Interface for class capable of handling events.
| HandleEvent | Handle an 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. |
| SendEvent | Sends an event to the event handler. |