public static void Unsubscribe <T>(T control)
     where T : FrameworkElement
 {
     FlexibleDataContextChangedHelper <T> .Unsubscribe(control);
 }
 public static void Subscribe <T>(T control, Action <T> callback)
     where T : FrameworkElement
 {
     FlexibleDataContextChangedHelper <T> .Subscribe(control, callback);
 }