如何在unity 5中使用鼠标拖动刚性对象

using UnityEngine;

public class DragObject : MonoBehaviour
{
    private Vector3 offset;

    void OnMouseDown()
    {
        offset = gameObject.transform.position - Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, 10.0f));
    }

    void OnMouseDrag()
    {
        Vector3 newPosition = new Vector3(Input.mousePosition.x, Input.mousePosition.y, 10.0f);
        transform.position = Camera.main.ScreenToWorldPoint(newPosition) + offset;
    }
}