下面是我用nui实现的一个虚拟摇杆。

1，示图

2、代码如下，都有比较详细的注释，就不说明了。

using UnityEngine;
using System.Collections;
using System.Collections.Generic;


[ExecuteInEditMode]
public class Joystick : MonoBehaviour
{    
    #region Delegate & Event
    public delegate void JoystickEventHandler(Joystick joystick);
    /// <summary>
    /// 开如
    /// </summary>
    public static event JoystickEventHandler On_JoystickMoveStart;
    /// <summary>
    /// Occurs when the joystick move.
    /// </summary>
    public static event JoystickEventHandler On_JoystickMove;
    /// <summary>
    /// thumb偏离中心位置，并牌按住时，每帧的回调
    /// </summary>
    public static event JoystickEventHandler On_JoystickHolding;
    /// <summary>
    /// Occurs when the joystick stops move
    /// </summary>
    public static event JoystickEventHandler On_JoystickMoveEnd;
    
    #endregion    

    #region   property
    [SerializeField] bool isRunInEditor = false;
    [SerializeField]private string joystickName = "NguiJoystick";
    public string JoystickName { get { return this.joystickName; } }
    [HideInInspector]private bool isLimitInCircle = true;
    public bool IsLimitInCircle { get { return this.isLimitInCircle; } }
    [SerializeField]private int radius = 100;
    public int Radius { get{ return this.radius; } }

    [SerializeField]
    private float minAlpha = 0.3f;
    public float MinAlpha { get { return this.minAlpha; } }

    private Vector2 joystickAxis = Vector2.zero;
    /// <summary>
    /// Gets the joystick axis value between -1 & 1...
    /// </summary>
    /// <value>
    /// The joystick axis.
    /// </value>
    public Vector2 JoystickAxis { get { return this.joystickAxis; } }

    private Vector2 lastJoystickAxis = Vector2.zero;
    public Vector2 LastJoystickAxis { get { return this.lastJoystickAxis; } }
    
    bool isForBid = false;
    /// <summary>
    /// 判断joystick是否被禁用
    /// </summary>
    public bool IsForBid { get { return this.isForBid; } }
    bool isHolding = false;
    public bool IsHolding { get { return this.isHolding; } }
    #endregion

    UIWidget root;
	[SerializeField]UISprite bg;
	[SerializeField]UISprite thumb;

    void Awake()
    {
        this.name = this.JoystickName;
        root = this.GetComponent<UIWidget>();
        Init();
    }
	
	
	// Update is called once per frame   
	void Update ()
    {
        if (isRunInEditor && Application.isEditor && !Application.isPlaying)
        {
            SetJoystickSize(radius);
        }

        if (!isForBid && isHolding)
        {
            Debug.Log("111111");
            if (On_JoystickHolding != null)
            {
                On_JoystickHolding(this);
            }
        }
	}

    void Init()
    {
        bg.transform.localPosition = Vector3.zero;
        thumb.transform.localPosition = Vector3.zero;
        SetJoystickSize(radius);
        Lighting(minAlpha);
    }

	#region ngui event
    ///// <summary>
    ///// test
    ///// </summary>
    //void OnClick ()
    //{
    //    Debug.Log("mouse pos :" + Input.mousePosition + " -- touch pos :" + ScreenPos_to_NGUIPos(Input.mousePosition));
    //    thumb.transform.localPosition = ScreenPos_to_NGUIPos(Input.mousePosition);
    //}
    void OnPress (bool isPressed)
    {
        if (isForBid)
        {
            Debug.Log("joystick is forbid!");
            return;
        }
        Debug.Log("OnPress:" + isPressed.ToString());
        if(isPressed)
        {
            Lighting(1f);
            CalculateJoystickAxis();
            if (On_JoystickMoveStart != null)
            {
                On_JoystickMoveStart(this);
            }
            isHolding = true;
        }
        else
        {
            CalculateJoystickAxis();
            if (On_JoystickMoveEnd != null)
            {
                On_JoystickMoveEnd(this);
            }
            thumb.transform.localPosition = Vector3.zero;
            FadeOut(1f, minAlpha);
            isHolding = false;
        }
    }

    //void OnDragStart ()
    //{
    //    if (isForBid)
    //    {
    //        Debug.Log("joystick is forbid!");
    //        return;
    //    }

    //    Debug.Log("OnDragStart");
    //    Lighting(1f);
    //    CalculateJoystickAxis();
    //    if(On_JoystickMoveStart!=null)
    //    {
    //        On_JoystickMoveStart(this);
    //    }
	//    isHolding = true;
    //    Debug.Log(string.Format("time:{0} - axis:{1}", Time.time, joystickAxis));
    //}

    void OnDrag(Vector2 delta)
    {
        if (isForBid)
        {
            return;
        }

