkWidget *window;
+
+ if (!window)
+   {
+     GtkWidget *header, *box, *grid, *button;
+
+     window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
+     gtk_window_set_display (GTK_WINDOW (window), gtk_widget_get_display (do_widget));
+
+     header = gtk_header_bar_new ();
+     gtk_header_bar_set_title (GTK_HEADER_BAR (header), "Constraints");
+     gtk_header_bar_set_show_title_buttons (GTK_HEADER_BAR (header), FALSE);
+     gtk_window_set_titlebar (GTK_WINDOW (window), header);
+     g_signal_connect (window, "destroy",
+                       G_CALLBACK (gtk_widget_destroyed), &window);
+
+     box = gtk_box_new (GTK_ORIENTATION_VERTICAL, 12);
+     gtk_container_add (GTK_CONTAINER (window), box);
+
+     grid = g_object_new (interactive_grid_get_type (), NULL);
+     gtk_widget_set_hexpand (grid, TRUE);
+     gtk_widget_set_vexpand (grid, TRUE);
+     gtk_container_add (GTK_CONTAINER (box), grid);
+
+     button = gtk_button_new_with_label ("Close");
+     gtk_container_add (GTK_CONTAINER (box), button);
+     gtk_widget_set_hexpand (grid, TRUE);
+     g_signal_connect_swapped (button, "clicked",
+                               G_CALLBACK (gtk_widget_destroy), window);
+   }
+
+ if (!gtk_widget_get_visible (window))
+   gtk_widget_show (window);
+ else
+   gtk_widget_destroy (window);
+
+ return window;
+}

demos/gtk-demo/demo.gresource.xml

@@ -150,6 +150,8 @@
     <file>clipboard.c</file>
     <file>colorsel.c</file>
     <file>combobox.c</file>
+    <file>constraints.c</file>
+    <file>constraints2.c</file>
     <file>css_accordion.c</file>
     <file>css_basics.c</file>
     <file>css_blendmodes.c</file>

demos/gtk-demo/meson.build

@@ -8,6 +8,8 @@ demos = files([
   'clipboard.c',
   'colorsel.c',
   'combobox.c',
+  'constraints.c',
+  'constraints2.c',
   'css_accordion.c',
   'css_basics.c',
   'css_blendmodes.c',

gtk/gtk.h

@@ -82,6 +82,8 @@
 #include <gtk/gtkcolorutils.h>
 #include <gtk/gtkcombobox.h>
 #include <gtk/gtkcomboboxtext.h>
+#include <gtk/gtkconstraintlayout.h>
+#include <gtk/gtkconstraint.h>
 #include <gtk/gtkcontainer.h>
 #include <gtk/gtkcssprovider.h>
 #include <gtk/gtkcustomlayout.h>

gtk/gtkconstraint.c

@@ -0,0 +1,617 @@
+/* gtkconstraint.c: Constraint between two widgets
+ * Copyright 2019  GNOME Foundation
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Emmanuele Bassi
+ */
+
+/**
+ * SECTION:gtkconstraint
+ * @Title: GtkConstraint
+ * @Short_description: The description of a constraint
+ *
+ * #GtkConstraint describes a constraint between an attribute on a widget
+ * and another attribute on another widget, expressed as a linear equation
+ * like:
+ *
+ * |[
+ *   target.attr1 = source.attr2 × multiplier + constant
+ * ]|
+ *
+ * Each #GtkConstraint is part of a system that will be solved by a
+ * #GtkConstraintLayout in order to allocate and position each child widget.
+ *
+ * The source and target widgets, as well as their attributes, of a
+ * #GtkConstraint instance are immutable after creation.
+ */
+
+#include "config.h"
+
+#include "gtkconstraintprivate.h"
+#include "gtkconstraintsolverprivate.h"
+#include "gtkintl.h"
+#include "gtktypebuiltins.h"
+#include "gtkwidget.h"
+
+enum {
+  PROP_TARGET = 1,
+  PROP_TARGET_ATTRIBUTE,
+  PROP_RELATION,
+  PROP_SOURCE,
+  PROP_SOURCE_ATTRIBUTE,
+  PROP_MULTIPLIER,
+  PROP_CONSTANT,
+  PROP_STRENGTH,
+
+  N_PROPERTIES
+};
+
+static GParamSpec *obj_props[N_PROPERTIES];
+
+G_DEFINE_TYPE (GtkConstraint, gtk_constraint, G_TYPE_OBJECT)
+
+static void
+gtk_constraint_set_property (GObject      *gobject,
+                             guint         prop_id,
+                             const GValue *value,
+                             GParamSpec   *pspec)
+{
+  GtkConstraint *self = GTK_CONSTRAINT (gobject);
+
+  switch (prop_id)
+    {
+    case PROP_TARGET:
+      self->target = g_value_get_object (value);
+      break;
+
+    case PROP_TARGET_ATTRIBUTE:
+      self->target_attribute = g_value_get_enum (value);
+      break;
+
+    case PROP_RELATION:
+      self->relation = g_value_get_enum (value);
+      break;
+
+    case PROP_SOURCE:
+      self->source = g_value_get_object (value);
+      break;
+
+    case PROP_SOURCE_ATTRIBUTE:
+      self->source_attribute = g_value_get_enum (value);
+      break;
+
+    case PROP_MULTIPLIER:
+      self->multiplier = g_value_get_double (value);
+      break;
+
+    case PROP_CONSTANT:
+      self->constant = g_value_get_double (value);
+      break;
+
+    case PROP_STRENGTH:
+      self->strength = g_value_get_int (value);
+      break;
+
+    default:
+      G_OBJECT_WARN_INVALID_PROPERTY_ID (gobject, prop_id, pspec);
+      break;
+    }
+}
+
+static void
+gtk_constraint_get_property (GObject    *gobject,
+                             guint       prop_id,
+                             GValue     *value,
+                             GParamSpec *pspec)
+{
+  GtkConstraint *self = GTK_CONSTRAINT (gobject);
+
+  switch (prop_id)
+    {
+    case PROP_TARGET:
+      g_value_set_object (value, self->target);
+      break;
+
+    case PROP_TARGET_ATTRIBUTE:
+      g_value_set_enum (value, self->target_attribute);
+      break;
+
+    case PROP_RELATION:
+      g_value_set_enum (value, self->relation);
+      break;
+
+    case PROP_SOURCE:
+      g_value_set_object (value, self->source);
+      break;
+
+    case PROP_SOURCE_ATTRIBUTE:
+      g_value_set_enum (value, self->source_attribute);
+      break;
+
+    case PROP_MULTIPLIER:
+      g_value_set_double (value, self->multiplier);
+      break;
+
+    case PROP_CONSTANT:
+      g_value_set_double (value, self->constant);
+      break;
+
+    case PROP_STRENGTH:
+      g_value_set_int (value, self->strength);
+      break;
+
+    default:
+      G_OBJECT_WARN_INVALID_PROPERTY_ID (gobject, prop_id, pspec);
+      break;
+    }
+}
+
+static void
+gtk_constraint_finalize (GObject *gobject)
+{
+  GtkConstraint *self = GTK_CONSTRAINT (gobject);
+
+  gtk_constraint_detach (self);
+
+  G_OBJECT_CLASS (gtk_constraint_parent_class)->finalize (gobject);
+}
+
+static void
+gtk_constraint_class_init (GtkConstraintClass *klass)
+{
+  GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
+
+  gobject_class->set_property = gtk_constraint_set_property;
+  gobject_class->get_property = gtk_constraint_get_property;
+  gobject_class->finalize = gtk_constraint_finalize;
+
+  /**
+   * GtkConstraint:target:
+   *
+   * The target of the constraint.
+   *
+   * The constraint will set the #GtkConstraint:target-attribute of the
+   * target using the #GtkConstraint:source-attribute of the source
+   * widget.
+   */
+  obj_props[PROP_TARGET] =
+    g_param_spec_object ("target",
+                         P_("Target"),
+                         P_("The target of the constraint"),
+                         GTK_TYPE_CONSTRAINT_TARGET,
+                         G_PARAM_READWRITE |
+                         G_PARAM_STATIC_STRINGS |
+                         G_PARAM_CONSTRUCT_ONLY);
+  /**
+   * GtkConstraint:target-attribute:
+   *
+   * The attribute of the #GtkConstraint:target set by the constraint.
+   */
+  obj_props[PROP_TARGET_ATTRIBUTE] =
+    g_param_spec_enum ("target-attribute",
+                       P_("Target Attribute"),
+                       P_("The attribute of the target set by the constraint"),
+                       GTK_TYPE_CONSTRAINT_ATTRIBUTE,
+                       GTK_CONSTRAINT_ATTRIBUTE_NONE,
+                       G_PARAM_READWRITE |
+                       G_PARAM_STATIC_STRINGS |
+                       G_PARAM_CONSTRUCT_ONLY);
+  /**
+   * GtkConstraint:relation:
+   *
+   * The relation order between the terms of the constraint.
+   */
+  obj_props[PROP_RELATION] =
+    g_param_spec_enum ("relation",
+                       P_("Relation"),
+                       P_("The relation between the source and target attributes"),
+                       GTK_TYPE_CONSTRAINT_RELATION,
+                       GTK_CONSTRAINT_RELATION_EQ,
+                       G_PARAM_READWRITE |
+                       G_PARAM_STATIC_STRINGS |
+                       G_PARAM_CONSTRUCT_ONLY);
+  /**
+   * GtkConstraint:source:
+   *
+   * The source of the constraint.
+   *
+   * The constraint will set the #GtkConstraint:target-attribute of the
+   * target using the #GtkConstraint:source-attribute of the source.
+   */
+  obj_props[PROP_SOURCE] =
+    g_param_spec_object ("source",
+                         P_("Source"),
+                         P_("The source of the constraint"),
+                         GTK_TYPE_CONSTRAINT_TARGET,
+                         G_PARAM_READWRITE |
+                         G_PARAM_STATIC_STRINGS |
+                         G_PARAM_CONSTRUCT_ONLY);
+  /**
+   * GtkConstraint:source-attribute:
+   *
+   * The attribute of the #GtkConstraint:source read by the constraint.
+   */
+  obj_props[PROP_SOURCE_ATTRIBUTE] =
+    g_param_spec_enum ("source-attribute",
+                       P_("Source Attribute"),
+                       P_("The attribute of the source widget set by the constraint"),
+                       GTK_TYPE_CONSTRAINT_ATTRIBUTE,
+                       GTK_CONSTRAINT_ATTRIBUTE_NONE,
+                       G_PARAM_READWRITE |
+                       G_PARAM_STATIC_STRINGS |
+                       G_PARAM_CONSTRUCT_ONLY);
+  /**
+   * GtkConstraint:multiplier:
+   *
+   * The multiplication factor to be applied to the
+   * #GtkConstraint:source-attribue.
+   */
+  obj_props[PROP_MULTIPLIER] =
+    g_param_spec_double ("multiplier",
+                         P_("Multiplier"),
+                         P_("The multiplication factor to be applied to the source attribute"),
+                         -G_MAXDOUBLE, G_MAXDOUBLE, 1.0,
+                         G_PARAM_READWRITE |
+                         G_PARAM_STATIC_STRINGS |
+                         G_PARAM_CONSTRUCT_ONLY);
+  /**
+   * GtkConstraint:constant:
+   *
+   * The constant value to be added to the #GtkConstraint:source-attribute.
+   */
+  obj_props[PROP_CONSTANT] =
+    g_param_spec_double ("constant",
+                         P_("Constant"),
+                         P_("The constant to be added to the source attribute"),
+                         -G_MAXDOUBLE, G_MAXDOUBLE, 0.0,
+                         G_PARAM_READWRITE |
+                         G_PARAM_STATIC_STRINGS |
+                         G_PARAM_CONSTRUCT_ONLY);
+  /**
+   * GtkConstraint:strength:
+   *
+   * The strength of the constraint.
+   *
+   * The strength can be expressed either using one of the symbolic values
+   * of the #GtkConstraintStrength enumeration, or any positive integer
+   * value.
+   */
+  obj_props[PROP_STRENGTH] =
+    g_param_spec_int ("strength",
+                      P_("Strength"),
+                      P_("The strength of the constraint"),
+                      GTK_CONSTRAINT_STRENGTH_WEAK, G_MAXINT,
+                      GTK_CONSTRAINT_STRENGTH_REQUIRED,
+                      G_PARAM_READWRITE |
+                      G_PARAM_STATIC_STRINGS |
+                      G_PARAM_CONSTRUCT_ONLY);
+
+  g_object_class_install_properties (gobject_class, N_PROPERTIES, obj_props);
+}
+
+static void
+gtk_constraint_init (GtkConstraint *self)
+{
+  self->multiplier = 1.0;
+  self->constant = 0.0;
+
+  self->target_attribute = GTK_CONSTRAINT_ATTRIBUTE_NONE;
+  self->source_attribute = GTK_CONSTRAINT_ATTRIBUTE_NONE;
+  self->relation = GTK_CONSTRAINT_RELATION_EQ;
+  self->strength = GTK_CONSTRAINT_STRENGTH_REQUIRED;
+}
+
+/**
+ * gtk_constraint_new:
+ * @target: (nullable) (type GtkConstraintTarget): a #GtkConstraintTarget
+ * @target_attribute: the attribute of @target to be set
+ * @relation: the relation equivalence between @target_attribute and @source_attribute
+ * @source: (nullable) (type GtkConstraintTarget): a #GtkConstraintTarget
+ * @source_attribute: the attribute of @source to be read
+ * @multiplier: a multiplication factor to be applied to @source_attribute
+ * @constant: a constant factor to be added to @source_attribute
+ * @strength: the strength of the constraint
+ *
+ * Creates a new #GtkConstraint representing a relation between a layout
+ * attribute on a source and a layout attribute on a target.
+ *
+ * Returns: the newly created #GtkConstraint
+ */
+GtkConstraint *
+gtk_constraint_new (gpointer                target,
+                    GtkConstraintAttribute  target_attribute,
+                    GtkConstraintRelation   relation,
+                    gpointer                source,
+                    GtkConstraintAttribute  source_attribute,
+                    double                  multiplier,
+                    double                  constant,
+                    int                     strength)
+{
+  g_return_val_if_fail (target == NULL || GTK_IS_CONSTRAINT_TARGET (target), NULL);
+  g_return_val_if_fail (source == NULL || GTK_IS_CONSTRAINT_TARGET (source), NULL);
+
+  return g_object_new (GTK_TYPE_CONSTRAINT,
+                       "target", target,
+                       "target-attribute", target_attribute,
+                       "relation", relation,
+                       "source", source,
+                       "source-attribute", source_attribute,
+                       "multiplier", multiplier,
+                       "constant", constant,
+                       "strength", strength,
+                       NULL);
+}
+
+/**
+ * gtk_constraint_new_constant:
+ * @target: (nullable) (type GtkConstraintTarget): a #GtkConstraintTarget
+ * @target_attribute: the attribute of @target to be set
+ * @relation: the relation equivalence between @target_attribute and @constant
+ * @constant: a constant factor to be set on @target_attribute
+ * @strength: the strength of the constraint
+ *
+ * Creates a new #GtkConstraint representing a relation between a layout
+ * attribute on a target and a constant value.
+ *
+ * Returns: the newly created #GtkConstraint
+ */
+GtkConstraint *
+gtk_constraint_new_constant (gpointer                target,
+                             GtkConstraintAttribute  target_attribute,
+                             GtkConstraintRelation   relation,
+                             double                  constant,
+                             int                     strength)
+{
+  g_return_val_if_fail (target == NULL || GTK_IS_CONSTRAINT_TARGET (target), NULL);
+
+  return g_object_new (GTK_TYPE_CONSTRAINT,
+                       "target", target,
+                       "target-attribute", target_attribute,
+                       "relation", relation,
+                       "source-attribute", GTK_CONSTRAINT_ATTRIBUTE_NONE,
+                       "constant", constant,
+                       "strength", strength,
+                       NULL);
+}
+
+/**
+ * gtk_constraint_get_target_widget:
+ * @constraint: a #GtkConstraint
+ *
+ * Retrieves the target widget for the @constraint.
+ *
+ * Returns: (transfer none) (nullable): a #GtkWidget
+ */
+GtkWidget *
+gtk_constraint_get_target_widget (GtkConstraint *constraint)
+{
+  g_return_val_if_fail (GTK_IS_CONSTRAINT (constraint), NULL);
+
+  if (GTK_IS_WIDGET (constraint->target))
+    return GTK_WIDGET (constraint->target);
+
+  return NULL;
+}
+
+GtkConstraintTarget *
+gtk_constraint_get_target (GtkConstraint *constraint)
+{
+  g_return_val_if_fail (GTK_IS_CONSTRAINT (constraint), NULL);
+
+  return constraint->target;
+}
+
+GtkConstraintAttribute
+gtk_constraint_get_target_attribute (GtkConstraint *constraint)
+{
+  g_return_val_if_fail (GTK_IS_CONSTRAINT (constraint), GTK_CONSTRAINT_ATTRIBUTE_NONE);
+
+  return constraint->target_attribute;
+}
+
+/**
+ * gtk_constraint_get_source_widget:
+ * @constraint: a #GtkConstraint
+ *
+ * Retrieves the source widget for the @constraint.
+ *
+ * Returns: (transfer none) (nullable): a #GtkWidget
+ */
+GtkWidget *
+gtk_constraint_get_source_widget (GtkConstraint *constraint)
+{
+  g_return_val_if_fail (GTK_IS_CONSTRAINT (constraint), NULL);
+
+  if (GTK_IS_WIDGET (constraint->source))
+    return GTK_WIDGET (constraint->source);
+
+  return NULL;
+}
+
+GtkConstraintTarget *
+gtk_constraint_get_source (GtkConstraint *constraint)
+{
+  g_return_val_if_fail (GTK_IS_CONSTRAINT (constraint), NULL);
+
+  return constraint->source;
+}
+
+GtkConstraintAttribute
+gtk_constraint_get_source_attribute (GtkConstraint *constraint)
+{
+  g_return_val_if_fail (GTK_IS_CONSTRAINT (constraint), GTK_CONSTRAINT_ATTRIBUTE_NONE);
+
+  return constraint->source_attribute;
+}
+
+GtkConstraintRelation
+gtk_constraint_get_relation (GtkConstraint *constraint)
+{
+  g_return_val_if_fail (GTK_IS_CONSTRAINT (constraint), GTK_CONSTRAINT_RELATION_EQ);
+
+  return constraint->relation;
+}
+
+double
+gtk_constraint_get_multiplier (GtkConstraint *constraint)
+{
+  g_return_val_if_fail (GTK_IS_CONSTRAINT (constraint), 1.0);
+
+  return constraint->multiplier;
+}
+
+double
+gtk_constraint_get_constant (GtkConstraint *constraint)
+{
+  g_return_val_if_fail (GTK_IS_CONSTRAINT (constraint), 0.0);
+
+  return constraint->constant;
+}
+
+int
+gtk_constraint_get_strength (GtkConstraint *constraint)
+{
+  g_return_val_if_fail (GTK_IS_CONSTRAINT (constraint), GTK_CONSTRAINT_STRENGTH_REQUIRED);
+
+  return constraint->strength;
+}
+
+/*< private >
+ * gtk_constraint_get_weight:
+ * @constraint: a #GtkConstraint
+ *
+ * Computes the weight of the @constraint to be used with
+ * #GtkConstraintSolver.
+ *
+ * Returns: the weight of the constraint
+ */
+double
+gtk_constraint_get_weight (GtkConstraint *constraint)
+{
+  if (constraint->strength > 0)
+    return constraint->strength;
+
+  switch (constraint->strength)
+    {
+    case GTK_CONSTRAINT_STRENGTH_REQUIRED:
+      return GTK_CONSTRAINT_WEIGHT_REQUIRED;
+
+    case GTK_CONSTRAINT_STRENGTH_STRONG:
+      return GTK_CONSTRAINT_WEIGHT_STRONG;
+
+    case GTK_CONSTRAINT_STRENGTH_MEDIUM:
+      return GTK_CONSTRAINT_WEIGHT_MEDIUM;
+
+    case GTK_CONSTRAINT_STRENGTH_WEAK:
+      return GTK_CONSTRAINT_WEIGHT_WEAK;
+
+    default:
+      g_assert_not_reached ();
+    }
+
+  return 0;
+}
+
+/**
+ * gtk_constraint_is_required:
+ * @constraint: a #GtkConstraint
+ *
+ * Checks whether the @constraint is a required relation for solving the
+ * constraint layout.
+ *
+ * Returns: %TRUE if the constraint is required
+ */
+gboolean
+gtk_constraint_is_required (GtkConstraint *constraint)
+{
+  g_return_val_if_fail (GTK_IS_CONSTRAINT (constraint), FALSE);
+
+  return constraint->strength == GTK_CONSTRAINT_STRENGTH_REQUIRED;
+}
+
+/**
+ * gtk_constraint_is_attached:
+ * @constraint: a #GtkConstraint
+ *
+ * Checks whether the @constraint is attached to a #GtkConstraintLayout,
+ * and it is contributing to the layout.
+ *
+ * Returns: %TRUE if the constraint is attached
+ */
+gboolean
+gtk_constraint_is_attached (GtkConstraint *constraint)
+{
+  g_return_val_if_fail (GTK_IS_CONSTRAINT (constraint), FALSE);
+
+  return constraint->constraint_ref != NULL;
+}
+
+/**
+ * gtk_constraint_is_constant:
+ * @constraint: a #GtkConstraint
+ *
+ * Checks whether the @constraint describes a relation between an attribute
+ * on the #GtkConstraint:target-widget and a constant value.
+ *
+ * Returns: %TRUE if the constraint is a constant relation
+ */
+gboolean
+gtk_constraint_is_constant (GtkConstraint *constraint)
+{
+  g_return_val_if_fail (GTK_IS_CONSTRAINT (constraint), FALSE);
+
+  return constraint->source == NULL &&
+         constraint->source_attribute == GTK_CONSTRAINT_ATTRIBUTE_NONE;
+}
+
+void
+gtk_constraint_attach (GtkConstraint       *constraint,
+                       GtkConstraintSolver *solver,
+                       GtkConstraintRef    *ref)
+{
+  g_return_if_fail (GTK_IS_CONSTRAINT (constraint));
+  g_return_if_fail (GTK_IS_CONSTRAINT_SOLVER (solver));
+  g_return_if_fail (ref != NULL);
+
+  constraint->constraint_ref = ref;
+  constraint->solver = solver;
+}
+
+void
+gtk_constraint_detach (GtkConstraint *constraint)
+{
+  g_return_if_fail (GTK_IS_CONSTRAINT (constraint));
+
+  if (constraint->constraint_ref == NULL)
+    return;
+
+  gtk_constraint_solver_remove_constraint (constraint->solver, constraint->constraint_ref);
+  constraint->constraint_ref = NULL;
+  constraint->solver = NULL;
+}
+
+typedef struct _GtkConstraintTargetInterface GtkConstraintTargetInterface;
+
+struct _GtkConstraintTargetInterface
+{
+  GTypeInterface g_iface;
+};
+
+G_DEFINE_INTERFACE (GtkConstraintTarget, gtk_constraint_target, G_TYPE_OBJECT)
+
+static void
+gtk_constraint_target_default_init (GtkConstraintTargetInterface *iface)
+{
+}

