[gtk/wip/otte/dnd] gtk-demo: Add a simple peg solitaire DND demo

[Benjamin Otte <otte src gnome org>]

commit 267823ef5d40a58d4757188b019725a522f7c140
Author: Benjamin Otte <otte redhat com>
Date:   Sun Mar 1 08:45:15 2020 +0100

    gtk-demo: Add a simple peg solitaire DND demo

 demos/gtk-demo/demo.gresource.xml |   1 +
 demos/gtk-demo/meson.build        |   1 +
 demos/gtk-demo/peg_solitaire.c    | 356 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 358 insertions(+)
---
diff --git a/demos/gtk-demo/demo.gresource.xml b/demos/gtk-demo/demo.gresource.xml
index a60a21721b..4dde63322a 100644
--- a/demos/gtk-demo/demo.gresource.xml
+++ b/demos/gtk-demo/demo.gresource.xml
@@ -199,6 +199,7 @@
     <file>paintable_mediastream.c</file>
     <file>panes.c</file>
     <file>password_entry.c</file>
+    <file>peg_solitaire.c</file>
     <file>pickers.c</file>
     <file>pixbufs.c</file>
     <file>popover.c</file>
diff --git a/demos/gtk-demo/meson.build b/demos/gtk-demo/meson.build
index eee93be78e..98bc765b3e 100644
--- a/demos/gtk-demo/meson.build
+++ b/demos/gtk-demo/meson.build
@@ -51,6 +51,7 @@ demos = files([
   'paintable_mediastream.c',
   'panes.c',
   'password_entry.c',
+  'peg_solitaire.c',
   'pickers.c',
   'pixbufs.c',
   'popover.c',
diff --git a/demos/gtk-demo/peg_solitaire.c b/demos/gtk-demo/peg_solitaire.c
new file mode 100644
index 0000000000..2330ebfc01
--- /dev/null
+++ b/demos/gtk-demo/peg_solitaire.c
@@ -0,0 +1,356 @@
+/* Peg Solitaire
+ *
+ * This demo demonstrates how to use drag'n'drop to implement peg solitaire.
+ *
+ */
+
+#include <gtk/gtk.h>
+
+static GtkWidget *window = NULL;
+
+/* Create an object for the pegs that get moved around in the game.
+ *
+ * We implement the GdkPaintable interface for them, so we can use GtkPicture
+ * objects for the wholes we put the pegs into.
+ */
+#define SOLITAIRE_TYPE_PEG (solitaire_peg_get_type ())
+G_DECLARE_FINAL_TYPE (SolitairePeg, solitaire_peg, SOLITAIRE, PEG, GObject)
+
+/* Declare the struct. */
+struct _SolitairePeg
+{
+  GObject parent_instance;
+
+  int x;
+  int y;
+};
+
+struct _SolitairePegClass
+{
+  GObjectClass parent_class;
+};
+
+/* Here, we implement the functionality required by the GdkPaintable interface */
+static void
+solitaire_peg_snapshot (GdkPaintable *paintable,
+                        GdkSnapshot  *snapshot,
+                        double        width,
+                        double        height)
+{
+  /* The snapshot function is the only function we need to implement.
+   * It does the actual drawing of the paintable.
+   */
+  gtk_snapshot_append_color (snapshot,
+                             &(GdkRGBA) { 0.6, 0.3, 0.0, 1.0 },
+                             &GRAPHENE_RECT_INIT (0, 0, width, height));
+}
+
+static GdkPaintableFlags
+solitaire_peg_get_flags (GdkPaintable *paintable)
+{
+  /* The flags are very useful to let GTK know that this image
+   * is never going to change.
+   * This allows many optimizations and should therefore always
+   * be set.
+   */
+  return GDK_PAINTABLE_STATIC_CONTENTS | GDK_PAINTABLE_STATIC_SIZE;
+}
+
+static void
+solitaire_peg_paintable_init (GdkPaintableInterface *iface)
+{
+  iface->snapshot = solitaire_peg_snapshot;
+  iface->get_flags = solitaire_peg_get_flags;
+}
+
+/* When defining the GType, we need to implement the GdkPaintable interface */
+G_DEFINE_TYPE_WITH_CODE (SolitairePeg, solitaire_peg, G_TYPE_OBJECT,
+                         G_IMPLEMENT_INTERFACE (GDK_TYPE_PAINTABLE,
+                                                solitaire_peg_paintable_init))
+
+/* Here's the boilerplate for the GObject declaration.
+ * We don't need to do anything special here, because we keep no
+ * data of our own.
+ */
+static void
+solitaire_peg_class_init (SolitairePegClass *klass)
+{
+}
+
+static void
+solitaire_peg_init (SolitairePeg *peg)
+{
+}
+
+/* Add a little setter for the peg's position.
+ * We want to track those so that we can check for legal moves
+ * during drag'n'drop operations.
+ */
+static void
+solitaire_peg_set_position (SolitairePeg *peg,
+                            guint         x,
+                            guint         y)
+{
+  peg->x = x;
+  peg->y = y;
+}
+
+/* And finally, we add a simple constructor.
