Unity拖拽三维实体

public class DragTarget : MonoBehaviour {
    //防止鼠标按下开始拖拽时，物体移动，需要计算偏移量
    Vector3 offset;
    private void OnMouseDown()
    {        
        Vector3 objSreen = Camera.main.WorldToScreenPoint(transform.position);

        //物体距相机的距离，需要用到objSreen.z,移动时不改变Z值
        Vector3 mouseSreen = new Vector3(Input.mousePosition.x, Input.mousePosition.y, objSreen.z);
        offset = objSreen - mouseSreen;
    }
    private void OnMouseDrag()
    {
        Vector3 objSreen = Camera.main.WorldToScreenPoint(transform.position);

        //物体距相机的距离，需要用到objSreen.z,移动时不改变Z值
        Vector3 mousePosition = new Vector3(Input.mousePosition.x, Input.mousePosition.y, objSreen.z);
        Vector3 objPostion = Camera.main.ScreenToWorldPoint(mousePosition+offset);
        transform.position = objPostion;
    }
}