最近一直在倒腾用UGUI做虚拟摇杆，网上普遍的的做法就是使用以下的代码，但是这个有些注意事项，第一点就是Canvas的Render Mode必须是Screen Space Overlay，第二点就是挂载这个脚本的锚点的x,y必须是0.5，如图下：

using UnityEngine;
using UnityEngine.EventSystems;
public class JoyStick : MonoBehaviour, IDragHandler, IEndDragHandler
{
 Transform point;
 Vector3 startPos;//开始位置
 Vector3 dir;//方向
 float radius = 0;//需要移动的半径
 void Start()
 {
 point = transform.GetChild(0);
 radius = (transform as RectTransform).sizeDelta.x * 0.5f;
 startPos = point.position;
 }
 public void OnDrag(PointerEventData eventData)
 {
 point.position = eventData.position;
 dir = (point.position - startPos).normalized;
 if (Vector3.SqrMagnitude(point.position - startPos) > radius * radius)
 point.position = startPos + dir * radius;
 }
 public void OnEndDrag(PointerEventData eventData)
 {
 point.localPosition = Vector3.zero;
 }
}

如果Canvas的Render Mode是Screen Space Camera，这样的话上面的代码是不能满足要求的，花了一点时间才发现是这个原因，导致上面的代码不适用的，最后把代码重写了一下，终于可以成功了！

public class JoyStick : MonoBehaviour, IDragEvent
{
 private Canvas canvas;
 private RectTransform rectTransform;//坐标
 private static Quaternion amendAngle;
 private static float mRadius = 0,v=0, h=0;
 private static Transform point;
 private static Vector3 initPos;
 private static Vector2 startPos;
 private void Start()
 {
 point = transform.GetChild(0);
 canva【本文由：香港大带宽服务器提供】s = GameObject.Find("UIRoot").GetComponent<Canvas>();
 rectTransform = transform as RectTransform; //也可以写成this.GetComponent<RectTransform>(),但是不建议；
 mRadius = (transform as RectTransform).sizeDelta.x * 0.5f;
 initPos = point.localPosition;
 h = v = 0;
 }
 
 public void OnBeginDrag(PointerEventData eventData)
 {
 RectTransformUtility.ScreenPointToLocalPointInRectangle(rectTransform, eventData.position, canvas.worldCamera, out startPos);
 startPos = eventData.position - startPos;
 h = v = 0; 
 }
 
 public void OnDrag(PointerEventData eventData)
 {
 point.localPosition = eventData.position - startPos;
 Vector3 dir = (point.localPosition - initPos).normalized;
 v = dir.normalized.x; h = dir.normalized.y;
 if (Vector3.SqrMagnitude(point.localPosition - initPos) > mRadius * mRadius)
 point.localPosition = initPos + dir * mRadius;
 }
 
 public void OnEndDrag(PointerEventData eventData)
 {
 point.localPosition = Vector3.zero;
 h = v = 0; 
 }
}

RectTransformUtility.ScreenPointToLocalPointInRectangle(rectTransform, eventData.position, canvas.worldCamera, out startPos)这个的startPos返回的是点击屏幕的坐标，rectTransform是这个脚本挂载物体上的RectTransform的组件，然后减去eventData.position就知道坐标的偏移值了，看一下代码应该都可以了解意思，这里就不过多的解释了。

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