Unity 拖动UI物体

using UnityEngine;
using System.Collections;
using UnityEngine.EventSystems;

public class Tool : MonoBehaviour, IDragHandler,IPointerDownHandler,IPointerUpHandler{

// Use this for initialization
void Start () {
}

// Update is called once per frame
void Update () {
}

public void OnPointerDown(PointerEventData eventDate) {
Debug.Log ("start");
}

public void OnDrag (PointerEventData eventData) {
GameObject pointerDrag = eventData.pointerDrag;

Vector3 globalMousePosition;
if (RectTransformUtility.ScreenPointToWorldPointInRectangle(pointerDrag.GetComponent<RectTransform>(), eventData.position, Camera.main, out globalMousePosition))
{
pointerDrag.transform.position = globalMousePosition;
}
}

public void OnPointerUp(PointerEventData eventData) {
Debug.Log ("end");
}
}