UGUI实现的虚拟摇杆，可改变摇杆位置

实现方式主要参考这篇文章：http://www.cnblogs.com/plateFace/p/4687896.html。

主要代码如下：

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

 public delegate void JoystickMoveDelegate(JoystickData data);

 public class Joystick : MonoBehaviour, IPointerDownHandler, IPointerUpHandler
 {

     public GameObject joystickUI;                   //摇杆整体UI,方便Active
     public RectTransform joystickCenter;            //摇杆重心
     public RectTransform joystickBackground;        //摇杆背景

     public RectTransform joystickRect;
     private float radius;
     bool isClick;
     Vector2 clickPosition;

     public static event JoystickMoveDelegate JoystickMoveEvent;

     // Use this for initialization
     void Start()
     {
         radius = ;
     }

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

     public void JoystickController()
     {
         if (isClick)
         {
             clickPosition = GetClickPosition();
             float distance = Vector2.Distance(new Vector2(clickPosition.x, clickPosition.y) / Screen.width * , joystickRect.anchoredPosition);

             if (distance < radius)
             {
                 //当距离小于半径就开始移动 摇杆重心
                 joystickCenter.anchoredPosition = new Vector2(clickPosition.x / Screen.width *  - joystickRect.anchoredPosition.x, clickPosition.y / Screen.width *  - joystickRect.anchoredPosition.y);
             }
             else
             {
                 //求圆上的一点：(目标点-原点) * 半径/原点到目标点的距离
                 Vector2 endPosition = (new Vector2(clickPosition.x, clickPosition.y) / Screen.width *  - joystickRect.anchoredPosition) * radius / distance;
                 joystickCenter.anchoredPosition = endPosition;
             }

             if (JoystickMoveEvent != null)
             {
                 JoystickMoveEvent(new JoystickData() { x = joystickCenter.anchoredPosition.x - joystickBackground.anchoredPosition.x, y = joystickCenter.anchoredPosition.y - joystickBackground.anchoredPosition.y });
             }

         }
     }

     public void OnPointerDown(PointerEventData eventData)
     {
         ChangeAlpha();
         clickPosition = GetClickPosition();
         joystickRect.anchoredPosition = clickPosition / Screen.width * ;
         isClick = true;
     }

     public void OnPointerUp(PointerEventData eventData)
     {
         ChangeAlpha(0.3f);
         joystickCenter.anchoredPosition = Vector2.zero;
         isClick = false;
     }

     /// <summary>
     /// 根据平台不同获取点击位置坐标
     /// </summary>
     /// <returns></returns>
     Vector2 GetClickPosition()
     {
         if (Application.platform == RuntimePlatform.Android)
         {
             return Input.GetTouch().position;

         }
         else if (Application.platform == RuntimePlatform.WindowsEditor)
         {
             return Input.mousePosition;
         }
         return Vector2.zero;
     }

     /// <summary>
     /// 改变图片alpha值
     /// </summary>
     /// <param name="alphaValue"></param>
     void ChangeAlpha(float alphaValue)
     {
         joystickBackground.GetComponent<RawImage>().color = new Color(,,,alphaValue);
         joystickCenter.GetComponent<Image>().color = new Color(, , , alphaValue);
     }
 }

 public class JoystickData
 {
     public float x;
     public float y;

 }

主要实现了两个接口：IPointerDownHandler, IPointerUpHandler，监测按下和抬起事件。