Unity+NGUI、タッチした所が始点になるバーチャルジョイスティック

JoyStick.cs

using UnityEngine;
using System.Collections;

public class JoyStick : MonoBehaviour {

	public GameObject AttachJoyStickSprite, AttachJoyStickBackSprite;
	public Camera AttachGUICamera;

	private Vector2 _position;
	public  Vector2 Position{
		get{return _position;}
	}

	private float _maxRadius;
	private const float MAX_RADIUS_RATE = 0.55f;

	//=================================================================================
	//初期化
	//=================================================================================

	private void Awake (){
		_position  = Vector2.zero;
		_maxRadius = AttachJoyStickBackSprite.GetComponent<UISprite>().height * MAX_RADIUS_RATE;
	}

	//=================================================================================
	//更新
	//=================================================================================

	private void Update () {
		DisplayConfirmation();
		Move ();
	}

	//ジョイスティックの表示確認
	private void DisplayConfirmation(){

		if (Input.GetMouseButtonDown(0)){
			AttachJoyStickBackSprite.SetActive (true);

			AttachJoyStickBackSprite.transform.position = AttachGUICamera.ScreenToWorldPoint(Input.mousePosition);
			AttachJoyStickSprite.transform.position = Vector3.zero;
		}
		else if (Input.GetMouseButtonUp(0)){
			AttachJoyStickBackSprite.SetActive (false);
			_position = Vector2.zero;
		}

	}

	//ジョイスティックをタッチされている場所に伴って移動
	private void Move(){

		//表示されていなければ移動しない
		if(!AttachJoyStickBackSprite.activeSelf){
			return;
		}

		Vector3 touchPosition = AttachGUICamera.ScreenToWorldPoint(Input.mousePosition);
		AttachJoyStickSprite.transform.position = touchPosition;

		//半径が制限を超えてる場合は制限内に抑える
		float radius = Vector3.Distance (Vector3.zero, AttachJoyStickSprite.transform.localPosition);
		if(radius > _maxRadius){

			//角度
			float radian = CalcRadian(
				Vector3.zero, 
				AttachJoyStickSprite.transform.localPosition
			);

			Vector3 setVec = Vector3.zero;
			setVec.x = _maxRadius * Mathf.Cos (radian);
			setVec.y = _maxRadius * Mathf.Sin (radian);

			AttachJoyStickSprite.transform.localPosition = setVec;
		}

		//-1〜1に正規化
		_position = new Vector2 (
			AttachJoyStickSprite.transform.localPosition.x / _maxRadius,
			AttachJoyStickSprite.transform.localPosition.y / _maxRadius
		);

	}

	//2点間の角度を求める
	private float CalcRadian(Vector3 from, Vector3 to) {
		float dx = to.x - from.x;
		float dy = to.y - from.y;
		float radian = Mathf.Atan2(dy, dx);
		return radian;
	}

}