        //Debug.Log("OnDrag:"+delta.ToString());        
        CalculateJoystickAxis();
        if (On_JoystickMoveStart != null)
        {
            On_JoystickMoveStart(this);
        }
    }


    //void OnDragEnd ()
    //{
    //    if (isForBid)
    //    {
    //        return;
    //    }

    //    Debug.Log("OnDragEnd");             
    //    CalculateJoystickAxis();
    //    if (On_JoystickMoveEnd != null)
    //    {
    //        On_JoystickMoveEnd(this);
    //    }
    //    thumb.transform.localPosition = Vector3.zero;
    //    FadeOut(1f, minAlpha);
    //    isHolding = false;
    //}
	#endregion

    #region utile

    /// <summary>
    /// 计算JoystickAxis
    /// </summary>
    /// <returns></returns>
    void CalculateJoystickAxis()
    {
        Vector3 offset = ScreenPos_to_NGUIPos(UICamera.currentTouch.pos);
        offset -= transform.localPosition;
        if (isLimitInCircle)
        {
            if (offset.magnitude > radius)
            {
                offset = offset.normalized * radius;
            }
        }
        thumb.transform.localPosition = offset;

        lastJoystickAxis = joystickAxis;
        joystickAxis = new Vector2(offset.x / radius, offset.y / radius);
    }

    /// <summary>
    /// Axis2s the angle.
    /// </summary>
    /// <returns>
    /// The angle.
    /// </returns>
    public float Axis2Angle(bool inDegree = true)
    {
        float angle = Mathf.Atan2(joystickAxis.x, joystickAxis.y);

        if (inDegree)
        {
            return angle * Mathf.Rad2Deg;
        }
        else
        {
            return angle;
        }
    }

    /// <summary>
    /// Axis2s the angle.
    /// </summary>
    /// <returns>
    /// The angle.
    /// </returns>
    public float Axis2Angle(Vector2 axis, bool inDegree = true)
    {
        float angle = Mathf.Atan2(axis.x, axis.y);

        if (inDegree)
        {
            return angle * Mathf.Rad2Deg;
        }
        else
        {
            return angle;
        }
    }



    /// <summary>
    /// 屏幕坐标-->ui坐标
    /// </summary>
    /// <param name="screenPos"></param>
    /// <returns></returns>
    Vector3 ScreenPos_to_NGUIPos(Vector3 screenPos)
    {
        Vector3 uiPos = UICamera.currentCamera.ScreenToWorldPoint(screenPos);
        uiPos = UICamera.currentCamera.transform.InverseTransformPoint(uiPos);
        return uiPos;
    }

    /// <summary>
    /// 屏幕坐标-->ngui坐标
    /// </summary>
    /// <param name="screenPos"></param>
    /// <returns></returns>
    Vector3 ScreenPos_to_NGUIPos(Vector2 screenPos)
    {
        return ScreenPos_to_NGUIPos(new Vector3(screenPos.x, screenPos.y, 0f));
    }

    /// <summary>
    /// 设置摇杆的大小
    /// </summary>
    /// <param name="radius"></param>
    void SetJoystickSize(int radius)
    {
        root.width = 2 * radius;
        root.height = 2 * radius;
        thumb.width = (int)(40f / 100f * root.width);
        thumb.height = (int)(40f / 100f * root.height);
    }

    /// <summary>
    /// 点亮摇杆
    /// </summary>
    void Lighting(float alpha)
    {
        iTween.Stop(this.gameObject, "value");
        root.alpha = alpha;
    }

    /// <summary>
    /// 渐变摇杆的透明度
    /// </summary>
    void FadeOut(float fromAlpha, float toAlpha)
    {
        Hashtable itweenArgs = new Hashtable();
        itweenArgs.Add("easetype", iTween.EaseType.linear);
        itweenArgs.Add("from", fromAlpha);
        itweenArgs.Add("to", toAlpha);       
        itweenArgs.Add("time", 0.5f);
        itweenArgs.Add("onupdate", "OnFadeOutTween");
        iTween.ValueTo(this.gameObject, itweenArgs);
    }
    void OnFadeOutTween(float value)
    {
        root.alpha = value;
    }

    #endregion

    #region 激活、禁用的控制
    List<string> keys = new List<string>();
    
   /// <summary>
    /// 禁用
   /// </summary>
   /// <returns>返回值是，取消这个禁用要用到的key</returns>
    public string ForbidJosystick()
    {
        string key = System.Guid.NewGuid().ToString();
        keys.Add(key);
        isForBid = true;
        return key;
    }

    /// <summary>
    /// 启用
    /// </summary>
    /// <param name="key"></param>
    public void ActivizeJosystick(string key)
    {
        if(keys.Contains(key))
        {
            keys.Remove(key);
        }

        isForBid = true;
        if(keys.Count==0)
        {
            isForBid = false;
        }
    }

    #endregion
}

NGUI版虚拟摇杆