Unity 两张背景的切换平移

using UnityEngine;
using System.Collections;

/**
 *两个背景图片平移 
 */
public class BackgroundController : MonoBehaviour {


    public SpriteRenderer back1;        
    public SpriteRenderer back2;        
    public Sprite [] textureList;           //背景图片集合
     public int speed = 1;                   //背景平移的速度
     public Transform endPosition;           //到达该位置,把背景移动到开始位置
     public Transform startPosition;         //背景从开始位置移动到结束位置
        
    private int backIndex;                  

    void Start() 
    {
        back1.sprite = textureList[0];
        back2.sprite = textureList[1];
        backIndex = 2;
    }

    void Update() 
    {
        if (backIndex >= textureList.Length) 
        {
            backIndex = textureList.Length - 1;
        }

        //两个图片的平移
        back1.transform.position = new Vector3(back1.transform.position.x - speed * Time.deltaTime, back1.transform.position.y, back1.transform.position.z);
        back2.transform.position = new Vector3(back2.transform.position.x - speed * Time.deltaTime, back2.transform.position.y, back2.transform.position.z);

        //到达结束位置,回到开始位置,切换图片
        if (back1.transform.position.x <= endPosition.transform.position.x) 
        {
            back1.transform.position = startPosition.position;
            back1.sprite = textureList[backIndex];
            backIndex++;
        }

        if (back2.transform.position.x <= endPosition.transform.position.x)
        {
            back2.transform.position = startPosition.position;
            back2.sprite = textureList[backIndex];
            backIndex++;
        }
    }

}