|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.