gtk/gtkconstraint.h

@@ -0,0 +1,104 @@
+/* gtkconstraint.h: Constraint between two widgets
+ * Copyright 2019  GNOME Foundation
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Emmanuele Bassi
+ */
+
+#pragma once
+
+#include <gtk/gtktypes.h>
+#include <gtk/gtkenums.h>
+
+G_BEGIN_DECLS
+
+typedef struct _GtkConstraintTarget GtkConstraintTarget;
+
+#define GTK_TYPE_CONSTRAINT_TARGET (gtk_constraint_target_get_type ())
+
+/**
+ * GtkConstraintTarget:
+ *
+ * The GtkConstraintTarget interface is implemented by objects that
+ * can be used as source or target in #GtkConstraints. Besides
+ * #GtkWidget, it is also implemented by #GtkConstraintGuide.
+ */
+
+GDK_AVAILABLE_IN_ALL
+G_DECLARE_INTERFACE (GtkConstraintTarget, gtk_constraint_target, GTK, CONSTRAINT_TARGET, GObject)
+
+#define GTK_TYPE_CONSTRAINT (gtk_constraint_get_type ())
+
+/**
+ * GtkConstraint:
+ *
+ * An object describing the relation between two widget attributes.
+ *
+ * All relations are in the form:
+ *
+ * |[<!-- language=plain -->
+ *   target.attr_name = source.attr_name × multiplier + constant
+ * ]|
+ *
+ * A #GtkConstraint is immutable once it's created.
+ */
+GDK_AVAILABLE_IN_ALL
+G_DECLARE_FINAL_TYPE (GtkConstraint, gtk_constraint, GTK, CONSTRAINT, GObject)
+
+GDK_AVAILABLE_IN_ALL
+GtkConstraint *         gtk_constraint_new                      (gpointer                target,
+                                                                 GtkConstraintAttribute  target_attribute,
+                                                                 GtkConstraintRelation   relation,
+                                                                 gpointer                source,
+                                                                 GtkConstraintAttribute  source_attribute,
+                                                                 double                  multiplier,
+                                                                 double                  constant,
+                                                                 int                     strength);
+GDK_AVAILABLE_IN_ALL
+GtkConstraint *         gtk_constraint_new_constant             (gpointer                target,
+                                                                 GtkConstraintAttribute  target_attribute,
+                                                                 GtkConstraintRelation   relation,
+                                                                 double                  constant,
+                                                                 int                     strength);
+
+GDK_AVAILABLE_IN_ALL
+GtkWidget *             gtk_constraint_get_target_widget        (GtkConstraint          *constraint);
+GDK_AVAILABLE_IN_ALL
+GtkConstraintTarget *   gtk_constraint_get_target               (GtkConstraint          *constraint);
+GDK_AVAILABLE_IN_ALL
+GtkConstraintAttribute  gtk_constraint_get_target_attribute     (GtkConstraint          *constraint);
+GDK_AVAILABLE_IN_ALL
+GtkWidget *             gtk_constraint_get_source_widget        (GtkConstraint          *constraint);
+GDK_AVAILABLE_IN_ALL
+GtkConstraintTarget *   gtk_constraint_get_source               (GtkConstraint          *constraint);
+GDK_AVAILABLE_IN_ALL
+GtkConstraintAttribute  gtk_constraint_get_source_attribute     (GtkConstraint          *constraint);
+GDK_AVAILABLE_IN_ALL
+GtkConstraintRelation   gtk_constraint_get_relation             (GtkConstraint          *constraint);
+GDK_AVAILABLE_IN_ALL
+double                  gtk_constraint_get_multiplier           (GtkConstraint          *constraint);
+GDK_AVAILABLE_IN_ALL
+double                  gtk_constraint_get_constant             (GtkConstraint          *constraint);
+GDK_AVAILABLE_IN_ALL
+int                     gtk_constraint_get_strength             (GtkConstraint          *constraint);
+
+GDK_AVAILABLE_IN_ALL
+gboolean                gtk_constraint_is_required              (GtkConstraint          *constraint);
+GDK_AVAILABLE_IN_ALL
+gboolean                gtk_constraint_is_attached              (GtkConstraint          *constraint);
+GDK_AVAILABLE_IN_ALL
+gboolean                gtk_constraint_is_constant              (GtkConstraint          *constraint);
+
+G_END_DECLS

