鼠标屏幕坐标命中世界物体

可用 射线，和摄像机 来判断

    Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);

                Physics.Raycast(ray, out hit);

                string tag = hit.collider.gameObject.tag;
                Debug.Log("click is " + tag);

                if (tag == "enemy")
                {
                    _target = hit.collider.gameObject;  
                    this.applyTarget();


                }