class in UnityEngine.Experimental.UIElements
实现接口：IManipulator
切换到手册Base class for objects that define user interaction with a VisualElement. Most often manipulators hold state for complex interactions, for example interactions that require a mouse down, mouse move mouse up sequence.
| target | The element that handles the interaction. |
| RegisterCallbacksOnTarget | Register event callbacks on the target visual element. |
| UnregisterCallbacksFromTarget | Unregister event callbacks on the target visual element. |