bool isExit = false;

        // 构造函数

        public MainPage()

        {

            InitializeComponent();

            isExit = false;

            // 用于本地化 ApplicationBar 的示例代码

            //BuildLocalizedApplicationBar();

        }

        protected override void OnBackKeyPress(CancelEventArgs e)

        {

            if (!isExit)

            {

                isExit = true;

                var toast = new ToastPrompt {Message = "再按一次退出程序"};

                toast.Completed += (o, ex) => { isExit = false; };

                toast.Show();

                e.Cancel = true;

            }

            else

            {

                NavigationService.RemoveBackEntry();

            }

        }