+ */
+static SolitairePeg *
+solitaire_peg_new (void)
+{
+  return g_object_new (SOLITAIRE_TYPE_PEG, NULL);
+}
+
+/*** DRAG AND DROP ***/
+
+/* The user tries to start a drag operation.
+ * We check if the image contains a peg, and if so, we return the
+ * peg as the content to be dragged.
+ */
+static GdkContentProvider *
+drag_prepare (GtkDragSource *source,
+              double         x,
+              double         y,
+              GtkWidget     *image)
+{
+  GdkPaintable *paintable = gtk_image_get_paintable (GTK_IMAGE (image));
+
+  if (!SOLITAIRE_IS_PEG (paintable))
+    return NULL;
+
+  return gdk_content_provider_new_typed (SOLITAIRE_TYPE_PEG, paintable);
+}
+
+/* This notifies us that the drag has begun.
+ * We can now set up the icon and the widget for the ongoing drag.
+ */
+static void
+drag_begin (GtkDragSource *source,
+            GdkDrag       *drag,
+            GtkWidget     *image)
+{
+  GdkPaintable *paintable = gtk_image_get_paintable (GTK_IMAGE (image));
+
+  /* We guaranteed in the drag_prepare function above that we
+   * only start a drag if a peg is available.
+   * So let's make sure we did not screw that up.
+   */
+  g_assert (SOLITAIRE_IS_PEG (paintable));
+
+  /* We use the peg as the drag icon.
+   */
+  gtk_drag_source_set_icon (source, paintable, -2, -2);
+
+  /* We also attach it to the drag operation as custom user data,
+   * so that we can get it back later if the drag fails.
+   */
+  g_object_set_data (G_OBJECT (drag), "the peg", paintable);
+
+  /* Because we are busy dragging the peg, we want to unset it
+   * on the image.
+   */
+  gtk_image_clear (GTK_IMAGE (image));
+}
+
+/* Thie is called once a drag operation has ended (successfully or not).
+ * We want to undo what we did in drag_begin() above and react
+ * to a potential move of the peg.
+ */
+static void
+drag_end (GtkDragSource *source,
+          GdkDrag       *drag,
+          gboolean       delete_data,
+          GtkWidget     *image)
+{
+  SolitairePeg *peg;
+
+  /* If the drag was successful, we should now delete the peg.
+   * We did this in drag_begin() above to prepare for the drag, so
+   * there's no need to do anything anymore.
+   */
+  if (delete_data)
+    return;
+
+  /* However, if the drag did not succeed, we need to undo what
+   * we did in drag_begin() and reinsert the peg here.
+   * Because we used it as the drag data
+   */
+  peg = g_object_get_data (G_OBJECT (drag), "the peg");
+  gtk_image_set_from_paintable (GTK_IMAGE (image), GDK_PAINTABLE (peg));
+}
+
+/* Whenever a new drop operation starts, we need to check if we can
+ * accept it.
+ * The default check unfortunately is not good enough, because it only
+ * checks the data type. But we also need to check if our image can
+ * even accept data.
+ */
+static gboolean
+drop_accept (GtkDropTarget *target,
+             GdkDrop       *drop,
+             GtkWidget     *image)
+{
+  /* First, check the drop is actually trying to drop a peg */
+  if (!gdk_content_formats_contain_gtype (gdk_drop_get_formats (drop), SOLITAIRE_TYPE_PEG))
+    return FALSE;
+
+  /* If the image already contains a peg, we cannot accept another one */
+  if (SOLITAIRE_IS_PEG (gtk_image_get_paintable (GTK_IMAGE (image))))
+    return FALSE;
+
+  return TRUE;
+}
+
+static gboolean
+drop_drop (GtkDropTarget *target,
+           const GValue  *value,
+           double         x,
+           double         y,
+           GtkWidget     *image)
+{
+  GtkGrid *grid;
+  SolitairePeg *peg;
+  int image_x, image_y;
+  GtkWidget *jumped;
+
+  grid = GTK_GRID (gtk_widget_get_parent (image));
+  /* The value contains the data in the type we demanded.
+   * We demanded a SolitairePeg, so that's what we get.
+   */
+  peg = g_value_get_object (value);
+
+  /* Make sure this was a legal move. */
+  /* First, figure out the image's position in the grid. */
+  gtk_grid_query_child (grid,
+                        image,
+                        &image_x, &image_y,
+                        NULL, NULL);
+
+  /* If the peg was not moved 2 spaces horizontally or vertically,
+   * this was not a valid jump. Reject it.
+   */
+  if (!((ABS (image_x - peg->x) == 2 && image_y == peg->y) ||
+        (ABS (image_y - peg->y) == 2 && image_x == peg->x)))
+    return FALSE;
+
+  /* Get the widget that was jumped over
+   */
+  jumped = gtk_grid_get_child_at (grid,
+                                  (image_x + peg->x) / 2,
+                                  (image_y + peg->y) / 2);
+  /* If the jumped widget does not have a peg in it, this move
+   * isn't valid.