gtk/gtkconstraintexpressionprivate.h

@@ -0,0 +1,276 @@
+/* gtkconstraintequationprivate.h: Constraint expressions and variables
+ * Copyright 2019  GNOME Foundation
+ *
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Emmanuele Bassi
+ */
+
+#pragma once
+
+#include "gtkconstrainttypesprivate.h"
+
+G_BEGIN_DECLS
+
+GtkConstraintVariable *
+gtk_constraint_variable_new (const char *name);
+
+GtkConstraintVariable *
+gtk_constraint_variable_new_dummy (const char *name);
+
+GtkConstraintVariable *
+gtk_constraint_variable_new_objective (const char *name);
+
+GtkConstraintVariable *
+gtk_constraint_variable_new_slack (const char *name);
+
+GtkConstraintVariable *
+gtk_constraint_variable_ref (GtkConstraintVariable *variable);
+
+void
+gtk_constraint_variable_unref (GtkConstraintVariable *variable);
+
+void
+gtk_constraint_variable_set_value (GtkConstraintVariable *variable,
+                                   double value);
+
+double
+gtk_constraint_variable_get_value (const GtkConstraintVariable *variable);
+
+void
+gtk_constraint_variable_set_prefix (GtkConstraintVariable *variable,
+                                    const char *prefix);
+
+char *
+gtk_constraint_variable_to_string (const GtkConstraintVariable *variable);
+
+gboolean
+gtk_constraint_variable_is_external (const GtkConstraintVariable *variable);
+
+gboolean
+gtk_constraint_variable_is_pivotable (const GtkConstraintVariable *variable);
+
+gboolean
+gtk_constraint_variable_is_restricted (const GtkConstraintVariable *variable);
+
+gboolean
+gtk_constraint_variable_is_dummy (const GtkConstraintVariable *variable);
+
+typedef struct {
+  GtkConstraintVariable *first;
+  GtkConstraintVariable *second;
+} GtkConstraintVariablePair;
+
+GtkConstraintVariablePair *
+gtk_constraint_variable_pair_new (GtkConstraintVariable *first,
+                                  GtkConstraintVariable *second);
+
+void
+gtk_constraint_variable_pair_free (GtkConstraintVariablePair *pair);
+
+typedef struct _GtkConstraintVariableSet        GtkConstraintVariableSet;
+
+GtkConstraintVariableSet *
+gtk_constraint_variable_set_new (void);
+
+void
+gtk_constraint_variable_set_free (GtkConstraintVariableSet *set);
+
+gboolean
+gtk_constraint_variable_set_add (GtkConstraintVariableSet *set,
+                                 GtkConstraintVariable *variable);
+
+gboolean
+gtk_constraint_variable_set_remove (GtkConstraintVariableSet *set,
+                                    GtkConstraintVariable *variable);
+
+int
+gtk_constraint_variable_set_size (GtkConstraintVariableSet *set);
+
+typedef struct {
+  /*< private >*/
+  gpointer dummy1;
+  gpointer dummy2;
+  gint64 dummy3;
+} GtkConstraintVariableSetIter;
+
+void
+gtk_constraint_variable_set_iter_init (GtkConstraintVariableSetIter *iter,
+                                       GtkConstraintVariableSet *set);
+
+gboolean
+gtk_constraint_variable_set_iter_next (GtkConstraintVariableSetIter *iter,
+                                       GtkConstraintVariable **variable_p);
+
+GtkConstraintExpression *
+gtk_constraint_expression_new (double constant);
+
+GtkConstraintExpression *
+gtk_constraint_expression_new_from_variable (GtkConstraintVariable *variable);
+
+GtkConstraintExpression *
+gtk_constraint_expression_ref (GtkConstraintExpression *expression);
+
+void
+gtk_constraint_expression_unref (GtkConstraintExpression *expression);
+
+GtkConstraintExpression *
+gtk_constraint_expression_clone (GtkConstraintExpression *expression);
+
+void
+gtk_constraint_expression_set_constant (GtkConstraintExpression *expression,
+                                        double constant);
+double
+gtk_constraint_expression_get_constant (const GtkConstraintExpression *expression);
+
+gboolean
+gtk_constraint_expression_is_constant (const GtkConstraintExpression *expression);
+
+void
+gtk_constraint_expression_add_expression (GtkConstraintExpression *a_expr,
+                                          GtkConstraintExpression *b_expr,
+                                          double n,
+                                          GtkConstraintVariable *subject,
+                                          GtkConstraintSolver *solver);
+
+void
+gtk_constraint_expression_add_variable (GtkConstraintExpression *expression,
+                                        GtkConstraintVariable *variable,
+                                        double coefficient,
+                                        GtkConstraintVariable *subject,
+                                        GtkConstraintSolver *solver);
+
+void
+gtk_constraint_expression_remove_variable (GtkConstraintExpression *expression,
+                                           GtkConstraintVariable *variable);
+
+void
+gtk_constraint_expression_set_variable (GtkConstraintExpression *expression,
+                                        GtkConstraintVariable *variable,
+                                        double coefficient);
+
+double
+gtk_constraint_expression_get_coefficient (GtkConstraintExpression *expression,
+                                           GtkConstraintVariable *variable);
+
+char *
+gtk_constraint_expression_to_string (const GtkConstraintExpression *expression);
+
+double
+gtk_constraint_expression_new_subject (GtkConstraintExpression *expression,
+                                       GtkConstraintVariable *subject);
+
+void
+gtk_constraint_expression_change_subject (GtkConstraintExpression *expression,
+                                          GtkConstraintVariable *old_subject,
+                                          GtkConstraintVariable *new_subject);
+
+void
+gtk_constraint_expression_substitute_out (GtkConstraintExpression *expression,
+                                          GtkConstraintVariable *out_var,
+                                          GtkConstraintExpression *expr,
+                                          GtkConstraintVariable *subject,
+                                          GtkConstraintSolver *solver);
+
+GtkConstraintVariable *
+gtk_constraint_expression_get_pivotable_variable (GtkConstraintExpression *expression);
+
+GtkConstraintExpression *
+gtk_constraint_expression_plus_constant (GtkConstraintExpression *expression,
+                                         double constant);
+
+GtkConstraintExpression *
+gtk_constraint_expression_minus_constant (GtkConstraintExpression *expression,
+                                          double constant);
+
+GtkConstraintExpression *
+gtk_constraint_expression_plus_variable (GtkConstraintExpression *expression,
+                                         GtkConstraintVariable *variable);
+
+GtkConstraintExpression *
+gtk_constraint_expression_minus_variable (GtkConstraintExpression *expression,
+                                          GtkConstraintVariable *variable);
+
+GtkConstraintExpression *
+gtk_constraint_expression_multiply_by (GtkConstraintExpression *expression,
+                                       double factor);
+
+GtkConstraintExpression *
+gtk_constraint_expression_divide_by (GtkConstraintExpression *expression,
+                                     double factor);
+
+struct _GtkConstraintExpressionBuilder
+{
+  /*< private >*/
+  gpointer dummy1;
+  gpointer dummy2;
+  int dummy3;
+};
+
+void
+gtk_constraint_expression_builder_init (GtkConstraintExpressionBuilder *builder,
+                                        GtkConstraintSolver *solver);
+
+void
+gtk_constraint_expression_builder_term (GtkConstraintExpressionBuilder *builder,
+                                        GtkConstraintVariable *term);
+
+void
+gtk_constraint_expression_builder_plus (GtkConstraintExpressionBuilder *builder);
+
+void
+gtk_constraint_expression_builder_minus (GtkConstraintExpressionBuilder *builder);
+
+void
+gtk_constraint_expression_builder_divide_by (GtkConstraintExpressionBuilder *builder);
+
+void
+gtk_constraint_expression_builder_multiply_by (GtkConstraintExpressionBuilder *builder);
+
+void
+gtk_constraint_expression_builder_constant (GtkConstraintExpressionBuilder *builder,
+                                            double value);
+
+GtkConstraintExpression *
+gtk_constraint_expression_builder_finish (GtkConstraintExpressionBuilder *builder) G_GNUC_WARN_UNUSED_RESULT;
+
+/*< private >
+ * GtkConstraintExpressionIter:
+ *
+ * An iterator object for terms inside a #GtkConstraintExpression.
+ */
+typedef struct {
+  /*< private >*/
+  gpointer dummy1;
+  gpointer dummy2;
+  gint64 dummy3;
+} GtkConstraintExpressionIter;
+
+void
+gtk_constraint_expression_iter_init (GtkConstraintExpressionIter *iter,
+                                     GtkConstraintExpression *equation);
+
+gboolean
+gtk_constraint_expression_iter_next (GtkConstraintExpressionIter *iter,
+                                     GtkConstraintVariable **variable,
+                                     double *coefficient);
+
+gboolean
+gtk_constraint_expression_iter_prev (GtkConstraintExpressionIter *iter,
+                                     GtkConstraintVariable **variable,
+                                     double *coefficient);
+
+G_END_DECLS

