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 = 2;

    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(2);
            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 == 0)
        {
            timesToTick = 2;
            timer.Tick -= timer_Tick;
            timer.Stop();
            ExitNotificationVisible = Visibility.Collapsed;  
        }
    }
    #endregion
}