+   */
+  if (!SOLITAIRE_IS_PEG (gtk_image_get_paintable (GTK_IMAGE (jumped))))
+    return FALSE;
+
+  /* Finally, we know it's a legal move. */
+
+  /* Clear the peg of the jumped-over image */
+  gtk_image_clear (GTK_IMAGE (jumped));
+
+  /* Add the peg to this image */
+  solitaire_peg_set_position (peg, image_x, image_y);
+  gtk_image_set_from_paintable (GTK_IMAGE (image), GDK_PAINTABLE (peg));
+
+  /* Success! */
+  return TRUE;
+}
+
+GtkWidget *
+do_peg_solitaire (GtkWidget *do_widget)
+{
+  if (!window)
+    {
+      GtkWidget *header;
+      GtkWidget *restart;
+      GtkWidget *grid;
+      GtkWidget *image;
+      int x, y;
+      GtkDragSource *source;
+      GtkDropTarget *target;
+
+      restart = gtk_button_new_from_icon_name ("view-refresh-symbolic");
+      g_signal_connect (restart, "clicked", G_CALLBACK (restart), NULL);
+
+      header = gtk_header_bar_new ();
+      gtk_header_bar_set_title (GTK_HEADER_BAR (header), "Peg Solitaire");
+      gtk_header_bar_set_show_title_buttons (GTK_HEADER_BAR (header), TRUE);
+      gtk_header_bar_pack_start (GTK_HEADER_BAR (header), restart);
+      window = gtk_window_new ();
+      gtk_window_set_display (GTK_WINDOW (window),
+                              gtk_widget_get_display (do_widget));
+      gtk_window_set_title (GTK_WINDOW (window), "Sliding Puzzle");
+      gtk_window_set_titlebar (GTK_WINDOW (window), header);
+      gtk_window_set_default_size (GTK_WINDOW (window), 400, 300);
+      g_signal_connect (window, "destroy",
+                        G_CALLBACK (gtk_widget_destroyed), &window);
+
+      grid = gtk_grid_new ();
+      gtk_widget_set_halign (grid, GTK_ALIGN_CENTER);
+      gtk_widget_set_valign (grid, GTK_ALIGN_CENTER);
+      gtk_grid_set_row_spacing (GTK_GRID (grid), 6);
+      gtk_grid_set_column_spacing (GTK_GRID (grid), 6);
+      gtk_grid_set_row_homogeneous (GTK_GRID (grid), TRUE);
+      gtk_grid_set_column_homogeneous (GTK_GRID (grid), TRUE);
+      gtk_container_add (GTK_CONTAINER (window), grid);
+
+      for (x = 0; x < 7; x++)
+        {
+          for (y = 0; y < 7; y++)
+            {
+              if ((x < 2 || x >= 5) && (y < 2 || y >= 5))
+                continue;
+
+              image = gtk_image_new ();
+              if (x != 3 || y != 3)
+                {
+                  SolitairePeg *peg = solitaire_peg_new ();
+                  solitaire_peg_set_position (peg, x, y);
+                  gtk_image_set_from_paintable (GTK_IMAGE (image), GDK_PAINTABLE (peg));
+                }
+
+              gtk_grid_attach (GTK_GRID (grid), image, x, y, 1, 1);
+
+              /* Set up the drag source.
+               * This is rather straightforward: Set the supported actions
+               * (in our case, pegs can only be moved) and connect all the
+               * relevant signals.
+               * And because all drag'n'drop handling is done via event controllers,
+               * we need to add the controller to the widget.
+               */
+              source = gtk_drag_source_new ();
+              gtk_drag_source_set_actions (source, GDK_ACTION_MOVE);
+              g_signal_connect (source, "prepare", G_CALLBACK (drag_prepare), image);
+              g_signal_connect (source, "drag-begin", G_CALLBACK (drag_begin), image);
+              g_signal_connect (source, "drag-end", G_CALLBACK (drag_end), image);
+              gtk_widget_add_controller (image, GTK_EVENT_CONTROLLER (source));
+
+              /* Set up the drop target.
+               * This is more involved, because the game logic goes here.
+               */
+
+              /* First we specify the data we accept: pegs.
+               * And we only want moves.
+               */
+              target = gtk_drop_target_new (SOLITAIRE_TYPE_PEG, GDK_ACTION_MOVE);
+              /* Then we connect our signals.
+               */
+              g_signal_connect (target, "accept", G_CALLBACK (drop_accept), image);
+              g_signal_connect (target, "drop", G_CALLBACK (drop_drop), image);
+              /* Finally, like above, we add it to the widget.
+               */
+              gtk_widget_add_controller (image, GTK_EVENT_CONTROLLER (target));
+            }
+        }
+    }
+
+  if (!gtk_widget_get_visible (window))
+    gtk_widget_show (window);
+  else
+    gtk_widget_destroy (window);
+
+  return window;
+}