gtk/gtkconstraintguide.c

@@ -0,0 +1,498 @@
+/* gtkconstraintguide.c: Flexible space for constraints
+ * Copyright 2019 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Matthias Clasen
+ */
+
+#include "config.h"
+
+#include "gtkconstraintguide.h"
+
+#include "gtkconstraintguideprivate.h"
+#include "gtkconstraintlayoutprivate.h"
+#include "gtkconstraintexpressionprivate.h"
+#include "gtkconstraintsolverprivate.h"
+
+#include "gtkdebug.h"
+#include "gtkintl.h"
+#include "gtkprivate.h"
+
+
+typedef enum {
+  GUIDE_MIN_WIDTH,
+  GUIDE_MIN_HEIGHT,
+  GUIDE_NAT_WIDTH,
+  GUIDE_NAT_HEIGHT,
+  GUIDE_MAX_WIDTH,
+  GUIDE_MAX_HEIGHT,
+  LAST_GUIDE_VALUE
+} GuideValue;
+
+struct _GtkConstraintGuide
+{ 
+  GObject parent_instance;
+
+  int values[LAST_GUIDE_VALUE];
+
+  GtkConstraintLayout *layout;
+
+  /* HashTable<static string, Variable>; a hash table of variables,
+   * one for each attribute; we use these to query and suggest the
+   * values for the solver. The string is static and does not need
+   * to be freed.
+   */
+  GHashTable *bound_attributes;
+
+  GtkConstraintRef *constraints[LAST_GUIDE_VALUE];
+};
+
+
+struct _GtkConstraintGuideClass {
+  GObjectClass parent_class;
+};
+
+enum {
+  GUIDE_PROP_MIN_WIDTH = 1,
+  GUIDE_PROP_MIN_HEIGHT,
+  GUIDE_PROP_NAT_WIDTH,
+  GUIDE_PROP_NAT_HEIGHT,
+  GUIDE_PROP_MAX_WIDTH,
+  GUIDE_PROP_MAX_HEIGHT,
+  LAST_GUIDE_PROP
+};
+
+static GParamSpec *guide_props[LAST_GUIDE_PROP];
+
+static void
+gtk_constraint_guide_constraint_target_iface_init (GtkConstraintTargetInterface *iface)
+{
+}
+
+G_DEFINE_TYPE_WITH_CODE (GtkConstraintGuide, gtk_constraint_guide, G_TYPE_OBJECT,
+                         G_IMPLEMENT_INTERFACE (GTK_TYPE_CONSTRAINT_TARGET,
+                                                gtk_constraint_guide_constraint_target_iface_init))
+
+static void
+gtk_constraint_guide_init (GtkConstraintGuide *guide)
+{
+  guide->bound_attributes =
+    g_hash_table_new_full (g_str_hash, g_str_equal,
+                           NULL,
+                           (GDestroyNotify) gtk_constraint_variable_unref);
+}
+
+static void
+gtk_constraint_guide_update_constraint (GtkConstraintGuide *guide,
+                                        GuideValue          index)
+{
+  GtkConstraintSolver *solver;
+  GtkConstraintVariable *var;
+  int attr[LAST_GUIDE_VALUE] = {
+    GTK_CONSTRAINT_ATTRIBUTE_WIDTH,
+    GTK_CONSTRAINT_ATTRIBUTE_HEIGHT,
+    GTK_CONSTRAINT_ATTRIBUTE_WIDTH,
+    GTK_CONSTRAINT_ATTRIBUTE_HEIGHT,
+    GTK_CONSTRAINT_ATTRIBUTE_WIDTH,
+    GTK_CONSTRAINT_ATTRIBUTE_HEIGHT,
+  };
+  int relation[LAST_GUIDE_VALUE] = {
+    GTK_CONSTRAINT_RELATION_GE,
+    GTK_CONSTRAINT_RELATION_GE,
+    GTK_CONSTRAINT_RELATION_EQ,
+    GTK_CONSTRAINT_RELATION_EQ,
+    GTK_CONSTRAINT_RELATION_LE,
+    GTK_CONSTRAINT_RELATION_LE,
+  };
+  double weight[LAST_GUIDE_VALUE] = {
+    GTK_CONSTRAINT_WEIGHT_REQUIRED,
+    GTK_CONSTRAINT_WEIGHT_REQUIRED,
+    GTK_CONSTRAINT_WEIGHT_MEDIUM,
+    GTK_CONSTRAINT_WEIGHT_MEDIUM,
+    GTK_CONSTRAINT_WEIGHT_REQUIRED,
+    GTK_CONSTRAINT_WEIGHT_REQUIRED,
+  };
+
+  if (!guide->layout)
+    return;
+
+  solver = gtk_constraint_layout_get_solver (guide->layout);
+  if (!solver)
+    return;
+
+  if (guide->constraints[index] != NULL)
+    gtk_constraint_solver_remove_constraint (solver, guide->constraints[index]);
+
+  var = gtk_constraint_layout_get_attribute (guide->layout, attr[index], "guide", NULL, guide->bound_attributes);
+  guide->constraints[index] =
+    gtk_constraint_solver_add_constraint (solver,
+                                          var,
+                                          relation[index],
+                                          gtk_constraint_expression_new (guide->values[index]),
+                                          weight[index]);
+}
+
+void
+gtk_constraint_guide_update (GtkConstraintGuide *guide)
+{
+  int i;
+
+  for (i = 0; i < LAST_GUIDE_VALUE; i++)
+    gtk_constraint_guide_update_constraint (guide, i);
+}
+
+void
+gtk_constraint_guide_detach (GtkConstraintGuide *guide)
+{
+  GtkConstraintSolver *solver;
+  int i;
+
+  if (!guide->layout)
+    return;
+
+  solver = gtk_constraint_layout_get_solver (guide->layout);
+  if (!solver)
+    return;
+
+  for (i = 0; i < LAST_GUIDE_VALUE; i++)
+    {
+      gtk_constraint_solver_remove_constraint (solver, guide->constraints[i]);
+      guide->constraints[i] = NULL;
+    }
+
+  g_hash_table_remove_all (guide->bound_attributes);
+}
+
+GtkConstraintVariable *
+gtk_constraint_guide_get_attribute (GtkConstraintGuide     *guide,
+                                    GtkConstraintAttribute  attr)
+{
+  GtkLayoutManager *manager = GTK_LAYOUT_MANAGER (guide->layout);
+  GtkWidget *widget = gtk_layout_manager_get_widget (manager);
+
+  return gtk_constraint_layout_get_attribute (guide->layout, attr, "guide", widget, guide->bound_attributes);
+}
+
+GtkConstraintLayout *
+gtk_constraint_guide_get_layout (GtkConstraintGuide *guide)
+{
+  return guide->layout;
+}
+
+void
+gtk_constraint_guide_set_layout (GtkConstraintGuide  *guide,
+                                 GtkConstraintLayout *layout)
+{
+  guide->layout = layout;
+}
+
+static void
+gtk_constraint_guide_set_property (GObject      *gobject,
+                                   guint         prop_id,
+                                   const GValue *value,
+                                   GParamSpec   *pspec)
+{
+  GtkConstraintGuide *self = GTK_CONSTRAINT_GUIDE (gobject);
+  int val;
+  GuideValue index;
+
+  switch (prop_id)
+    {
+    case GUIDE_PROP_MIN_WIDTH:
+    case GUIDE_PROP_MIN_HEIGHT:
+    case GUIDE_PROP_NAT_WIDTH:
+    case GUIDE_PROP_NAT_HEIGHT:
+    case GUIDE_PROP_MAX_WIDTH:
+    case GUIDE_PROP_MAX_HEIGHT:
+      val = g_value_get_int (value);
+      index = prop_id - 1;
+      if (self->values[index] != val)
+        {
+          self->values[index] = val;
+          g_object_notify_by_pspec (gobject, pspec);
+          gtk_constraint_guide_update_constraint (self, index);
+        }
+      break;
+
+    default:
+      G_OBJECT_WARN_INVALID_PROPERTY_ID (gobject, prop_id, pspec);
+      break;
+    }
+}
+
+static void
+gtk_constraint_guide_get_property (GObject    *gobject,
+                                   guint       prop_id,
+                                   GValue     *value,
+                                   GParamSpec *pspec)
+{
+  GtkConstraintGuide *self = GTK_CONSTRAINT_GUIDE (gobject);
+
+  switch (prop_id)
+    {
+    case GUIDE_PROP_MIN_WIDTH:
+    case GUIDE_PROP_MIN_HEIGHT:
+    case GUIDE_PROP_NAT_WIDTH:
+    case GUIDE_PROP_NAT_HEIGHT:
+    case GUIDE_PROP_MAX_WIDTH:
+    case GUIDE_PROP_MAX_HEIGHT:
+      g_value_set_int (value, self->values[prop_id - 1]);
+      break;
+
+    default:
+      G_OBJECT_WARN_INVALID_PROPERTY_ID (gobject, prop_id, pspec);
+      break;
+    }
+}
+
+static void
+gtk_constraint_guide_finalize (GObject *object)
+{
+  GtkConstraintGuide *self = GTK_CONSTRAINT_GUIDE (object);
+
+  g_clear_pointer (&self->bound_attributes, g_hash_table_unref);
+
+  G_OBJECT_CLASS (gtk_constraint_guide_parent_class)->finalize (object);
+}
+
+static void
+gtk_constraint_guide_class_init (GtkConstraintGuideClass *class)
+{
+  GObjectClass *object_class = G_OBJECT_CLASS (class);
+
+  object_class->finalize = gtk_constraint_guide_finalize;
+  object_class->set_property = gtk_constraint_guide_set_property;
+  object_class->get_property = gtk_constraint_guide_get_property;
+
+  guide_props[GUIDE_PROP_MIN_WIDTH] =
+      g_param_spec_int ("min-width",
+                        "Minimum width",
+                        "Minimum width",
+                        0, G_MAXINT, 0,
+                        G_PARAM_READWRITE|
+                        G_PARAM_EXPLICIT_NOTIFY);
+  guide_props[GUIDE_PROP_MIN_HEIGHT] =
+      g_param_spec_int ("min-height",
+                        "Minimum height",
+                        "Minimum height",
+                        0, G_MAXINT, 0,
+                        G_PARAM_READWRITE|
+                        G_PARAM_EXPLICIT_NOTIFY);
+  guide_props[GUIDE_PROP_NAT_WIDTH] =
+      g_param_spec_int ("nat-width",
+                        "Natural width",
+                        "Natural width",
+                        0, G_MAXINT, 0,
+                        G_PARAM_READWRITE|
+                        G_PARAM_EXPLICIT_NOTIFY);
+  guide_props[GUIDE_PROP_NAT_HEIGHT] =
+      g_param_spec_int ("nat-height",
+                        "Natural height",
+                        "Natural height",
+                        0, G_MAXINT, 0,
+                        G_PARAM_READWRITE|
+                        G_PARAM_EXPLICIT_NOTIFY);
+  guide_props[GUIDE_PROP_MAX_WIDTH] =
+      g_param_spec_int ("max-width",
+                        "Maximum width",
+                        "Maximum width",
+                        0, G_MAXINT, G_MAXINT,
+                        G_PARAM_READWRITE|
+                        G_PARAM_EXPLICIT_NOTIFY);
+  guide_props[GUIDE_PROP_MAX_HEIGHT] =
+      g_param_spec_int ("max-height",
+                        "Maximum height",
+                        "Maximum height",
+                        0, G_MAXINT, G_MAXINT,
+                        G_PARAM_READWRITE|
+                        G_PARAM_EXPLICIT_NOTIFY);
+
+  g_object_class_install_properties (object_class, LAST_GUIDE_PROP, guide_props);
+}
+
+/**
+ * gtk_constraint_guide_new:
+ *
+ * Creates a new #GtkConstraintGuide object.
+ *
+ * Return: a new #GtkConstraintGuide object.
+ */
+GtkConstraintGuide *
+gtk_constraint_guide_new (void)
+{
+  return g_object_new (GTK_TYPE_CONSTRAINT_GUIDE, NULL);
+}
+
+/**
+ * gtk_constraint_guide_set_min_size:
+ * @guide: a #GtkConstraintGuide object
+ * @width: the new minimum width, or -1 to not change it
+ * @height: the new minimum height, or -1 to not change it
+ *
+ * Sets the minimum size of @guide.
+ *
+ * If @guide is attached to a #GtkConstraintLayout,
+ * the constraints will be updated to reflect the new size.
+ */
+void
+gtk_constraint_guide_set_min_size (GtkConstraintGuide *guide,
+                                   int                 width,
+                                   int                 height)
+{
+  g_return_if_fail (GTK_IS_CONSTRAINT_GUIDE (guide));
+  g_return_if_fail (width >= -1);
+  g_return_if_fail (height >= -1);
+
+  g_object_freeze_notify (G_OBJECT (guide));
+
+  if (width != -1)
+    g_object_set (guide, "min-width", width, NULL);
+
+  if (height != -1)
+    g_object_set (guide, "min-height", height, NULL);
+
+  g_object_thaw_notify (G_OBJECT (guide));
+}
+
+/**
+ * gtk_constraint_guide_get_min_size:
+ * @guide: a #GtkContraintGuide object
+ * @width: (allow-none): return location for the minimum width,
+ *     or %NULL
+ * @height: (allow-none): return location for the minimum height,
+ *     or %NULL
+ *
+ * Gets the minimum size of @guide.
+ */
+void
+gtk_constraint_guide_get_min_size (GtkConstraintGuide *guide,
+                                   int                *width,
+                                   int                *height)
+{
+  g_return_if_fail (GTK_IS_CONSTRAINT_GUIDE (guide));
+
+  if (width)
+    *width = guide->values[GUIDE_MIN_WIDTH];
+  if (height)
+    *height = guide->values[GUIDE_MIN_HEIGHT];
+}
+
+/**
+ * gtk_constraint_guide_set_nat_size:
+ * @guide: a #GtkConstraintGuide object
+ * @width: the new natural width, or -1 to not change it
+ * @height: the new natural height, or -1 to not change it
+ *
+ * Sets the natural size of @guide.
+ *
+ * If @guide is attached to a #GtkConstraintLayout,
+ * the constraints will be updated to reflect the new size.
+ */
+void
+gtk_constraint_guide_set_nat_size (GtkConstraintGuide *guide,
+                                   int                 width,
+                                   int                 height)
+{
+  g_return_if_fail (GTK_IS_CONSTRAINT_GUIDE (guide));
+  g_return_if_fail (width >= -1);
+  g_return_if_fail (height >= -1);
+
+  g_object_freeze_notify (G_OBJECT (guide));
+
+  if (width != -1)
+    g_object_set (guide, "nat-width", width, NULL);
+
+  if (height != -1)
+    g_object_set (guide, "nat-height", height, NULL);
+
+  g_object_thaw_notify (G_OBJECT (guide));
+}
+
+/**
+ * gtk_constraint_guide_get_nat_size:
+ * @guide: a #GtkContraintGuide object
+ * @width: (allow-none): return location for the natural width,
+ *     or %NULL
+ * @height: (allow-none): return location for the natural height,
+ *     or %NULL
+ *
+ * Gets the natural size of @guide.
+ */
+void
+gtk_constraint_guide_get_nat_size (GtkConstraintGuide *guide,
+                                   int                *width,
+                                   int                *height)
+{
+  g_return_if_fail (GTK_IS_CONSTRAINT_GUIDE (guide));
+
+  if (width)
+    *width = guide->values[GUIDE_NAT_WIDTH];
+  if (height)
+    *height = guide->values[GUIDE_NAT_HEIGHT];
+}
+
+/**
+ * gtk_constraint_guide_set_max_size:
+ * @guide: a #GtkConstraintGuide object
+ * @width: the new maximum width, or -1 to not change it
+ * @height: the new maximum height, or -1 to not change it
+ *
+ * Sets the maximum size of @guide.
+ *
+ * If @guide is attached to a #GtkConstraintLayout,
+ * the constraints will be updated to reflect the new size.
+ */
+void
+gtk_constraint_guide_set_max_size (GtkConstraintGuide *guide,
+                                   int                 width,
+                                   int                 height)
+{
+  g_return_if_fail (GTK_IS_CONSTRAINT_GUIDE (guide));
+  g_return_if_fail (width >= -1);
+  g_return_if_fail (height >= -1);
+
+  g_object_freeze_notify (G_OBJECT (guide));
+
+  if (width != -1)
+    g_object_set (guide, "max-width", width, NULL);
+
+  if (height != -1)
+    g_object_set (guide, "max-height", height, NULL);
+
+  g_object_thaw_notify (G_OBJECT (guide));
+}
+
+/**
+ * gtk_constraint_guide_get_max_size:
+ * @guide: a #GtkContraintGuide object
+ * @width: (allow-none): return location for the maximum width,
+ *     or %NULL
+ * @height: (allow-none): return location for the maximum height,
+ *     or %NULL
+ *
+ * Gets the maximum size of @guide.
+ */
+void
+gtk_constraint_guide_get_max_size (GtkConstraintGuide *guide,
+                                   int                *width,
+                                   int                *height)
+{
+  g_return_if_fail (GTK_IS_CONSTRAINT_GUIDE (guide));
+
+  if (width)
+    *width = guide->values[GUIDE_MAX_WIDTH];
+  if (height)
+    *height = guide->values[GUIDE_MAX_HEIGHT];
+}

gtk/gtkconstraintguide.h

@@ -0,0 +1,71 @@
+/* gtkconstraintguide.h: Flexible space for constraints
+ * Copyright 2019 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Matthias Clasen
+ */
+
+#pragma once
+
+#include <gtk/gtktypes.h>
+#include <gtk/gtkenums.h>
+
+G_BEGIN_DECLS
+
+
+#define GTK_TYPE_CONSTRAINT_GUIDE (gtk_constraint_guide_get_type ())
+
+/**
+ * GtkConstraintGuide:
+ *
+ * An object that can be added to a #GtkConstraintLayout and be
+ * used in constraints like a widget, without being drawn.
+ *
+ * Guides have a minimum, maximum and natural size. Depending
+ * on the constraints that are applied, they can act like a
+ * guideline that widgets can be aligned to, or like 'flexible space'.
+ */
+GDK_AVAILABLE_IN_ALL
+G_DECLARE_FINAL_TYPE (GtkConstraintGuide, gtk_constraint_guide, GTK, CONSTRAINT_GUIDE, GObject)
+
+GDK_AVAILABLE_IN_ALL
+GtkConstraintGuide *    gtk_constraint_guide_new                (void);
+
+GDK_AVAILABLE_IN_ALL
+void                    gtk_constraint_guide_set_min_size       (GtkConstraintGuide *guide,
+                                                                 int                 width,
+                                                                 int                 height);
+GDK_AVAILABLE_IN_ALL
+void                    gtk_constraint_guide_get_min_size       (GtkConstraintGuide *guide,
+                                                                 int                *width,
+                                                                 int                *height);
+GDK_AVAILABLE_IN_ALL
+void                    gtk_constraint_guide_set_nat_size       (GtkConstraintGuide *guide,
+                                                                 int                 width,
+                                                                 int                 height);
+GDK_AVAILABLE_IN_ALL
+void                    gtk_constraint_guide_get_nat_size       (GtkConstraintGuide *guide,
+                                                                 int                *width,
+                                                                 int                *height);
+GDK_AVAILABLE_IN_ALL
+void                    gtk_constraint_guide_set_max_size       (GtkConstraintGuide *guide,
+                                                                 int                 width,
+                                                                 int                 height);
+GDK_AVAILABLE_IN_ALL
+void                    gtk_constraint_guide_get_max_size       (GtkConstraintGuide *guide,
+                                                                 int                *width,
+                                                                 int                *height);
+
+G_END_DECLS

gtk/gtkconstraintguideprivate.h

@@ -0,0 +1,38 @@
+/* gtkconstraintguideprivate.h: Constraint between two widgets
+ * Copyright 2019 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Matthias Clasen
+ */
+
+#pragma once
+
+#include "gtkconstraintguide.h"
+#include "gtkconstraintlayout.h"
+#include "gtkconstrainttypesprivate.h"
+
+G_BEGIN_DECLS
+
+void                   gtk_constraint_guide_update        (GtkConstraintGuide     *guide);
+void                   gtk_constraint_guide_detach        (GtkConstraintGuide     *guide);
+
+GtkConstraintVariable *gtk_constraint_guide_get_attribute (GtkConstraintGuide      *guide,
+                                                           GtkConstraintAttribute  attr);
+
+GtkConstraintLayout   *gtk_constraint_guide_get_layout    (GtkConstraintGuide     *guide);
+void                   gtk_constraint_guide_set_layout    (GtkConstraintGuide     *guide,
+                                                           GtkConstraintLayout    *layout);
+
+G_END_DECLS

gtk/gtkconstraintlayout.h

@@ -0,0 +1,64 @@
+/* gtkconstraintlayout.h: Layout manager using constraints
+ * Copyright 2019  GNOME Foundation
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Emmanuele Bassi
+ */
+#pragma once
+
+#include <gtk/gtklayoutmanager.h>
+#include <gtk/gtkconstraint.h>
+#include <gtk/gtkconstraintguide.h>
+
+G_BEGIN_DECLS
+
+#define GTK_TYPE_CONSTRAINT_LAYOUT (gtk_constraint_layout_get_type ())
+#define GTK_TYPE_CONSTRAINT_LAYOUT_CHILD (gtk_constraint_layout_child_get_type ())
+
+/**
+ * GtkConstraintLayoutChild:
+ *
+ * A #GtkLayoutChild in a #GtkConstraintLayout.
+ */
+GDK_AVAILABLE_IN_ALL
+G_DECLARE_FINAL_TYPE (GtkConstraintLayoutChild, gtk_constraint_layout_child, GTK, CONSTRAINT_LAYOUT_CHILD, GtkLayoutChild)
+
+/**
+ * GtkConstraintLayout:
+ *
+ * A layout manager using #GtkConstraint to describe
+ * relations between widgets.
+ */
+GDK_AVAILABLE_IN_ALL
+G_DECLARE_FINAL_TYPE (GtkConstraintLayout, gtk_constraint_layout, GTK, CONSTRAINT_LAYOUT, GtkLayoutManager)
+
+GDK_AVAILABLE_IN_ALL
+GtkLayoutManager *      gtk_constraint_layout_new               (void);
+
+GDK_AVAILABLE_IN_ALL
+void                    gtk_constraint_layout_add_constraint    (GtkConstraintLayout *manager,
+                                                                 GtkConstraint       *constraint);
+GDK_AVAILABLE_IN_ALL
+void                    gtk_constraint_layout_remove_constraint (GtkConstraintLayout *manager,
+                                                                 GtkConstraint       *constraint);
+
+GDK_AVAILABLE_IN_ALL
+void                    gtk_constraint_layout_add_guide         (GtkConstraintLayout *manager,
+                                                                 GtkConstraintGuide  *guide);
+GDK_AVAILABLE_IN_ALL
+void                    gtk_constraint_layout_remove_guide      (GtkConstraintLayout *manager,
+                                                                 GtkConstraintGuide  *guide);
+
+G_END_DECLS

