问windows phone 10上的硬件后退按钮不起作用

EN

protected override void OnLaunched(LaunchActivatedEventArgs e)
        {
#if DEBUG
            if (System.Diagnostics.Debugger.IsAttached)
            {
                this.DebugSettings.EnableFrameRateCounter = true;
            }
#endif
            Frame rootFrame = Window.Current.Content as Frame;

            // Do not repeat app initialization when the Window already has content,
            // just ensure that the window is active
            if (rootFrame == null)
            {
                // Create a Frame to act as the navigation context and navigate to the first page
                rootFrame = new Frame();

                rootFrame.NavigationFailed += OnNavigationFailed;
                rootFrame.Navigated += RootFrame_Navigated;

                if (e.PreviousExecutionState == ApplicationExecutionState.Terminated)
                {
                    //TODO: Load state from previously suspended application
                }

                // Place the frame in the current Window
                Window.Current.Content = rootFrame;

                // Register a handler for BackRequested events and set the  
                // visibility of the Back button  
                SystemNavigationManager.GetForCurrentView().BackRequested += OnBackRequested;

                SystemNavigationManager.GetForCurrentView().AppViewBackButtonVisibility =
                    rootFrame.CanGoBack ?
                    AppViewBackButtonVisibility.Visible :
                    AppViewBackButtonVisibility.Collapsed;
            }

            if (e.PrelaunchActivated == false)
            {
                if (rootFrame.Content == null)
                {
                    // When the navigation stack isn't restored navigate to the first page,
                    // configuring the new page by passing required information as a navigation
                    // parameter
                    rootFrame.Navigate(typeof(BackgroundMusic), e.Arguments);
                }
                // Ensure the current window is active
                Window.Current.Activate();
            }
        }

private void RootFrame_Navigated(object sender, NavigationEventArgs e)
        {
            // Each time a navigation event occurs, update the Back button's visibility  
            SystemNavigationManager.GetForCurrentView().AppViewBackButtonVisibility =
                ((Frame)sender).CanGoBack ?
                AppViewBackButtonVisibility.Visible :
                AppViewBackButtonVisibility.Collapsed;
        }

        private void OnBackRequested(object sender, BackRequestedEventArgs e)
        {
            Frame rootFrame = Window.Current.Content as Frame;

            if (App.DetectPlatform() == Platform.WindowsPhone)
            {
                HardwareButtons.BackPressed += new EventHandler<BackPressedEventArgs>((s, a) =>
                {
                    if (rootFrame.CanGoBack)
                    {

                        rootFrame.GoBack();
                        a.Handled = true;
                    }
                });
            }
            else if (App.DetectPlatform() == Platform.Windows)
            {
                if (rootFrame.CanGoBack)
                {
                    e.Handled = true;
                    rootFrame.GoBack();
                }
            }
        }

        public static Platform DetectPlatform()
        {
            bool isHardwareButtonsAPIPresent = ApiInformation.IsTypePresent("Windows.Phone.UI.Input.HardwareButtons");

            if (isHardwareButtonsAPIPresent)
            {
                return Platform.WindowsPhone;
            }
            else
            {
                return Platform.Windows;
            }
        }

public static MediaElement GlobalMediaElement
        {
            get { return Current.Resources["MyPlayer"] as MediaElement; }
        }

        private void mediaended(object sender, RoutedEventArgs e)
        {
            var AppMediaElement = App.GlobalMediaElement;
            AppMediaElement.Position = TimeSpan.Zero;
            AppMediaElement.Play();
        }

const string soundTrackToken = "soundtrack";
    int flag = 1;


    public BackgroundMusic()
    {
        this.InitializeComponent();
    }

    protected async override void OnNavigatedTo(NavigationEventArgs e)
    {
        //navigationHelper.OnNavigatedTo(e);
        mainFrame.Navigate(typeof(MainPage));

        if (StorageApplicationPermissions.FutureAccessList.ContainsItem(soundTrackToken))
        {
            StorageFile audioFile = await StorageApplicationPermissions.FutureAccessList.GetFileAsync(soundTrackToken);
            if (audioFile != null)
            {
                await StartAudio(audioFile);
            }
        }
    }