class in UnityEngine.Experimental.UIElements
实现接口：IEventHandler
切换到手册使类能够让回调处理事件的接口。
| 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。 |