1.過載Activity的OnkeyDown事件

2.在裡面判斷兩次back間隔的時間是否大於設定的時間間隔， 大於則提示，小於則直接退出。

    public class MainActivity : Activity

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           private DateTime _firstBackTime;

        private static TimeSpan BackExitTimeSpan = new TimeSpan (0, 0, 0, 0, 800); //ms

       public override bool OnKeyDown(Keycode keyCode, KeyEvent e)

        {

            if (e.KeyCode == Keycode .Back)

            {

                var secondBackTime = DateTime .Now;

                if (secondBackTime - _firstBackTime > BackExitTimeSpan)

                {

                    Toast.MakeText(_view.Context, "再按一次退出程式..." , ToastLength.Short).Show();

                    _firstBackTime = secondBackTime;

                    return true ;

                }

                else

                {

                    System. Environment.Exit(0);

                }

            }

            return base .OnKeyDown(keyCode, e);

        }

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