Xamarin 自定义OnKeyDown 再按一次退出程序的实现

private DateTime? lastBackKeyDownTime;

public override bool OnKeyDown(Keycode keyCode, KeyEvent e)
        {
            if (keyCode == Keycode.Back && e.Action == KeyEventActions.Down && e.RepeatCount == )
            {
                var backPressed = DateTime.Now;
                var navigation = App.Current.MainPage.Navigation;
                if (navigation.NavigationStack.Count > )
                {
                    navigation.PopAsync();
                    OnBackPressed();
                    return false;
                }
                else if (!lastBackKeyDownTime.HasValue || backPressed.Subtract(lastBackKeyDownTime.Value).Seconds > )
                {
                    lastBackKeyDownTime = backPressed;
                    var toast = Toast.MakeText(this, "再按一次退出程序", ToastLength.Short);
                    toast.SetGravity(GravityFlags.Center, , );
                    toast.Show();
                }
                else
                {
                    Finish();
                }
                return true;
            }
            return base.OnKeyDown(keyCode, e);
        }

   public override void OnBackPressed()
   {
            Rg.Plugins.Popup.Popup.SendBackPressed(base.OnBackPressed);
   }