clutter/actor: Allow specifying the layout manager for an actor type

Some actors have a well-defined layout manager other than FixedLayout.

If they do, we can handle the layout manager creation at the
ClutterActor instantiation, like GTK does for widget layout managers.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3445>

clutter/clutter/clutter-actor.c

@@ -5654,11 +5654,19 @@ clutter_actor_constructor (GType gtype,
 
   if (self->priv->layout_manager == NULL)
     {
+      ClutterActorClass *actor_class;
       ClutterLayoutManager *default_layout;
+      GType layout_manager_type;
+
+      actor_class = CLUTTER_ACTOR_GET_CLASS (self);
+
+      layout_manager_type = clutter_actor_class_get_layout_manager_type (actor_class);
+      if (layout_manager_type == G_TYPE_INVALID)
+        layout_manager_type = CLUTTER_TYPE_FIXED_LAYOUT;
 
       CLUTTER_NOTE (LAYOUT, "Creating default layout manager");
 
-      default_layout = clutter_fixed_layout_new ();
+      default_layout = g_object_new (layout_manager_type, NULL);
       clutter_actor_set_layout_manager (self, default_layout);
     }
 
@@ -5711,6 +5719,8 @@ clutter_actor_class_init (ClutterActorClass *klass)
   klass->paint = clutter_actor_real_paint;
   klass->destroy = clutter_actor_real_destroy;
 
+  klass->layout_manager_type = G_TYPE_INVALID;
+
   /**
    * ClutterActor:x:
    *
@@ -15413,12 +15423,25 @@ clutter_actor_set_layout_manager (ClutterActor         *self,
                                   ClutterLayoutManager *manager)
 {
   ClutterActorPrivate *priv;
+  GType expected_type, manager_type;
 
   g_return_if_fail (CLUTTER_IS_ACTOR (self));
   g_return_if_fail (manager == NULL || CLUTTER_IS_LAYOUT_MANAGER (manager));
 
   priv = self->priv;
 
+  expected_type = clutter_actor_class_get_layout_manager_type (CLUTTER_ACTOR_GET_CLASS (self));
+  manager_type = manager != NULL ? G_TYPE_FROM_INSTANCE (manager) : G_TYPE_INVALID;
+
+  if (expected_type != G_TYPE_INVALID &&
+      manager_type != G_TYPE_INVALID &&
+      !g_type_is_a (manager_type, expected_type))
+    {
+      g_warning ("Trying to set layout manager of type %s, but actor only accepts %s",
+                 g_type_name (manager_type), g_type_name (expected_type));
+      return;
+    }
+
   if (priv->layout_manager != NULL)
     {
       g_clear_signal_handler (&priv->layout_changed_id, priv->layout_manager);
@@ -18616,3 +18639,44 @@ clutter_actor_notify_transform_invalid (ClutterActor *self)
   if (!graphene_matrix_equal (&old_transform, &priv->transform))
     clutter_actor_queue_redraw (self);
 }
+
+/**
+ * clutter_actor_class_set_layout_manager_type
+ * @actor_class: A #ClutterActor class
+ * @type: A #GType
+ *
+ * Sets the type to be used for creating layout managers for
+ * actors of @actor_class.
+ *
+ * The given @type must be a subtype of [class@Clutter.LayoutManager].
+ *
+ * This function should only be called from class init functions of actors.
+ */
+void
+clutter_actor_class_set_layout_manager_type (ClutterActorClass *actor_class,
+                                             GType              type)
+{
+  g_return_if_fail (CLUTTER_IS_ACTOR_CLASS (actor_class));
+  g_return_if_fail (g_type_is_a (type, CLUTTER_TYPE_LAYOUT_MANAGER));
+
+  actor_class->layout_manager_type = type;
+}
+
+/**
+ * clutter_actor_class_get_layout_manager_type
+ * @actor_class: A #ClutterActor class
+ *
+ * Retrieves the type of the [class@Clutter.LayoutManager]
+ * used by actors of class @actor_class.
+ *
+ * See also: [method@Clutter.ActorClass.set_layout_manager_type].
+ *
+ * Returns: type of a `ClutterLayoutManager` subclass, or %G_TYPE_INVALID
+ */
+GType
+clutter_actor_class_get_layout_manager_type (ClutterActorClass *actor_class)
+{
+  g_return_val_if_fail (CLUTTER_IS_ACTOR_CLASS (actor_class), G_TYPE_INVALID);
+
+  return actor_class->layout_manager_type;
+}

clutter/clutter/clutter-actor.h

@@ -251,6 +251,9 @@ struct _ClutterActorClass
                                      ClutterActor         *child);
   void     (* child_removed)        (ClutterActor         *self,
                                      ClutterActor         *child);
+
+  /* private */
+  GType layout_manager_type;
 };
 
 /**
@@ -883,4 +886,10 @@ void clutter_actor_invalidate_transform (ClutterActor *self);
 CLUTTER_EXPORT
 void clutter_actor_invalidate_paint_volume (ClutterActor *self);
 
+CLUTTER_EXPORT
+void clutter_actor_class_set_layout_manager_type (ClutterActorClass *actor_class,
+                                                  GType              type);
+CLUTTER_EXPORT
+GType clutter_actor_class_get_layout_manager_type (ClutterActorClass *actor_class);
+
 G_END_DECLS