Interface for classes capable of having callbacks to handle events.
| HandleEvent | Handle an event, most often by executing the callbacks associated with the event. |
| HasBubbleUpHandlers | Return true if event handlers for the event propagation BubbleUp phase have been attached on this object. |
| HasTrickleDownHandlers | Returns true if event handlers, for the event propagation TrickleDown phase, are attached to this object. |
| RegisterCallback | 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. |
| SendEvent | Sends an event to the event handler. |
| UnregisterCallback | Remove callback from the instance. |