gtk/gtkconstraintprivate.h

@@ -0,0 +1,62 @@
+/* gtkconstraintprivate.h: Constraint between two widgets
+ * Copyright 2019  GNOME Foundation
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Emmanuele Bassi
+ */
+
+#pragma once
+
+#include "gtkconstraint.h"
+#include "gtkconstrainttypesprivate.h"
+
+G_BEGIN_DECLS
+
+struct _GtkConstraint
+{
+  GObject parent_instance;
+
+  GtkConstraintAttribute target_attribute;
+  GtkConstraintAttribute source_attribute;
+
+  GtkConstraintTarget *target;
+  GtkConstraintTarget *source;
+
+  GtkConstraintRelation relation;
+
+  double multiplier;
+  double constant;
+
+  int strength;
+
+  /* A reference to the real constraint inside the
+   * GtkConstraintSolver, so we can remove it when
+   * finalizing the GtkConstraint instance
+   */
+  GtkConstraintRef *constraint_ref;
+
+  GtkConstraintSolver *solver;
+
+  guint active : 1;
+};
+
+double  gtk_constraint_get_weight       (GtkConstraint       *constraint);
+
+void    gtk_constraint_attach           (GtkConstraint       *constraint,
+                                         GtkConstraintSolver *solver,
+                                         GtkConstraintRef    *ref);
+void    gtk_constraint_detach           (GtkConstraint       *constraint);
+
+G_END_DECLS

gtk/gtkconstraintsolverprivate.h

@@ -0,0 +1,165 @@
+/* gtkconstraintsolverprivate.h: Constraint solver based on the Cassowary method
+ * Copyright 2019  GNOME Foundation
+ *
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Emmanuele Bassi
+ */
+
+#pragma once
+
+#include "gtkconstrainttypesprivate.h"
+
+G_BEGIN_DECLS
+
+#ifdef GTK_TEST_EXTERNAL
+
+#define GTK_TYPE_CONSTRAINT_SOLVER (g_type_from_name ("GtkConstraintSolver"))
+#define GTK_CONSTRAINT_SOLVER(solver)                (G_TYPE_CHECK_INSTANCE_CAST ((solver), GTK_TYPE_CONSTRAINT_SOLVER, GtkConstraintSolver))
+#define GTK_CONSTRAINT_SOLVER_CLASS(klass)           (G_TYPE_CHECK_CLASS_CAST ((klass), GTK_TYPE_CONSTRAINT_SOLVER, GtkConstraintSolverClass))
+#define GTK_IS_CONSTRAINT_SOLVER(solver)             (G_TYPE_CHECK_INSTANCE_TYPE ((solver), GTK_TYPE_CONSTRAINT_SOLVER))
+#define GTK_IS_CONSTRAINT_SOLVER_CLASS(klass)        (G_TYPE_CHECK_CLASS_TYPE ((klass), GTK_TYPE_CONSTRAINT_SOLVER))
+#define GTK_CONSTRAINT_SOLVER_GET_CLASS(obj)         (G_TYPE_INSTANCE_GET_CLASS ((obj), GTK_TYPE_CONSTRAINT_SOLVER, GtkConstraintSolverClass))
+
+typedef struct _GtkConstraintSolver GtkConstraintSolver;
+typedef struct _GtkConstraintSolverClass GtkConstraintSolverClass;
+
+struct _GtkConstraintSolverClass {
+  GObjectClass parent_class;
+};
+
+#else
+
+#define GTK_TYPE_CONSTRAINT_SOLVER (gtk_constraint_solver_get_type())
+
+G_DECLARE_FINAL_TYPE (GtkConstraintSolver, gtk_constraint_solver, GTK, CONSTRAINT_SOLVER, GObject)
+
+#endif
+
+/* Symbolic weight thresholds
+ *
+ * Constraint weights live on a continuum, but we use thresholds for simplicity's
+ * sake, so we don't have to necessarily reason in terms of numeric values.
+ *
+ * The public API has a similar approach, where the symbolic constants are negative
+ * values, and positive values are explicit weights. We map those values into
+ * numeric values that the GtkConstraintSolver can plug into the linear equations
+ * tableau.
+ */
+#define GTK_CONSTRAINT_WEIGHT_REQUIRED  (make_weight (1000, 1000, 1000, 1))
+#define GTK_CONSTRAINT_WEIGHT_STRONG    (make_weight (   1,    0,    0, 1))
+#define GTK_CONSTRAINT_WEIGHT_MEDIUM    (make_weight (   0,    1,    0, 1))
+#define GTK_CONSTRAINT_WEIGHT_WEAK      (make_weight (   0,    0,    1, 1))
+
+G_GNUC_PURE
+static inline double
+make_weight (double a,
+             double b,
+             double c,
+             double w)
+{
+  double res = 0;
+
+  res += CLAMP (a * w, 0, 1000) * 1000000;
+  res += CLAMP (b * w, 0, 1000) * 1000;
+  res += CLAMP (c * w, 0, 1000);
+
+  return res;
+}
+
+GtkConstraintSolver *
+gtk_constraint_solver_new (void);
+
+void
+gtk_constraint_solver_freeze (GtkConstraintSolver *solver);
+
+void
+gtk_constraint_solver_thaw (GtkConstraintSolver *solver);
+
+void
+gtk_constraint_solver_resolve (GtkConstraintSolver *solver);
+
+GtkConstraintVariable *
+gtk_constraint_solver_create_variable (GtkConstraintSolver *solver,
+                                       const char          *prefix,
+                                       const char          *name,
+                                       double               value);
+
+GtkConstraintRef *
+gtk_constraint_solver_add_constraint (GtkConstraintSolver     *solver,
+                                      GtkConstraintVariable   *variable,
+                                      GtkConstraintRelation    relation,
+                                      GtkConstraintExpression *expression,
+                                      double                   strength);
+
+void
+gtk_constraint_solver_remove_constraint (GtkConstraintSolver *solver,
+                                         GtkConstraintRef    *reference);
+
+GtkConstraintRef *
+gtk_constraint_solver_add_stay_variable (GtkConstraintSolver   *solver,
+                                         GtkConstraintVariable *variable,
+                                         double                 strength);
+
+void
+gtk_constraint_solver_remove_stay_variable (GtkConstraintSolver   *solver,
+                                            GtkConstraintVariable *variable);
+
+gboolean
+gtk_constraint_solver_has_stay_variable (GtkConstraintSolver   *solver,
+                                         GtkConstraintVariable *variable);
+
+GtkConstraintRef *
+gtk_constraint_solver_add_edit_variable (GtkConstraintSolver   *solver,
+                                         GtkConstraintVariable *variable,
+                                         double                 strength);
+
+void
+gtk_constraint_solver_remove_edit_variable (GtkConstraintSolver   *solver,
+                                            GtkConstraintVariable *variable);
+
+gboolean
+gtk_constraint_solver_has_edit_variable (GtkConstraintSolver   *solver,
+                                         GtkConstraintVariable *variable);
+
+void
+gtk_constraint_solver_suggest_value (GtkConstraintSolver   *solver,
+                                     GtkConstraintVariable *variable,
+                                     double                 value);
+
+void
+gtk_constraint_solver_begin_edit (GtkConstraintSolver *solver);
+
+void
+gtk_constraint_solver_end_edit (GtkConstraintSolver *solver);
+
+void
+gtk_constraint_solver_note_added_variable (GtkConstraintSolver *self,
+                                           GtkConstraintVariable *variable,
+                                           GtkConstraintVariable *subject);
+
+void
+gtk_constraint_solver_note_removed_variable (GtkConstraintSolver *self,
+                                             GtkConstraintVariable *variable,
+                                             GtkConstraintVariable *subject);
+
+void
+gtk_constraint_solver_clear (GtkConstraintSolver *solver);
+
+char *
+gtk_constraint_solver_to_string (GtkConstraintSolver *solver);
+
+G_END_DECLS

gtk/gtkconstrainttypesprivate.h

@@ -0,0 +1,50 @@
+/* gtkconstrainttypesprivate.h: Private types for the constraint solver
+ * Copyright 2019  GNOME Foundation
+ *
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Emmanuele Bassi
+ */
+
+#pragma once
+
+#include <gtk/gtktypes.h>
+#include <gtk/gtkenums.h>
+
+G_BEGIN_DECLS
+
+typedef struct _GtkConstraintVariable           GtkConstraintVariable;
+typedef struct _GtkConstraintExpression         GtkConstraintExpression;
+typedef struct _GtkConstraintExpressionBuilder  GtkConstraintExpressionBuilder;
+
+/*< private >
+ * GtkConstraintRef:
+ *
+ * A reference to a constraint stored inside the solver; while #GtkConstraint
+ * represent the public API, a #GtkConstraintRef represents data stored inside
+ * the solver. A #GtkConstraintRef is completely opaque, and should only be
+ * used to remove a constraint from the solver.
+ */
+typedef struct _GtkConstraintRef                GtkConstraintRef;
+
+/*< private >
+ * GtkConstraintSolver:
+ *
+ * A simplex solver using the Cassowary constraint solving algorithm.
+ */
+typedef struct _GtkConstraintSolver             GtkConstraintSolver;
+
+G_END_DECLS

gtk/gtkenums.h

@@ -1053,4 +1053,78 @@ typedef enum {
   GTK_PICK_NON_TARGETABLE = 1 << 1
 } GtkPickFlags;
 
+/**
+ * GtkConstraintRelation:
+ * @GTK_CONSTRAINT_RELATION_EQ: Equal
+ * @GTK_CONSTRAINT_RELATION_LE: Less than, or equal
+ * @GTK_CONSTRAINT_RELATION_GE: Greater than, or equal
+ *
+ * The relation between two terms of a constraint.
+ */
+typedef enum {
+  GTK_CONSTRAINT_RELATION_LE = -1,
+  GTK_CONSTRAINT_RELATION_EQ = 0,
+  GTK_CONSTRAINT_RELATION_GE = 1
+} GtkConstraintRelation;
+
+/**
+ * GtkConstraintStrength:
+ * @GTK_CONSTRAINT_STRENGTH_REQUIRED: The constraint is required towards solving the layout
+ * @GTK_CONSTRAINT_STRENGTH_STRONG: A strong constraint
+ * @GTK_CONSTRAINT_STRENGTH_MEDIUM: A medium constraint
+ * @GTK_CONSTRAINT_STRENGTH_WEAK: A weak constraint
+ *
+ * The strength of a constraint, expressed as a symbolic constant.
+ *
+ * The strength of a #GtkConstraint can be expressed with any positive
+ * integer; the values of this enumeration can be used for readability.
+ */
+typedef enum {
+  GTK_CONSTRAINT_STRENGTH_REQUIRED = 0,
+  GTK_CONSTRAINT_STRENGTH_STRONG = -1,
+  GTK_CONSTRAINT_STRENGTH_MEDIUM = -2,
+  GTK_CONSTRAINT_STRENGTH_WEAK = -3
+} GtkConstraintStrength;
+
+/**
+ * GtkConstraintAttribute:
+ * @GTK_CONSTRAINT_ATTRIBUTE_NONE: No attribute, used for constant
+ *   relations
+ * @GTK_CONSTRAINT_ATTRIBUTE_LEFT: The left edge of a widget, regardless of
+ *   text direction
+ * @GTK_CONSTRAINT_ATTRIBUTE_RIGHT: The right edge of a widget, regardless
+ *   of text direction
+ * @GTK_CONSTRAINT_ATTRIBUTE_TOP: The top edge of a widget
+ * @GTK_CONSTRAINT_ATTRIBUTE_BOTTOM: The bottom edge of a widget
+ * @GTK_CONSTRAINT_ATTRIBUTE_START: The leading edge of a widget, depending
+ *   on text direction; equivalent to %GTK_CONSTRAINT_ATTRIBUTE_LEFT for LTR
+ *   languages, and %GTK_CONSTRAINT_ATTRIBUTE_RIGHT for RTL ones
+ * @GTK_CONSTRAINT_ATTRIBUTE_END: The trailing edge of a widget, depending
+ *   on text direction; equivalent to %GTK_CONSTRAINT_ATTRIBUTE_RIGHT for LTR
+ *   languages, and %GTK_CONSTRAINT_ATTRIBUTE_LEFT for RTL ones
+ * @GTK_CONSTRAINT_ATTRIBUTE_WIDTH: The width of a widget
+ * @GTK_CONSTRAINT_ATTRIBUTE_HEIGHT: The height of a widget
+ * @GTK_CONSTRAINT_ATTRIBUTE_CENTER_X: The center of a widget, on the
+ *   horizontal axis
+ * @GTK_CONSTRAINT_ATTRIBUTE_CENTER_Y: The center of a widget, on the
+ *   vertical axis
+ * @GTK_CONSTRAINT_ATTRIBUTE_BASELINE: The baseline of a widget
+ *
+ * The widget attributes that can be used when creating a #GtkConstraint.
+ */
+typedef enum {
+  GTK_CONSTRAINT_ATTRIBUTE_NONE,
+  GTK_CONSTRAINT_ATTRIBUTE_LEFT,
+  GTK_CONSTRAINT_ATTRIBUTE_RIGHT,
+  GTK_CONSTRAINT_ATTRIBUTE_TOP,
+  GTK_CONSTRAINT_ATTRIBUTE_BOTTOM,
+  GTK_CONSTRAINT_ATTRIBUTE_START,
+  GTK_CONSTRAINT_ATTRIBUTE_END,
+  GTK_CONSTRAINT_ATTRIBUTE_WIDTH,
+  GTK_CONSTRAINT_ATTRIBUTE_HEIGHT,
+  GTK_CONSTRAINT_ATTRIBUTE_CENTER_X,
+  GTK_CONSTRAINT_ATTRIBUTE_CENTER_Y,
+  GTK_CONSTRAINT_ATTRIBUTE_BASELINE
+} GtkConstraintAttribute;
+
 #endif /* __GTK_ENUMS_H__ */

gtk/gtklayoutmanager.c

@@ -91,6 +91,7 @@
 
 typedef struct {
   GtkWidget *widget;
+  GtkRoot *root;
 
   /* HashTable<Widget, LayoutChild> */
   GHashTable *layout_children;
@@ -98,6 +99,16 @@ typedef struct {
 
 G_DEFINE_ABSTRACT_TYPE_WITH_PRIVATE (GtkLayoutManager, gtk_layout_manager, G_TYPE_OBJECT)
 
+static void
+gtk_layout_manager_real_root (GtkLayoutManager *manager)
+{
+}
+
+static void
+gtk_layout_manager_real_unroot (GtkLayoutManager *manager)
+{
+}
+
 static GtkSizeRequestMode
 gtk_layout_manager_real_get_request_mode (GtkLayoutManager *manager,
                                           GtkWidget        *widget)
@@ -188,13 +199,30 @@ gtk_layout_manager_real_create_layout_child (GtkLayoutManager *manager,
                        NULL);
 }
 
