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);

   }