class in UnityEngine.Experimental.UIElements
/
继承自：Experimental.UIElements.CallbackEventHandler
切换到手册可获得焦点的对象的基类。
焦点用于指定将接收键盘事件的元素。
| canGrabFocus | 如果元素可以获得焦点，则返回 true。 |
| focusController | 返回此元素的焦点控制器。 |
| focusIndex | 用于对焦点环中可获得焦点的元素排序的整数。负值表示元素不能获得焦点。 |
| HandleEvent | 处理事件，通常是通过执行与事件关联的回调处理事件。 |
| 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 | 从此实例删除回调。 |