+static void
+gtk_layout_manager_finalize (GObject *gobject)
+{
+  GtkLayoutManager *self = GTK_LAYOUT_MANAGER (gobject);
+  GtkLayoutManagerPrivate *priv = gtk_layout_manager_get_instance_private (self);
+
+  g_clear_pointer (&priv->layout_children, g_hash_table_unref);
+
+  G_OBJECT_CLASS (gtk_layout_manager_parent_class)->finalize (gobject);
+}
+
 static void
 gtk_layout_manager_class_init (GtkLayoutManagerClass *klass)
 {
+  GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
+
+  gobject_class->finalize = gtk_layout_manager_finalize;
+
   klass->get_request_mode = gtk_layout_manager_real_get_request_mode;
   klass->measure = gtk_layout_manager_real_measure;
   klass->allocate = gtk_layout_manager_real_allocate;
   klass->create_layout_child = gtk_layout_manager_real_create_layout_child;
+  klass->root = gtk_layout_manager_real_root;
+  klass->unroot = gtk_layout_manager_real_unroot;
 }
 
 static void
@@ -226,6 +254,38 @@ gtk_layout_manager_set_widget (GtkLayoutManager *layout_manager,
     }
 
   priv->widget = widget;
+
+  if (widget != NULL)
+    gtk_layout_manager_set_root (layout_manager, gtk_widget_get_root (widget));
+}
+
+/*< private >
+ * gtk_layout_manager_set_root:
+ * @layout_manager: a #GtkLayoutManager
+ * @root: (nullable): a #GtkWidget implementing #GtkRoot
+ *
+ * Sets a back pointer from @root to @layout_manager.
+ *
+ * This function is called by #GtkWidget when getting rooted and unrooted,
+ * and will call #GtkLayoutManagerClass.root() or #GtkLayoutManagerClass.unroot()
+ * depending on whether @root is a #GtkWidget or %NULL.
+ */
+void
+gtk_layout_manager_set_root (GtkLayoutManager *layout_manager,
+                             GtkRoot          *root)
+{
+  GtkLayoutManagerPrivate *priv = gtk_layout_manager_get_instance_private (layout_manager);
+  GtkRoot *old_root = priv->root;
+
+  priv->root = root;
+
+  if (old_root != root)
+    {
+      if (priv->root != NULL)
+        GTK_LAYOUT_MANAGER_GET_CLASS (layout_manager)->root (layout_manager);
+      else
+        GTK_LAYOUT_MANAGER_GET_CLASS (layout_manager)->unroot (layout_manager);
+    }
 }
 
 /**

gtk/gtklayoutmanager.h

@@ -42,6 +42,10 @@ G_DECLARE_DERIVABLE_TYPE (GtkLayoutManager, gtk_layout_manager, GTK, LAYOUT_MANA
  * @layout_child_type: the type of #GtkLayoutChild used by this layout manager
  * @create_layout_child: a virtual function, used to create a #GtkLayoutChild
  *   meta object for the layout properties
+ * @root: a virtual function, called when the widget using the layout
+ *   manager is attached to a #GtkRoot
+ * @unroot: a virtual function, called when the widget using the layout
+ *   manager is detached from a #GtkRoot
  *
  * The `GtkLayoutManagerClass` structure contains only private data, and
  * should only be accessed through the provided API, or when subclassing
@@ -77,6 +81,9 @@ struct _GtkLayoutManagerClass
                                               GtkWidget        *widget,
                                               GtkWidget        *for_child);
 
+  void               (* root)                (GtkLayoutManager *manager);
+  void               (* unroot)              (GtkLayoutManager *manager);
+
   /*< private >*/
   gpointer _padding[16];
 };

gtk/gtklayoutmanagerprivate.h

@@ -10,4 +10,7 @@ void gtk_layout_manager_set_widget (GtkLayoutManager *manager,
 void gtk_layout_manager_remove_layout_child (GtkLayoutManager *manager,
                                              GtkWidget        *widget);
 
+void gtk_layout_manager_set_root (GtkLayoutManager *manager,
+                                  GtkRoot          *root);
+
 G_END_DECLS

gtk/gtkroot.c

@@ -50,10 +50,18 @@ gtk_root_default_get_display (GtkRoot *self)
   return gdk_display_get_default ();
 }
 
+
+static GtkConstraintSolver *
+gtk_root_default_get_constraint_solver (GtkRoot *self)
+{
+  return NULL;
+}
+
 static void
 gtk_root_default_init (GtkRootInterface *iface)
 {
   iface->get_display = gtk_root_default_get_display;
+  iface->get_constraint_solver = gtk_root_default_get_constraint_solver;
 
   g_object_interface_install_property (iface,
       g_param_spec_object ("focus-widget",
@@ -82,6 +90,17 @@ gtk_root_get_display (GtkRoot *self)
   return iface->get_display (self);
 }
 
+GtkConstraintSolver *
+gtk_root_get_constraint_solver (GtkRoot *self)
+{
+  GtkRootInterface *iface;
+
+  g_return_val_if_fail (GTK_IS_ROOT (self), NULL);
+
+  iface = GTK_ROOT_GET_IFACE (self);
+  return iface->get_constraint_solver (self);
+}
+
 /**
  * gtk_root_set_focus:
  * @self: a #GtkRoot

gtk/gtkroot.h

@@ -34,20 +34,6 @@ G_BEGIN_DECLS
 GDK_AVAILABLE_IN_ALL
 G_DECLARE_INTERFACE (GtkRoot, gtk_root, GTK, ROOT, GtkWidget)
 
-/**
- * GtkRootIface:
- *
- * The list of functions that must be implemented for the #GtkRoot interface.
- */
-struct _GtkRootInterface
-{
-  /*< private >*/
-  GTypeInterface g_iface;
-
-  /*< public >*/
-  GdkDisplay * (* get_display)  (GtkRoot *self);
-};
-
 GDK_AVAILABLE_IN_ALL
 GdkDisplay * gtk_root_get_display (GtkRoot *self);
 

gtk/gtkrootprivate.h

@@ -3,8 +3,28 @@
 
 #include "gtkroot.h"
 
+#include "gtkconstraintsolverprivate.h"
+
 G_BEGIN_DECLS
 
+/**
+ * GtkRootIface:
+ *
+ * The list of functions that must be implemented for the #GtkRoot interface.
+ */
+struct _GtkRootInterface
+{
+  /*< private >*/
+  GTypeInterface g_iface;
+
+  /*< public >*/
+  GdkDisplay * (* get_display)  (GtkRoot *self);
+
+  GtkConstraintSolver * (* get_constraint_solver) (GtkRoot *self);
+};
+
+GtkConstraintSolver *   gtk_root_get_constraint_solver  (GtkRoot *self);
+
 enum {
   GTK_ROOT_PROP_FOCUS_WIDGET,
   GTK_ROOT_NUM_PROPERTIES

gtk/gtkwidget.c

@@ -73,6 +73,7 @@
 #include "gtkwindowgroup.h"
 #include "gtkwindowprivate.h"
 #include "gtknativeprivate.h"
+#include "gtkconstraint.h"
 
 #include "a11y/gtkwidgetaccessible.h"
 #include "inspector/window.h"
@@ -770,6 +771,13 @@ gtk_widget_get_type (void)
 	NULL /* interface data */
       };
 
+      const GInterfaceInfo constraint_target_info =
+      {
+	(GInterfaceInitFunc) NULL,
+	(GInterfaceFinalizeFunc) NULL,
+	NULL /* interface data */
+      };
+
       widget_type = g_type_register_static (G_TYPE_INITIALLY_UNOWNED, g_intern_static_string ("GtkWidget"),
                                             &widget_info, G_TYPE_FLAG_ABSTRACT);
 
@@ -782,6 +790,8 @@ gtk_widget_get_type (void)
                                    &accessibility_info) ;
       g_type_add_interface_static (widget_type, GTK_TYPE_BUILDABLE,
                                    &buildable_info) ;
+      g_type_add_interface_static (widget_type, GTK_TYPE_CONSTRAINT_TARGET,
+                                   &constraint_target_info) ;
     }
 
   return widget_type;
@@ -2893,6 +2903,9 @@ gtk_widget_root (GtkWidget *widget)
 
   _gtk_widget_update_parent_muxer (widget);
 
+  if (priv->layout_manager)
+    gtk_layout_manager_set_root (priv->layout_manager, priv->root);
+
   GTK_WIDGET_GET_CLASS (widget)->root (widget);
 
   if (!GTK_IS_ROOT (widget))
@@ -2920,6 +2933,9 @@ gtk_widget_unroot (GtkWidget *widget)
   if (priv->context)
     gtk_style_context_set_display (priv->context, gdk_display_get_default ());
 
+  if (priv->layout_manager)
+    gtk_layout_manager_set_root (priv->layout_manager, NULL);
+
   if (g_object_get_qdata (G_OBJECT (widget), quark_pango_context))
     g_object_set_qdata (G_OBJECT (widget), quark_pango_context, NULL);
 

gtk/gtkwindow.c

@@ -281,6 +281,8 @@ typedef struct
   GskRenderer *renderer;
 
   GList *foci;
+
+  GtkConstraintSolver *constraint_solver;
 } GtkWindowPrivate;
 
 #ifdef GDK_WINDOWING_X11
@@ -1883,6 +1885,9 @@ gtk_window_init (GtkWindow *window)
   g_signal_connect_swapped (priv->key_controller, "key-released",
                             G_CALLBACK (gtk_window_key_released), window);
   gtk_widget_add_controller (widget, priv->key_controller);
+
+  /* Shared constraint solver */
+  priv->constraint_solver = gtk_constraint_solver_new ();
 }
 
 static GtkGesture *
@@ -2353,6 +2358,15 @@ gtk_window_native_get_renderer (GtkNative *native)
   return priv->renderer;
 }
 
+static GtkConstraintSolver *
+gtk_window_root_get_constraint_solver (GtkRoot *root)
+{
+  GtkWindow *self = GTK_WINDOW (root);
+  GtkWindowPrivate *priv = gtk_window_get_instance_private (self);
+
+  return priv->constraint_solver;
+}
+
 static void
 gtk_window_native_get_surface_transform (GtkNative *native,
                                          int       *x,
@@ -2381,6 +2395,7 @@ static void
 gtk_window_root_interface_init (GtkRootInterface *iface)
 {
   iface->get_display = gtk_window_root_get_display;
+  iface->get_constraint_solver = gtk_window_root_get_constraint_solver;
 }
 
 static void
@@ -4723,6 +4738,7 @@ gtk_window_finalize (GObject *object)
       priv->mnemonics_display_timeout_id = 0;
     }
 
+  g_clear_object (&priv->constraint_solver);
   g_clear_object (&priv->renderer);
 
   G_OBJECT_CLASS (gtk_window_parent_class)->finalize (object);

gtk/meson.build

@@ -37,6 +37,8 @@ gtk_private_sources = files([
   'gtkcolorpickershell.c',
   'gtkcolorscale.c',
   'gtkcolorswatch.c',
+  'gtkconstraintexpression.c',
+  'gtkconstraintsolver.c',
   'gtkcssanimatedstyle.c',
   'gtkcssanimation.c',
   'gtkcssarrayvalue.c',
@@ -200,6 +202,9 @@ gtk_public_sources = files([
   'gtkcombobox.c',
   'gtkcomboboxtext.c',
   'gtkcomposetable.c',
+  'gtkconstraintguide.c',
+  'gtkconstraintlayout.c',
+  'gtkconstraint.c',
   'gtkcontainer.c',
   'gtkcssprovider.c',
   'gtkdialog.c',
@@ -459,6 +464,8 @@ gtk_public_headers = files([
   'gtkcolorutils.h',
   'gtkcombobox.h',
   'gtkcomboboxtext.h',
+  'gtkconstraintlayout.h',
+  'gtkconstraint.h',
   'gtkcontainer.h',
   'gtkcssprovider.h',
   'gtkcustomlayout.h',

