clutter: Add "revoked" property to ClutterGrab

Users of Clutter grabs may listen for notify::revoked changes in
order to know that their grab is no longer in charge of event
propagation, without the use of crossing events.

Since a ClutterGrab may stay in the stack and regain effects,
this notification also happens the other way around.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3420>

clutter/clutter/clutter-grab-private.h

@@ -42,4 +42,6 @@ ClutterGrab * clutter_grab_new (ClutterStage *stage,
                                 ClutterActor *actor,
                                 gboolean      owns_actor);
 
+void clutter_grab_notify (ClutterGrab *grab);
+
 G_END_DECLS

clutter/clutter/clutter-grab.c

@@ -23,6 +23,17 @@
 
 #include "clutter-grab-private.h"
 
+#include "clutter-private.h"
+
+enum
+{
+  PROP_0,
+  PROP_REVOKED,
+  N_PROPS,
+};
+
+static GParamSpec *props[N_PROPS] = { 0, };
+
 G_DEFINE_FINAL_TYPE (ClutterGrab, clutter_grab, G_TYPE_OBJECT)
 
 static void
@@ -43,6 +54,25 @@ clutter_grab_finalize (GObject *object)
   G_OBJECT_CLASS (clutter_grab_parent_class)->finalize (object);
 }
 
+static void
+clutter_grab_get_property (GObject    *object,
+                           guint       prop_id,
+                           GValue     *value,
+                           GParamSpec *pspec)
+{
+  ClutterGrab *grab = CLUTTER_GRAB (object);
+
+  switch (prop_id)
+    {
+    case PROP_REVOKED:
+      g_value_set_boolean (value, clutter_grab_is_revoked (grab));
+      break;
+    default:
+      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
+      break;
+    }
+}
+
 static void
 clutter_grab_class_init (ClutterGrabClass *klass)
 {
@@ -50,6 +80,16 @@ clutter_grab_class_init (ClutterGrabClass *klass)
 
   object_class->dispose = clutter_grab_dispose;
   object_class->finalize = clutter_grab_finalize;
+  object_class->get_property = clutter_grab_get_property;
+
+  props[PROP_REVOKED] =
+    g_param_spec_boolean ("revoked", NULL, NULL,
+                          FALSE,
+                          G_PARAM_READABLE |
+                          G_PARAM_STATIC_STRINGS |
+                          G_PARAM_EXPLICIT_NOTIFY);
+
+  g_object_class_install_properties (object_class, N_PROPS, props);
 }
 
 static void
@@ -73,3 +113,17 @@ clutter_grab_new (ClutterStage *stage,
 
   return grab;
 }
+
+void
+clutter_grab_notify (ClutterGrab *grab)
+{
+  g_object_notify (G_OBJECT (grab), "revoked");
+}
+
+gboolean
+clutter_grab_is_revoked (ClutterGrab *grab)
+{
+  g_return_val_if_fail (CLUTTER_IS_GRAB (grab), FALSE);
+
+  return grab->prev != NULL;
+}

clutter/clutter/clutter-grab.h

@@ -41,3 +41,6 @@ void clutter_grab_dismiss (ClutterGrab *grab);
 
 CLUTTER_EXPORT
 ClutterGrabState clutter_grab_get_seat_state (ClutterGrab *grab);
+
+CLUTTER_EXPORT
+gboolean clutter_grab_is_revoked (ClutterGrab *grab);

clutter/clutter/clutter-stage.c

@@ -3930,6 +3930,9 @@ clutter_stage_grab_full (ClutterStage *stage,
   if (was_grabbed != !!priv->topmost_grab)
     g_object_notify_by_pspec (G_OBJECT (stage), obj_props[PROP_IS_GRABBED]);
 
+  if (grab->next)
+    clutter_grab_notify (grab->next);
+
   return grab;
 }
 
@@ -4035,6 +4038,9 @@ clutter_stage_unlink_grab (ClutterStage *stage,
 
   if (grab->owns_actor)
     g_clear_pointer (&grab->actor, clutter_actor_destroy);
+
+  if (priv->topmost_grab)
+    clutter_grab_notify (priv->topmost_grab);
 }
 
 /**