unity实现鼠标拖住3D物体

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ModelDrages : MonoBehaviour 
{

//发射射线的摄像机
private Camera cam;
//射线碰撞的物体
private GameObject go;
//射线碰撞物体的名字
public static string btnName;
private Vector3 screenSpace;
private Vector3 offset;
private bool isDrage = false;

// Use this for initialization
void Start ()
{
 cam = Camera.main;
}

// Update is called once per frame
 void Update ()
{
 //整体初始位置
 Ray ray = cam.ScreenPointToRay(Input.mousePosition);
 //从摄像机发出到点击坐标的射线
 RaycastHit hitInfo;
 if (isDrage == false)
 {
  if(Physics .Raycast (ray,out hitInfo))
  {
   //划出射线 只有在Scene视图中才能看到
   Debug.DrawLine(ray.origin, hitInfo.point);
   go = hitInfo.collider.gameObject;
   print(btnName);
   screenSpace = cam.WorldToScreenPoint(go.transform.position);
   offset = go.transform.position - cam.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, Input.mousePosition.z));
   //物体的名字
   btnName = go.name;
   //组件的名字
  }
  else
  {
   btnName = null;
  }
 }
 if(Input.GetMouseButton(0))
 {
  Vector3 currentScreenSpace = new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenSpace.z);
  Vector3 currentPosition = cam.ScreenToWorldPoint(currentScreenSpace) + offset;
  if (btnName != null)
  {
   go.transform.position = currentPosition;
  }
  isDrage = true;
 }
 else
 {
  isDrage = false;
 }
 }

}