tests/constrainttree.c

@@ -0,0 +1,381 @@
+#include <gtk/gtk.h>
+
+#include "../../gtk/gtkconstrainttypesprivate.h"
+#include "../../gtk/gtkconstraintsolverprivate.h"
+#include "../../gtk/gtkconstraintexpressionprivate.h"
+
+typedef struct _Node Node;
+
+static GtkConstraintSolver *solver;
+static Node *tree;
+static Node *drag_node;
+static double drag_start_x;
+static double drag_start_y;
+static GtkConstraintVariable *width_var;
+static GtkConstraintVariable *height_var;
+
+struct _Node {
+  double x;
+  double y;
+  Node *parent;
+  Node *left;
+  Node *right;
+
+  GtkConstraintVariable *x_var;
+  GtkConstraintVariable *y_var;
+};
+
+static Node *
+make_tree (Node *parent,
+           int   depth,
+           int   x,
+           int   y,
+           int   dx,
+           int   dy)
+{
+  Node *node;
+
+  node = g_new0 (Node, 1);
+  node->parent = parent;
+
+  if (depth > 0)
+    {
+      node->left = make_tree (node, depth - 1, x - dx, y + dy, dx / 2, dy);
+      node->right = make_tree (node, depth - 1, x  + dx, y + dy, dx / 2, dy);
+    }
+
+  node->x = x;
+  node->y = y;
+
+  node->x_var = gtk_constraint_solver_create_variable (solver, NULL, "x", x);
+  node->y_var = gtk_constraint_solver_create_variable (solver, NULL, "y", y);
+
+  /* weak stay for the current position */
+  gtk_constraint_solver_add_stay_variable (solver, node->x_var, GTK_CONSTRAINT_WEIGHT_WEAK);
+  gtk_constraint_solver_add_stay_variable (solver, node->y_var, GTK_CONSTRAINT_WEIGHT_WEAK);
+
+  /* require to stay in area */
+  gtk_constraint_solver_add_constraint (solver,
+                                        node->x_var,
+                                        GTK_CONSTRAINT_RELATION_GE,
+                                        gtk_constraint_expression_new (0.0),
+                                        GTK_CONSTRAINT_WEIGHT_REQUIRED);
+  gtk_constraint_solver_add_constraint (solver,
+                                        node->x_var,
+                                        GTK_CONSTRAINT_RELATION_LE,
+                                        gtk_constraint_expression_new (1600.0),
+                                        //gtk_constraint_expression_new_from_variable (width_var),
+                                        GTK_CONSTRAINT_WEIGHT_REQUIRED);
+  gtk_constraint_solver_add_constraint (solver,
+                                        node->y_var,
+                                        GTK_CONSTRAINT_RELATION_GE,
+                                        gtk_constraint_expression_new (0.0),
+                                        GTK_CONSTRAINT_WEIGHT_REQUIRED);
+  gtk_constraint_solver_add_constraint (solver,
+                                        node->y_var,
+                                        GTK_CONSTRAINT_RELATION_LE,
+                                        gtk_constraint_expression_new (600.0),
+                                        //gtk_constraint_expression_new_from_variable (height_var),
+                                        GTK_CONSTRAINT_WEIGHT_REQUIRED);
+
+  if (node->left)
+    {
+      GtkConstraintExpressionBuilder builder;
+
+      /* left.y = right.y */
+      gtk_constraint_solver_add_constraint (solver,
+                                            node->left->y_var,
+                                            GTK_CONSTRAINT_RELATION_EQ,
+                                            gtk_constraint_expression_new_from_variable (node->right->y_var),
+                                            GTK_CONSTRAINT_WEIGHT_REQUIRED);
+
+      /* left.y >= parent.y + 10 */
+      gtk_constraint_expression_builder_init (&builder, solver);
+      gtk_constraint_expression_builder_term (&builder, node->y_var);
+      gtk_constraint_expression_builder_plus (&builder);
+      gtk_constraint_expression_builder_constant (&builder, 10.0);
+      gtk_constraint_solver_add_constraint (solver,
+                                            node->left->y_var,
+                                            GTK_CONSTRAINT_RELATION_GE,
+                                            gtk_constraint_expression_builder_finish (&builder),
+                                            GTK_CONSTRAINT_WEIGHT_REQUIRED);
+
+      /* right.y >= parent.y + 10 */
+      gtk_constraint_expression_builder_init (&builder, solver);
+      gtk_constraint_expression_builder_term (&builder, node->y_var);
+      gtk_constraint_expression_builder_plus (&builder);
+      gtk_constraint_expression_builder_constant (&builder, 10.0);
+      gtk_constraint_solver_add_constraint (solver,
+                                            node->right->y_var,
+                                            GTK_CONSTRAINT_RELATION_GE,
+                                            gtk_constraint_expression_builder_finish (&builder),
+                                            GTK_CONSTRAINT_WEIGHT_REQUIRED);
+      /* parent.x = (left.x + right.x) / 2 */
+      gtk_constraint_expression_builder_init (&builder, solver);
+      gtk_constraint_expression_builder_term (&builder, node->left->x_var);
+      gtk_constraint_expression_builder_plus (&builder);
+      gtk_constraint_expression_builder_term (&builder, node->right->x_var);
+      gtk_constraint_expression_builder_divide_by (&builder);
+      gtk_constraint_expression_builder_constant (&builder, 2.0);
+      gtk_constraint_solver_add_constraint (solver,
+                                            node->x_var,
+                                            GTK_CONSTRAINT_RELATION_EQ,
+                                            gtk_constraint_expression_builder_finish (&builder),
+                                            GTK_CONSTRAINT_WEIGHT_REQUIRED);
+    }
+
+  return node;
+}
+
+static void
+draw_node (Node *node, cairo_t *cr)
+{
+  if (node->left)
+    draw_node (node->left, cr);
+  if (node->right)
+    draw_node (node->right, cr);
+
+  if (node->parent)
+    {
+      cairo_set_source_rgb (cr, 0, 0, 0);
+      cairo_move_to (cr, node->parent->x, node->parent->y);
+      cairo_line_to (cr, node->x, node->y);
+      cairo_stroke (cr);
+    }
+
+  if (node == drag_node)
+    cairo_set_source_rgb (cr, 1, 0, 0);
+  else
+    cairo_set_source_rgb (cr, 0, 0, 0);
+
+  cairo_move_to (cr, node->x, node->y);
+  cairo_arc (cr, node->x, node->y, 5, 0, 2*M_PI);
+  cairo_close_path (cr);
+  cairo_fill (cr);
+}
+
+static void
+draw_func (GtkDrawingArea *da,
+           cairo_t        *cr,
+           int             width,
+           int             height,
+           gpointer        data)
+{
+  cairo_set_line_width (cr, 1);
+  cairo_set_source_rgb (cr, 1, 1, 1);
+  cairo_paint (cr);
+
+  draw_node (tree, cr);
+}
+
+static Node *
+find_node (Node   *node,
+           double  x,
+           double  y)
+{
+  Node *ret;
+
+  double dx = x - node->x;
+  double dy = y - node->y;
+
+  if (dx*dx + dy*dy < 10*10)
+    return node;
+
+  if (node->left)
+    {
+      ret = find_node (node->left, x, y);
+      if (ret)
+        return ret;
+    }
+
+  if (node->right)
+    {
+      ret = find_node (node->right, x, y);
+      if (ret)
+        return ret;
+    }
+
+  return NULL;
+}
+
+static void
+drag_begin (GtkGestureDrag *drag,
+            double          start_x,
+            double          start_y,
+            gpointer        data)
+{
+  drag_node = find_node (tree, start_x, start_y);
+  if (!drag_node)
+    return;
+
+  drag_start_x = start_x;
+  drag_start_y = start_y;
+  gtk_widget_queue_draw (gtk_event_controller_get_widget (GTK_EVENT_CONTROLLER (drag)));
+}
+
+static void
+update_tree (Node *node)
+{
+  if (!node)
+    return;
+
+  node->x = gtk_constraint_variable_get_value (node->x_var);
+  node->y = gtk_constraint_variable_get_value (node->y_var);
+
+  update_tree (node->left);
+  update_tree (node->right);
+}
+
+static void
+drag_update (GtkGestureDrag *drag,
+             double          offset_x,
+             double          offset_y,
+             gpointer        data)
+{
+  if (!drag_node)
+    return;
+
+  gtk_constraint_solver_add_edit_variable (solver,
+                                           drag_node->x_var,
+                                           GTK_CONSTRAINT_WEIGHT_REQUIRED);
+  gtk_constraint_solver_add_edit_variable (solver,
+                                           drag_node->y_var,
+                                           GTK_CONSTRAINT_WEIGHT_REQUIRED);
+  gtk_constraint_solver_begin_edit (solver);
+  gtk_constraint_solver_suggest_value (solver,
+                                       drag_node->x_var,
+                                       drag_start_x + offset_x);
+  gtk_constraint_solver_suggest_value (solver,
+                                       drag_node->y_var,
+                                       drag_start_y + offset_y);
+  gtk_constraint_solver_resolve (solver);
+
+  update_tree (tree);
+
+  gtk_constraint_solver_remove_edit_variable (solver, drag_node->x_var);
+  gtk_constraint_solver_remove_edit_variable (solver, drag_node->y_var);
+  gtk_constraint_solver_end_edit (solver);
+  gtk_widget_queue_draw (gtk_event_controller_get_widget (GTK_EVENT_CONTROLLER (drag)));
+}
+
+static void
+drag_end (GtkGestureDrag *drag,
+          double          offset_x,
+          double          offset_y,
+          gpointer        data)
+{
+  if (!drag_node)
+    return;
+
+
+  drag_node = NULL;
+
+  gtk_widget_queue_draw (gtk_event_controller_get_widget (GTK_EVENT_CONTROLLER (drag)));
+}
+
+static void
+size_change (GtkWidget *da,
+             int        width,
+             int        height,
+             int        baseline,
+             gpointer   data)
+{
+  gtk_constraint_variable_set_value (width_var, width);
+  gtk_constraint_variable_set_value (height_var, height);
+  gtk_constraint_solver_resolve (solver);
+}
+
+static void
+reset_tree (Node *node,
+            int   x,
+            int   y,
+            int   dx,
+            int   dy)
+{
+  node->x = x;
+  node->y = y;
+
+  gtk_constraint_solver_remove_stay_variable (solver, node->x_var);
+  gtk_constraint_solver_remove_stay_variable (solver, node->y_var);
+  gtk_constraint_variable_set_value (node->x_var, x);
+  gtk_constraint_variable_set_value (node->y_var, y);
+  gtk_constraint_solver_add_stay_variable (solver, node->x_var, GTK_CONSTRAINT_WEIGHT_WEAK);
+  gtk_constraint_solver_add_stay_variable (solver, node->y_var, GTK_CONSTRAINT_WEIGHT_WEAK);
+
+  if (node->left)
+    reset_tree (node->left, x - dx, y + dy, dx / 2, dy);
+  if (node->right)
+    reset_tree (node->right, x  + dx, y + dy, dx / 2, dy);
+}
+
+static void
+reset (GtkButton *button,
+       GtkWidget *da)
+{
+  int width, height;
+
+  width = gtk_widget_get_allocated_width (da);
+  height = gtk_widget_get_allocated_height (da);
+
+  gtk_constraint_solver_freeze (solver);
+  reset_tree (tree, width / 2, 20, width / 4 - 40, (height - 40) / 7);
+  gtk_constraint_solver_thaw (solver);
+
+  gtk_widget_queue_draw (da);
+}
+
+int
+main (int argc, char *argv[])
+{
+  GtkWidget *window;
+  GtkWidget *header;
+  GtkWidget *button;
+  GtkWidget *da;
+  GtkGesture *drag;
+  int width = 1600;
+  int height = 600;
+
+  gtk_init ();
+
+  da = gtk_drawing_area_new ();
+
+  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
+  header = gtk_header_bar_new ();
+  gtk_header_bar_set_show_title_buttons (GTK_HEADER_BAR (header), TRUE);
+  button = gtk_button_new_with_label ("Reset");
+  g_signal_connect (button, "clicked", G_CALLBACK (reset), da);
+  gtk_header_bar_pack_start (GTK_HEADER_BAR (header), button);
+  gtk_window_set_titlebar (GTK_WINDOW (window), header);
+
+  gtk_drawing_area_set_content_width (GTK_DRAWING_AREA (da), width);
+  gtk_drawing_area_set_content_height (GTK_DRAWING_AREA (da), height);
+  gtk_drawing_area_set_draw_func (GTK_DRAWING_AREA (da), draw_func, NULL, NULL);
+
+  gtk_container_add (GTK_CONTAINER (window), da);
+
+  drag = gtk_gesture_drag_new ();
+  g_signal_connect (drag, "drag-begin", G_CALLBACK (drag_begin), NULL);
+  g_signal_connect (drag, "drag-update", G_CALLBACK (drag_update), NULL);
+  g_signal_connect (drag, "drag-end", G_CALLBACK (drag_end), NULL);
+  gtk_widget_add_controller (da, GTK_EVENT_CONTROLLER (drag));
+
+  solver = g_object_new (g_type_from_name ("GtkConstraintSolver"), NULL);
+  gtk_constraint_solver_freeze (solver);
+
+  width_var = gtk_constraint_solver_create_variable (solver, NULL, "width", width);
+  height_var = gtk_constraint_solver_create_variable (solver, NULL, "height", height);
+  gtk_constraint_solver_add_stay_variable (solver, width_var, GTK_CONSTRAINT_WEIGHT_REQUIRED);
+  gtk_constraint_solver_add_stay_variable (solver, height_var, GTK_CONSTRAINT_WEIGHT_REQUIRED);
+
+  g_signal_connect (da, "size-allocate", G_CALLBACK (size_change), NULL);
+
+  tree = make_tree (NULL, 7, width / 2, 20, width / 4 - 40, (height - 40) / 7);
+
+  gtk_constraint_solver_thaw (solver);
+
+  gtk_widget_show (window);
+
+  gtk_main ();
+
+  return 0;
+}

tests/meson.build

@@ -1,5 +1,10 @@
 gtk_tests = [
   # testname, optional extra sources
+  ['constrainttree', [
+      '../gtk/gtkconstraintsolver.c',
+      '../gtk/gtkconstraintexpression.c',
+    ], ['-DGTK_COMPILATION', '-UG_ENABLE_DEBUG', '-DGTK_TEST_EXTERNAL']
+  ],
   ['rendernode'],
   ['rendernode-create-tests'],
   ['overlayscroll'],
@@ -142,9 +147,12 @@ test_args = ['-DGTK_SRCDIR="@0@"'.format(meson.current_source_dir())]
 foreach t: gtk_tests
   test_name = t.get(0)
   test_srcs = ['@0@.c'.format(test_name), t.get(1, [])]
+  test_extra_cargs = t.get(2, [])
+  test_extra_ldflags = t.get(3, [])
   executable(test_name, test_srcs,
              include_directories: [confinc, gdkinc],
-             c_args: test_args,
+             c_args: test_args + test_extra_cargs,
+             link_args : test_extra_ldflags,
              dependencies: [libgtk_dep, libm])
 endforeach
 

