using UnityEngine;
using System.Collections;

public class DragObject : MonoBehaviour {
  /// <summary>
  /// 將要拖動的物體
  /// </summary>
  private Transform dragGameObject;
  /// <summary>
  /// 獲取射線需要碰撞的層
  /// </summary>
  private LayerMask canDrag;
  /// <summary>
  /// 直接從外部定義好層，簡單理解
  /// </summary>
  public LayerMask canDrag2;
  /// <summary>
  /// 獲得鼠標(biāo)的位置和cube位置差
  /// </summary>
  private Vector3 offset;
  /// <summary>
  /// 是否點擊到cube
  /// </summary>
  private bool isClickCube;
  /// <summary>
  /// 目標(biāo)對象的屏幕坐標(biāo)
  /// </summary>
  private Vector3 targetScreenPoint;

  // Use this for initialization
  void Start () {
    // LayerMask.GetMask("Cube"); 得到 名字為 Cube 的層的 2 進(jìn)制
    // LayerMask.LayerToName(9); 得到一個 10 進(jìn)制表示的層 的名字 這里既第十層
    // LayerMask.NameToLayer("Cube"); 得到 名字為 Cube 的層的 10 進(jìn)制

    //使用位運算，因為 LayerMask （好像）是以2進(jìn)制存儲的 Layer 的層是以0開始
    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)
    {
      //當(dāng)前鼠標(biāo)所在的屏幕坐標(biāo)
      Vector3 curScreenPoint = new Vector3(Input.mousePosition.x, Input.mousePosition.y, targetScreenPoint.z);
      //把當(dāng)前鼠標(biāo)的屏幕坐標(biāo)轉(zhuǎn)換成世界坐標(biāo)
      Vector3 curWorldPoint = Camera.main.ScreenToWorldPoint(curScreenPoint);
      dragGameObject.position = curWorldPoint + offset;
    }

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

  /// <summary>
  /// 檢查是否點擊到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;
      //得到射線碰撞到的物體
      dragGameObject = hitInfo.collider.gameObject.transform;
      targetScreenPoint = Camera.main.WorldToScreenPoint(dragGameObject.position);
      return true;
    }
    return false;
  }

}