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;
  }
 }

}