| callback | 要添加的事件处理程序。 |
| useTrickleDown | By default, this callback is called during the BubbleUp phase. Pass TrickleDown.TrickleDown to call this callback during the TrickleDown phase. |
| userArgs | 要传递给回调的数据。 |
Adds an event handler to the instance. If the event handler has already been registered for the same phase (either TrickleDown or BubbleUp) then this method has no effect.