Base class for objects that can get the focus.
The focus is used to designate an element that will receive keyboard events.
| canGrabFocus | Return true if the element can be focused. |
| delegatesFocus | Whether the element should delegate the focus to its children. |
| focusable | True if the element can be focused. |
| focusController | Return the focus controller for this element. |
| tabIndex | An integer used to sort focusables in the focus ring. Must be greater than or equal to zero. |
| Blur | Tell the element to release the focus. |
| Focus | Attempt to give the focus to this element. |
| 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. |