//设置一个DispatcherTimer，控制三秒内再次点击返回键时执行退出逻辑

 public void ExitBy2Click(System.ComponentModel.CancelEventArgs e)        {            if (!IsExit)            {                IsExit = true;                e.Cancel = true;                _timer = new DispatcherTimer();                _timer.Start();                SystemTrayMessage.Instance.StartAdv("再按一次退出", 3, false);                _timer.Interval = TimeSpan.FromSeconds(3);                _timer.Tick += (sender, ee) =>                {                    IsExit = false;                    e.Cancel = false;                    _timer.Stop();                };                return;            }            else            {                       while (NavigationService.BackStack.Any())                    NavigationService.RemoveBackEntry();                base.OnBackKeyPress(new CancelEventArgs());               // App.Current.Terminate();                //new Microsoft.Xna.Framework.Game().Exit();//wp7            }