testsuite/gtk/constraint-solver.c

@@ -0,0 +1,368 @@
+#include <locale.h>
+
+#include "../../gtk/gtkconstrainttypesprivate.h"
+#include "../../gtk/gtkconstraintsolverprivate.h"
+#include "../../gtk/gtkconstraintexpressionprivate.h"
+
+static void
+constraint_solver_simple (void)
+{
+  GtkConstraintSolver *solver = gtk_constraint_solver_new ();
+
+  GtkConstraintVariable *x = gtk_constraint_solver_create_variable (solver, NULL, "x", 167.0);
+  GtkConstraintVariable *y = gtk_constraint_solver_create_variable (solver, NULL, "y", 2.0);
+
+  GtkConstraintExpression *e = gtk_constraint_expression_new_from_variable (y);
+
+  gtk_constraint_solver_add_constraint (solver,
+                                        x, GTK_CONSTRAINT_RELATION_EQ, e,
+                                        GTK_CONSTRAINT_WEIGHT_REQUIRED);
+
+  double x_value = gtk_constraint_variable_get_value (x);
+  double y_value = gtk_constraint_variable_get_value (y);
+
+  g_assert_cmpfloat_with_epsilon (x_value, y_value, 0.001);
+  g_assert_cmpfloat_with_epsilon (x_value, 0.0, 0.001);
+  g_assert_cmpfloat_with_epsilon (y_value, 0.0, 0.001);
+
+  gtk_constraint_variable_unref (y);
+  gtk_constraint_variable_unref (x);
+
+  g_object_unref (solver);
+}
+
+static void
+constraint_solver_stay (void)
+{
+  GtkConstraintSolver *solver = gtk_constraint_solver_new ();
+
+  GtkConstraintVariable *x = gtk_constraint_solver_create_variable (solver, NULL, "x", 5.0);
+  GtkConstraintVariable *y = gtk_constraint_solver_create_variable (solver, NULL, "y", 10.0);
+
+  gtk_constraint_solver_add_stay_variable (solver, x, GTK_CONSTRAINT_WEIGHT_WEAK);
+  gtk_constraint_solver_add_stay_variable (solver, y, GTK_CONSTRAINT_WEIGHT_WEAK);
+
+  double x_value = gtk_constraint_variable_get_value (x);
+  double y_value = gtk_constraint_variable_get_value (y);
+
+  g_assert_cmpfloat_with_epsilon (x_value, 5.0, 0.001);
+  g_assert_cmpfloat_with_epsilon (y_value, 10.0, 0.001);
+
+  gtk_constraint_variable_unref (x);
+  gtk_constraint_variable_unref (y);
+
+  g_object_unref (solver);
+}
+
+static void
+constraint_solver_variable_geq_constant (void)
+{
+  GtkConstraintSolver *solver = gtk_constraint_solver_new ();
+
+  GtkConstraintVariable *x = gtk_constraint_solver_create_variable (solver, NULL, "x", 10.0);
+  GtkConstraintExpression *e = gtk_constraint_expression_new (100.0);
+
+  gtk_constraint_solver_add_constraint (solver,
+                                        x, GTK_CONSTRAINT_RELATION_GE, e,
+                                        GTK_CONSTRAINT_WEIGHT_REQUIRED);
+
+  double x_value = gtk_constraint_variable_get_value (x);
+
+  g_assert_cmpfloat (x_value, >=, 100.0);
+
+  gtk_constraint_variable_unref (x);
+
+  g_object_unref (solver);
+}
+
+static void
+constraint_solver_variable_leq_constant (void)
+{
+  GtkConstraintSolver *solver = gtk_constraint_solver_new ();
+
+  GtkConstraintVariable *x = gtk_constraint_solver_create_variable (solver, NULL, "x", 100.0);
+  GtkConstraintExpression *e = gtk_constraint_expression_new (10.0);
+
+  gtk_constraint_solver_add_constraint (solver,
+                                        x, GTK_CONSTRAINT_RELATION_LE, e,
+                                        GTK_CONSTRAINT_WEIGHT_REQUIRED);
+
+  double x_value = gtk_constraint_variable_get_value (x);
+
+  g_assert_cmpfloat (x_value, <=, 10.0);
+
+  gtk_constraint_variable_unref (x);
+
+  g_object_unref (solver);
+}
+
+static void
+constraint_solver_variable_eq_constant (void)
+{
+  GtkConstraintSolver *solver = gtk_constraint_solver_new ();
+
+  GtkConstraintVariable *x = gtk_constraint_solver_create_variable (solver, NULL, "x", 10.0);
+  GtkConstraintExpression *e = gtk_constraint_expression_new (100.0);
+
+  gtk_constraint_solver_add_constraint (solver,
+                                        x, GTK_CONSTRAINT_RELATION_EQ, e,
+                                        GTK_CONSTRAINT_WEIGHT_REQUIRED);
+
+  double x_value = gtk_constraint_variable_get_value (x);
+
+  g_assert_cmpfloat_with_epsilon (x_value, 100.0, 0.001);
+
+  gtk_constraint_variable_unref (x);
+
+  g_object_unref (solver);
+}
+
+static void
+constraint_solver_eq_with_stay (void)
+{
+  GtkConstraintSolver *solver = gtk_constraint_solver_new ();
+
+  GtkConstraintVariable *x = gtk_constraint_solver_create_variable (solver, NULL, "x", 10.0);
+  GtkConstraintVariable *width = gtk_constraint_solver_create_variable (solver, NULL, "width", 10.0);
+  GtkConstraintVariable *right_min = gtk_constraint_solver_create_variable (solver, NULL, "rightMin", 100.0);
+
+  GtkConstraintExpressionBuilder builder;
+  gtk_constraint_expression_builder_init (&builder, solver);
+  gtk_constraint_expression_builder_term (&builder, x);
+  gtk_constraint_expression_builder_plus (&builder);
+  gtk_constraint_expression_builder_term (&builder, width);
+  GtkConstraintExpression *right = gtk_constraint_expression_builder_finish (&builder);
+
+  gtk_constraint_solver_add_stay_variable (solver, width, GTK_CONSTRAINT_WEIGHT_WEAK);
+  gtk_constraint_solver_add_stay_variable (solver, right_min, GTK_CONSTRAINT_WEIGHT_WEAK);
+  gtk_constraint_solver_add_constraint (solver,
+                                        right_min, GTK_CONSTRAINT_RELATION_EQ, right,
+                                        GTK_CONSTRAINT_WEIGHT_REQUIRED);
+
+  double x_value = gtk_constraint_variable_get_value (x);
+  double width_value = gtk_constraint_variable_get_value (width);
+
+  g_assert_cmpfloat_with_epsilon (x_value, 90.0, 0.001);
+  g_assert_cmpfloat_with_epsilon (width_value, 10.0, 0.001);
+
+  gtk_constraint_variable_unref (right_min);
+  gtk_constraint_variable_unref (width);
+  gtk_constraint_variable_unref (x);
+
+  g_object_unref (solver);
+}
+
+static void
+constraint_solver_cassowary (void)
+{
+  GtkConstraintSolver *solver = gtk_constraint_solver_new ();
+
+  GtkConstraintVariable *x = gtk_constraint_solver_create_variable (solver, NULL, "x", 0.0);
+  GtkConstraintVariable *y = gtk_constraint_solver_create_variable (solver, NULL, "y", 0.0);
+
+  GtkConstraintExpression *e;
+
+  e = gtk_constraint_expression_new_from_variable (y);
+  gtk_constraint_solver_add_constraint (solver,
+                                        x, GTK_CONSTRAINT_RELATION_LE, e,
+                                        GTK_CONSTRAINT_WEIGHT_REQUIRED);
+
+  e = gtk_constraint_expression_plus_constant (gtk_constraint_expression_new_from_variable (x), 3.0);
+  gtk_constraint_solver_add_constraint (solver,
+                                        y, GTK_CONSTRAINT_RELATION_EQ, e,
+                                        GTK_CONSTRAINT_WEIGHT_REQUIRED);
+
+  e = gtk_constraint_expression_new (10.0);
+  gtk_constraint_solver_add_constraint (solver,
+                                        x, GTK_CONSTRAINT_RELATION_EQ, e,
+                                        GTK_CONSTRAINT_WEIGHT_WEAK);
+
+  e = gtk_constraint_expression_new (10.0);
+  gtk_constraint_solver_add_constraint (solver,
+                                        y, GTK_CONSTRAINT_RELATION_EQ, e,
+                                        GTK_CONSTRAINT_WEIGHT_WEAK);
+
+  double x_val = gtk_constraint_variable_get_value (x);
+  double y_val = gtk_constraint_variable_get_value (y);
+
+  g_test_message ("x = %g, y = %g", x_val, y_val);
+
+  /* The system is unstable and has two possible solutions we need to test */
+  g_assert_true ((G_APPROX_VALUE (x_val, 10.0, 1e-8) &&
+                  G_APPROX_VALUE (y_val, 13.0, 1e-8)) ||
+                 (G_APPROX_VALUE (x_val,  7.0, 1e-8) &&
+                  G_APPROX_VALUE (y_val, 10.0, 1e-8)));
+
+  gtk_constraint_variable_unref (x);
+  gtk_constraint_variable_unref (y);
+
+  g_object_unref (solver);
+}
+
+static void
+constraint_solver_edit_var_required (void)
+{
+  GtkConstraintSolver *solver = gtk_constraint_solver_new ();
+
+  GtkConstraintVariable *a = gtk_constraint_solver_create_variable (solver, NULL, "a", 0.0);
+  gtk_constraint_solver_add_stay_variable (solver, a, GTK_CONSTRAINT_WEIGHT_STRONG);
+
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (a), 0.0, 0.001);
+
+  gtk_constraint_solver_add_edit_variable (solver, a, GTK_CONSTRAINT_WEIGHT_REQUIRED);
+  gtk_constraint_solver_begin_edit (solver);
+  gtk_constraint_solver_suggest_value (solver, a, 2.0);
+  gtk_constraint_solver_resolve (solver);
+
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (a), 2.0, 0.001);
+
+  gtk_constraint_solver_suggest_value (solver, a, 10.0);
+  gtk_constraint_solver_resolve (solver);
+
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (a), 10.0, 0.001);
+
+  gtk_constraint_solver_end_edit (solver);
+
+  gtk_constraint_variable_unref (a);
+
+  g_object_unref (solver);
+}
+
+static void
+constraint_solver_edit_var_suggest (void)
+{
+  GtkConstraintSolver *solver = gtk_constraint_solver_new ();
+
+  GtkConstraintVariable *a = gtk_constraint_solver_create_variable (solver, NULL, "a", 0.0);
+  GtkConstraintVariable *b = gtk_constraint_solver_create_variable (solver, NULL, "b", 0.0);
+
+  gtk_constraint_solver_add_stay_variable (solver, a, GTK_CONSTRAINT_WEIGHT_STRONG);
+
+  GtkConstraintExpression *e = gtk_constraint_expression_new_from_variable (b);
+  gtk_constraint_solver_add_constraint (solver,
+                                        a, GTK_CONSTRAINT_RELATION_EQ, e,
+                                        GTK_CONSTRAINT_WEIGHT_REQUIRED);
+
+  gtk_constraint_solver_resolve (solver);
+
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (a), 0.0, 0.001);
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (b), 0.0, 0.001);
+
+  gtk_constraint_solver_add_edit_variable (solver, a, GTK_CONSTRAINT_WEIGHT_REQUIRED);
+  gtk_constraint_solver_begin_edit (solver);
+
+  gtk_constraint_solver_suggest_value (solver, a, 2.0);
+  gtk_constraint_solver_resolve (solver);
+
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (a), 2.0, 0.001);
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (b), 2.0, 0.001);
+
+  gtk_constraint_solver_suggest_value (solver, a, 10.0);
+  gtk_constraint_solver_resolve (solver);
+
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (a), 10.0, 0.001);
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (b), 10.0, 0.001);
+
+  gtk_constraint_variable_unref (a);
+  gtk_constraint_variable_unref (b);
+
+  g_object_unref (solver);
+}
+
+static void
+constraint_solver_paper (void)
+{
+  GtkConstraintSolver *solver = gtk_constraint_solver_new ();
+
+  GtkConstraintVariable *left = gtk_constraint_solver_create_variable (solver, NULL, "left", 0.0);
+  GtkConstraintVariable *middle = gtk_constraint_solver_create_variable (solver, NULL, "middle", 0.0);
+  GtkConstraintVariable *right = gtk_constraint_solver_create_variable (solver, NULL, "right", 0.0);
+
+  GtkConstraintExpressionBuilder builder;
+  GtkConstraintExpression *expr;
+
+  gtk_constraint_expression_builder_init (&builder, solver);
+  gtk_constraint_expression_builder_term (&builder, left);
+  gtk_constraint_expression_builder_plus (&builder);
+  gtk_constraint_expression_builder_term (&builder, right);
+  gtk_constraint_expression_builder_divide_by (&builder);
+  gtk_constraint_expression_builder_constant (&builder, 2.0);
+  expr = gtk_constraint_expression_builder_finish (&builder);
+  gtk_constraint_solver_add_constraint (solver,
+                                        middle, GTK_CONSTRAINT_RELATION_EQ, expr,
+                                        GTK_CONSTRAINT_WEIGHT_REQUIRED);
+
+  gtk_constraint_expression_builder_init (&builder, solver);
+  gtk_constraint_expression_builder_term (&builder, left);
+  gtk_constraint_expression_builder_plus (&builder);
+  gtk_constraint_expression_builder_constant (&builder, 10.0);
+  expr = gtk_constraint_expression_builder_finish (&builder);
+  gtk_constraint_solver_add_constraint (solver,
+                                        right, GTK_CONSTRAINT_RELATION_EQ, expr,
+                                        GTK_CONSTRAINT_WEIGHT_REQUIRED);
+
+  expr = gtk_constraint_expression_new (100.0);
+  gtk_constraint_solver_add_constraint (solver,
+                                        right, GTK_CONSTRAINT_RELATION_LE, expr,
+                                        GTK_CONSTRAINT_WEIGHT_REQUIRED);
+
+  expr = gtk_constraint_expression_new (0.0);
+  gtk_constraint_solver_add_constraint (solver,
+                                        left, GTK_CONSTRAINT_RELATION_GE, expr,
+                                        GTK_CONSTRAINT_WEIGHT_REQUIRED);
+
+  g_test_message ("Check constraints hold");
+
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (middle),
+                                  (gtk_constraint_variable_get_value (left) + gtk_constraint_variable_get_value (right)) / 2.0,
+                                  0.001);
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (right),
+                                  gtk_constraint_variable_get_value (left) + 10.0,
+                                  0.001);
+  g_assert_cmpfloat (gtk_constraint_variable_get_value (right), <=, 100.0);
+  g_assert_cmpfloat (gtk_constraint_variable_get_value (left), >=, 0.0);
+
+  gtk_constraint_variable_set_value (middle, 45.0);
+  gtk_constraint_solver_add_stay_variable (solver, middle, GTK_CONSTRAINT_WEIGHT_WEAK);
+
+  g_test_message ("Check constraints hold after setting middle");
+
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (middle),
+                                  (gtk_constraint_variable_get_value (left) + gtk_constraint_variable_get_value (right)) / 2.0,
+                                  0.001);
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (right),
+                                  gtk_constraint_variable_get_value (left) + 10.0,
+                                  0.001);
+  g_assert_cmpfloat (gtk_constraint_variable_get_value (right), <=, 100.0);
+  g_assert_cmpfloat (gtk_constraint_variable_get_value (left), >=, 0.0);
+
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (left), 40.0, 0.001);
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (middle), 45.0, 0.001);
+  g_assert_cmpfloat_with_epsilon (gtk_constraint_variable_get_value (right), 50.0, 0.001);
+
+  gtk_constraint_variable_unref (left);
+  gtk_constraint_variable_unref (middle);
+  gtk_constraint_variable_unref (right);
+
+  g_object_unref (solver);
+}
+
+int
+main (int argc, char *argv[])
+{
+  g_test_init (&argc, &argv, NULL);
+  setlocale (LC_ALL, "C");
+
+  g_test_add_func ("/constraint-solver/paper", constraint_solver_paper);
+  g_test_add_func ("/constraint-solver/simple", constraint_solver_simple);
+  g_test_add_func ("/constraint-solver/constant/eq", constraint_solver_variable_eq_constant);
+  g_test_add_func ("/constraint-solver/constant/ge", constraint_solver_variable_geq_constant);
+  g_test_add_func ("/constraint-solver/constant/le", constraint_solver_variable_leq_constant);
+  g_test_add_func ("/constraint-solver/stay/simple", constraint_solver_stay);
+  g_test_add_func ("/constraint-solver/stay/eq", constraint_solver_eq_with_stay);
+  g_test_add_func ("/constraint-solver/cassowary", constraint_solver_cassowary);
+  g_test_add_func ("/constraint-solver/edit/required", constraint_solver_edit_var_required);
+  g_test_add_func ("/constraint-solver/edit/suggest", constraint_solver_edit_var_suggest);
+
+  return g_test_run ();
+}

testsuite/gtk/meson.build

@@ -17,6 +17,11 @@ tests = [
   ['builderparser'],
   ['cellarea'],
   ['check-icon-names'],
+  ['constraint-solver', [
+      '../../gtk/gtkconstraintsolver.c',
+      '../../gtk/gtkconstraintexpression.c',
+    ], ['-DGTK_COMPILATION', '-UG_ENABLE_DEBUG']
+  ],
   ['cssprovider'],
   ['rbtree-crash', ['../../gtk/gtkrbtree.c'], ['-DGTK_COMPILATION', '-UG_ENABLE_DEBUG']],
   ['defaultvalue'],