连续按两次提示退出功能

该类要继承Activity

　　　DateTime? lastBackKeyDownTime;

　　　public override Boolean OnKeyDown(Keycode keyCode, KeyEvent e)
        {
            if (keyCode == Keycode.Back && e.Action == KeyEventActions.Down)
            {
                if (!lastBackKeyDownTime.HasValue || DateTime.Now - lastBackKeyDownTime.Value > new TimeSpan(0, 0, 2))
                {
                    Toast.MakeText(this.ApplicationContext, "再按一次退出", ToastLength.Short).Show();
                    lastBackKeyDownTime = DateTime.Now;
                }
                else
                {
                    Finish();
                }
                return true;
            }
            return base.OnKeyDown(keyCode, e);
        }