public Camera camera;
    Ray ray;
    RaycastHit hitInfo;
    Vector3 offset;
    GameObject obj;//獲取點選到的物體
    public GameObject transformObj;
    Vector3 startMousePosition;
    // Use this for initialization
    void Start () {
        startMousePosition = Input.mousePosition;
    }

// Update is called once per frame
    void Update () {
        if (Input.GetMouseButtonDown (0)) {
            ray = camera.ScreenPointToRay (Input.mousePosition);
            if (Physics.Raycast (ray, out hitInfo)) {
                 obj = GameObject.Find (hitInfo.transform.name);
                obj.GetComponent<MeshRenderer>().material.color = new Color(Random.Range (0f, 1f),Random.Range (0f, 1f),Random.Range (0f, 1f),Random.Range (0f, 1f));
            }
        }
        if (Input.GetMouseButton (0)) {
            if (obj) {
                //將物體由世界座標轉為螢幕座標ScreenToWorldPoint
                Vector3 screenObj = Camera.main.WorldToScreenPoint(obj.transform.position);
                //儲存滑鼠的螢幕座標 z使用目標物體的螢幕座標
                Vector3 screenMouse = new Vector3(Input.mousePosition.x, Input.mousePosition.y,screenObj.z);
                offset =Camera.main.ScreenToWorldPoint(screenMouse)- obj.transform.position ;
                Vector3 currentPosition = obj.transform.position + offset;
                obj.transform.position = currentPosition;
            }
        }
    
        if (Input.mousePosition != startMousePosition && !Input.GetMouseButton(0)&& !Input.GetMouseButtonDown(0)) {
            Vector3 screenObj = Camera.main.WorldToScreenPoint(transformObj.transform.position);
            Vector3 screenMouse = Input.mousePosition;
            Vector3 newVector = screenMouse - screenObj;
            transformObj.transform.forward = newVector;
        }
    }
 

transform.position = new Vector3 (Mathf.MoveTowards (transform.position.x, ball.transform.position.x, speed * Time.deltaTime), Mathf.MoveTowards (transform.position.y, ball.transform.position.y- 2f, speed * Time.deltaTime), Mathf.MoveTowards (transform.position.z, ball.transform.position.z , speed * Time.deltaTime));