gdk: Add callback cursors

Add a variant of GdkCursor that obtains the texture for the cursor
via a callback. The callback gives us the flexibility to handle
fractional scales and update the cursor for cursor theme size
changes as well as scale changes.

gdk/gdkcursor.c

@@ -156,6 +156,9 @@ gdk_cursor_finalize (GObject *object)
   g_clear_object (&cursor->texture);
   g_clear_object (&cursor->fallback);
 
+  if (cursor->destroy)
+    cursor->destroy (cursor->data);
+
   G_OBJECT_CLASS (gdk_cursor_parent_class)->finalize (object);
 }
 
@@ -253,6 +256,11 @@ gdk_cursor_hash (gconstpointer pointer)
     hash ^= g_str_hash (cursor->name);
   else if (cursor->texture)
     hash ^= g_direct_hash (cursor->texture);
+  else if (cursor->callback)
+    {
+      hash ^= g_direct_hash (cursor->callback);
+      hash ^= g_direct_hash (cursor->data);
+    }
 
   hash ^= (cursor->hotspot_x << 8) | cursor->hotspot_y;
 
@@ -281,6 +289,10 @@ gdk_cursor_equal (gconstpointer a,
       ca->hotspot_y != cb->hotspot_y)
     return FALSE;
 
+  if (ca->callback != cb->callback ||
+      ca->data != cb->data)
+    return FALSE;
+
   return TRUE;
 }
 
@@ -355,6 +367,45 @@ gdk_cursor_new_from_texture (GdkTexture *texture,
                        NULL);
 }
 
+/**
+ * gdk_cursor_new_from_callback:
+ * @callback: the `GdkCursorGetTextureCallback`
+ * @data: data to pass to @callback
+ * @destroy: destroy notify for @data
+ * @fallback: (nullable): the `GdkCursor` to fall back to when
+ *   this one cannot be supported
+ *
+ * Creates a new callback-based cursor object.
+ *
+ * Cursors of this kind produce textures for the cursor
+ * image on demand, when the @callback is called.
+ *
+ * Returns: (nullable): a new `GdkCursor`
+ *
+ * Since: 4.16
+ */
+GdkCursor *
+gdk_cursor_new_from_callback (GdkCursorGetTextureCallback  callback,
+                              gpointer                     data,
+                              GDestroyNotify               destroy,
+                              GdkCursor                   *fallback)
+{
+  GdkCursor *cursor;
+
+  g_return_val_if_fail (callback != NULL, NULL);
+  g_return_val_if_fail (fallback == NULL || GDK_IS_CURSOR (fallback), NULL);
+
+  cursor = g_object_new (GDK_TYPE_CURSOR,
+                         "fallback", fallback,
+                         NULL);
+
+  cursor->callback = callback;
+  cursor->data = data;
+  cursor->destroy = destroy;
+
+  return cursor;
+}
+
 /**
  * gdk_cursor_get_fallback: (attributes org.gtk.Method.get_property=fallback)
  * @cursor: a `GdkCursor`
@@ -459,3 +510,22 @@ gdk_cursor_get_hotspot_y (GdkCursor *cursor)
 
   return cursor->hotspot_y;
 }
+
+GdkTexture *
+gdk_cursor_get_texture_for_size (GdkCursor *cursor,
+                                 int        cursor_size,
+                                 double     scale,
+                                 int       *width,
+                                 int       *height,
+                                 int       *hotspot_x,
+                                 int       *hotspot_y)
+{
+  if (cursor->callback == NULL)
+    return NULL;
+
+  return cursor->callback (cursor,
+                           cursor_size, scale,
+                           width, height,
+                           hotspot_x, hotspot_y,
+                           cursor->data);
+}

gdk/gdkcursor.h

@@ -51,6 +51,52 @@ GDK_AVAILABLE_IN_ALL
 GdkCursor*  gdk_cursor_new_from_name     (const char      *name,
                                           GdkCursor       *fallback);
 
+/**
+ * GdkCursorGetTestureCallback:
+ * @cursor: the `GdkCursor`
+ * @cursor_size: the nominal cursor size, in application pixels
+ * @scale: the device scale
+ * @width: (out): return location for the actual cursor width,
+ *   in application pixels
+ * @height: (out): return location for the actual cursor height,
+ *   in application pixels
+ * @hotspot_x: (out): return location for the hotspot X position,
+ *   in application pixels
+ * @hotspot_y: (out): return location for the hotspot Y position,
+ *   in application pixels
+ * @data: User data for the callback
+ *
+ * The type of callback used by a dynamic `GdkCursor` to generate
+ * a texture for the cursor image at the given @cursor_size
+ * and @scale.
+ *
+ * The actual cursor size in application pixels may be different
+ * from @cursor_size x @cursor_size, and will be returned in
+ * @width, @height. The returned texture should have a size that
+ * corresponds to the actual cursor size, in device pixels (i.e.
+ * application pixels, multiplied by @scale).
+ *
+ * This function may fail and return `NULL`, in which case
+ * the fallback cursor will be used.
+ *
+ * Returns: (nullable) (transfer full): the cursor image, or
+ *   `NULL` if none could be produced.
+ */
+typedef GdkTexture * (* GdkCursorGetTextureCallback) (GdkCursor   *cursor,
+                                                      int          cursor_size,
+                                                      double       scale,
+                                                      int         *width,
+                                                      int         *height,
+                                                      int         *hotspot_x,
+                                                      int         *hotspot_y,
+                                                      gpointer     data);
+
+GDK_AVAILABLE_IN_4_16
+GdkCursor * gdk_cursor_new_from_callback (GdkCursorGetTextureCallback  callback,
+                                          gpointer                     data,
+                                          GDestroyNotify               destroy,
+                                          GdkCursor                   *fallback);
+
 GDK_AVAILABLE_IN_ALL
 GdkCursor * gdk_cursor_get_fallback      (GdkCursor       *cursor);
 GDK_AVAILABLE_IN_ALL

gdk/gdkcursorprivate.h

@@ -44,6 +44,10 @@ struct _GdkCursor
   GdkTexture *texture;
   int hotspot_x;
   int hotspot_y;
+
+  GdkCursorGetTextureCallback callback;
+  gpointer data;
+  GDestroyNotify destroy;
 };
 
 struct _GdkCursorClass
@@ -55,5 +59,13 @@ guint                   gdk_cursor_hash                         (gconstpointer
 gboolean                gdk_cursor_equal                        (gconstpointer          a,
                                                                  gconstpointer          b);
 
+GdkTexture *            gdk_cursor_get_texture_for_size         (GdkCursor *cursor,
+                                                                 int        cursor_size,
+                                                                 double     scale,
+                                                                 int       *width,
+                                                                 int       *height,
+                                                                 int       *hotspot_x,
+                                                                 int       *hotspot_y);
+
 G_END_DECLS