Unity实现鼠标拖动3D物体

using UnityEngine;
using System.Collections;

public class DragObject : MonoBehaviour {
  /// <summary>
  /// 将要拖动的物体
  /// </summary>
  private Transform dragGameObject;
  /// <summary>
  /// 获取射线需要碰撞的层
  /// </summary>
  private LayerMask canDrag;
  /// <summary>
  /// 直接从外部定义好层，简单理解
  /// </summary>
  public LayerMask canDrag2;
  /// <summary>
  /// 获得鼠标的位置和cube位置差
  /// </summary>
  private Vector3 offset;
  /// <summary>
  /// 是否点击到cube
  /// </summary>
  private bool isClickCube;
  /// <summary>
  /// 目标对象的屏幕坐标
  /// </summary>
  private Vector3 targetScreenPoint;

  // Use this for initialization
  void Start () {
    // LayerMask.GetMask("Cube"); 得到 名字为 Cube 的层的 2 进制
    // LayerMask.LayerToName(9); 得到一个 10 进制表示的层 的名字 这里既第十层
    // LayerMask.NameToLayer("Cube"); 得到 名字为 Cube 的层的 10 进制

    //使用位运算，因为 LayerMask （好像）是以2进制存储的 Layer 的层是以0开始
    canDrag = 1 << LayerMask.NameToLayer("Cube");
  }

  // Update is called once per frame
  void Update () {
    if (Input.GetMouseButtonDown(0))
    {
      if (CheckGameObject())
      {
        offset = dragGameObject.transform.position - Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, targetScreenPoint.z));
      }
    }

    if (isClickCube)
    {
      //当前鼠标所在的屏幕坐标
      Vector3 curScreenPoint = new Vector3(Input.mousePosition.x, Input.mousePosition.y, targetScreenPoint.z);
      //把当前鼠标的屏幕坐标转换成世界坐标
      Vector3 curWorldPoint = Camera.main.ScreenToWorldPoint(curScreenPoint);
      dragGameObject.position = curWorldPoint + offset;
    }

    if (Input.GetMouseButtonUp(0))
    {
      isClickCube = false;
    }
  }

  /// <summary>
  /// 检查是否点击到cbue
  /// </summary>
  /// <returns></returns>
  bool CheckGameObject ()
  {
    Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
    RaycastHit hitInfo;
    if (Physics.Raycast(ray, out hitInfo, 100f, canDrag))
    {
      isClickCube = true;
      //得到射线碰撞到的物体
      dragGameObject = hitInfo.collider.gameObject.transform;
      targetScreenPoint = Camera.main.WorldToScreenPoint(dragGameObject.position);
      return true;
    }
    return false;
  }

}