wp8.1 app退出操作提示

微软的wp8.1 sdk相比之前wp8 sdk以及相关dll类库，微软又重新编译过，相关系统类库也经过精简，删改了部分传统dll库中的方法对象，很多常用方法对象被写进Windows.UI为前缀的命名空间中，可以看出微软wp8.1经过了一定的优化。

此处功能设计描述为，触摸一次返回键，提示是否退出app，再点一次即关闭app。

 <Grid Background="#F5F5F5" DataContext="{Binding Path=MainPageViewModel, Source={StaticResource Locator}}">
     <Grid Canvas.ZIndex="10000" Visibility="{Binding ExitNotificationVisible}" Height="30" Background="Red" VerticalAlignment="Top">
         <TextBlock FontSize="14" Text="提示: 再按一次返回键退出支付宝" VerticalAlignment="Center" Margin="10 0 0 0" />
     </Grid>
     <ContentControl HorizontalContentAlignment="Stretch" VerticalContentAlignment="Stretch" Content="{Binding DynamicContent}" />
 </Grid>

 using System;
 using Windows.UI.Xaml;
 using Windows.UI.Xaml.Controls;
 using Windows.UI.Popups;

 public class MainPageViewModel : Core.ViewModelBase
 {
     public MainPageViewModel()
     {
         InitData();
         if(DynamicContent == null)
         {
             DynamicContent = ViewControlLocator.Instance.RegisterViewControl<HomePage, HomePageViewModel>(true);
         }
         //---物理返回键api---
         Windows.Phone.UI.Input.HardwareButtons.BackPressed += HardwareButtons_BackPressed;
     }

     #region  attribute
     //---计时器---
     DispatcherTimer timer = null;
     int timesToTick = ;

     private UserControl m_DynamicContent;
     public UserControl DynamicContent
     {
         get { return m_DynamicContent; }
         set { m_DynamicContent = value; RaisePropertyChanged("DynamicContent"); }
     }

     private Visibility m_ExitNotificationVisible;
     public Visibility ExitNotificationVisible
     {
         get { return m_ExitNotificationVisible; }
         set { m_ExitNotificationVisible = value; RaisePropertyChanged("ExitNotificationVisible"); }
     }
     #endregion

     #region  method
     private void InitData()
     {
         ExitNotificationVisible = Visibility.Collapsed;
     }

     //---关闭事件---
     void HardwareButtons_BackPressed(object sender, Windows.Phone.UI.Input.BackPressedEventArgs e)
     {
         e.Handled = true;
         if (ExitNotificationVisible == Visibility.Collapsed)
         {
             ExitNotificationVisible = Visibility.Visible;

             if(timer == null)
             {
                 timer = new DispatcherTimer();
             }
             timer.Interval = TimeSpan.FromSeconds();
             timer.Tick += timer_Tick;
             timer.Start();
         }
         else
         {
             App.Current.Exit();
         }
     }

     //---到时没有执行关闭操作，隐藏关闭操作提示---
     void timer_Tick(object sender, object e)
     {
         timesToTick--;
         if (timesToTick == )
         {
             timesToTick = ;
             timer.Tick -= timer_Tick;
             timer.Stop();
             ExitNotificationVisible = Visibility.Collapsed;
         }
     }
     #endregion
 }