Unity 双击Esc或者返回退出游戏,有文字提示

第一次点击Esc或者返回,显示提示文字"再次按下返回键退出游戏",在文字消失之前再次点击Esc或者返回,退出游戏.

此脚本挂在Text文字提示上:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 public class ExitGame : MonoBehaviour
 {
     [Tooltip("ExitMessage拖进来")]
     Text ExitMessageObj;
     float fadingSpeed = ;
     bool fading;
     float startFadingTimep;
     Color originalColor;
     Color transparentColor;
     string sss;

     void Start()
     {
         ExitMessageObj = this.GetComponent<Text>();
         originalColor = ExitMessageObj.color;
         transparentColor = originalColor;
         transparentColor.a = ;
         ExitMessageObj.text = "再次按下返回键退出游戏";
         ExitMessageObj.color = transparentColor;
     }

     void Update()
     {
         if (Input.GetKeyDown(KeyCode.Escape))
         {
             if (startFadingTimep == )
             {
                 ExitMessageObj.color = originalColor;
                 startFadingTimep = Time.time;
                 fading = true;
             }
             else
             {                                Application.Quit();//退出游戏
             }
         }
         if (fading)
         {
             ExitMessageObj.color = Color.Lerp(originalColor, transparentColor, (Time.time - startFadingTimep) * fadingSpeed);
             if (ExitMessageObj.color.a < 2.0 / )
             {
                 ExitMessageObj.color = transparentColor;
                 startFadingTimep = ;
                 fading = false;
             }
         }
     }
 }