Xamarin表单- UWP后退按钮？

public App()
{
    InitializeComponent();
    MainPage = new NavigationPage(new MainPage());
    MainPage.SetValue(NavigationPage.BackButtonTitleProperty, ""); // 隐藏后退按钮的标题
    MainPage.SetValue(NavigationPage.HasBackButtonProperty, true); // 显示后退按钮
    MainPage.PropertyChanged += MainPage_PropertyChanged;
}

private void MainPage_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
{
    if (e.PropertyName == "CurrentPage")
    {
        var navigationPage = (NavigationPage)sender;
        var currentPage = navigationPage.CurrentPage;

        if (currentPage != null && navigationPage.Navigation.NavigationStack.Count > 1)
        {
            SystemNavigationManager.GetForCurrentView().AppViewBackButtonVisibility = AppViewBackButtonVisibility.Visible;
            SystemNavigationManager.GetForCurrentView().BackRequested += OnBackRequested;
        }
        else
        {
            SystemNavigationManager.GetForCurrentView().AppViewBackButtonVisibility = AppViewBackButtonVisibility.Collapsed;
            SystemNavigationManager.GetForCurrentView().BackRequested -= OnBackRequested;
        }
    }
}

private void OnBackRequested(object sender, BackRequestedEventArgs e)
{
    var navigationPage = (NavigationPage)MainPage;
    if (navigationPage.Navigation.NavigationStack.Count > 1)
    {
        navigationPage.Navigation.PopAsync();
        e.Handled = true;
    }
}