UGUI的拖拽功能

需要实现这3个接口IBeginDragHandler, IDragHandler, IEndDragHandler

```private void SetDraggedPosition(PointerEventData eventData)    {        

var rt = gameObject.GetComponent();// transform the screen point to world point int rectangle

Vector3 globalMousePos;

if (RectTransformUtility.ScreenPointToWorldPointInRectangle(rt, eventData.position,            eventData.pressEventCamera, out globalMousePos)) {

rt.position = globalMousePos;

//rt.parent = rt.parent.parent;

}

}

}```