unity 在UI中 点击画布 返回鼠标所在画布的位置

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
public class aaa : MonoBehaviour {
Vector3 go ;
public GameObject bul;
public Canvas canvas;//将画布拖进来
// Use this for initialization
void Start () {

}
void Update () {

if (Input.GetMouseButton (0)) {

Vector2 _pos = Vector2.one;
RectTransformUtility.ScreenPointToLocalPointInRectangle (canvas.transform as RectTransform,
Input.mousePosition, canvas.worldCamera, out _pos);
Debug.Log ("pos:" + _pos);

}

}
}