private SafeArrayList<Spatial> getUpdateList() {
        if (updateListValid) {
            return updateList;
        }
        if (updateList == null) {
            updateList = new SafeArrayList<>(Spatial.class);
        } else {
            updateList.clear();
        }

        // Build the list
        addUpdateChildren(updateList);
        updateListValid = true;
        return updateList;
    }
Called to invalidate the root node's update list.  This is
 called whenever a spatial is attached/detached as well as
 when a control is added/removed from a Spatial in a way
 that would change state.
jme3-core/src/main/java/com/jme3/scene/Node.java