Unity中2D物体的拖拽

public class testButton : MonoBehaviour {
    public bool isMouseDown = false;
    private Vector3 lastMousePosition = Vector3.zero;
    private Vector3 offset;

    private void Update()
    {

        if(Input.GetMouseButtonDown(0))
        {
            isMouseDown = true;
        }
        if(Input.GetMouseButtonUp(0))
        {
            isMouseDown = false;
        }

        if(isMouseDown)
        {
            var hit = Physics2D.Raycast(Camera.main.ScreenToWorldPoint(Input.mousePosition), Vector2.zero, Mathf.Infinity);
            if(hit.collider!=null)
            {
                if(hit.collider.tag=="boot")
                {
                  //  print("OK");
                    MoveCube();
                }

            }
        } 
    }

    void MoveCube()
    {     
        offset = Camera.main.ScreenToWorldPoint(Input.mousePosition);
        offset.z = 0;
       // Debug.Log("offset:" + offset);
        transform.position = offset;
        lastMousePosition = transform.position;
    }
}