| callback | The event handler to add. |
| useTrickleDown | By default, this callback is called during the BubbleUp phase. Pass TrickleDown.TrickleDown to call this callback during the TrickleDown phase. |
| userArgs | Data to pass to the callback. |
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.