While working on the Khan Academy app, I realized early in the development that using the navigation pattern provided by the default project templates was not optimal. If you haven’t noticed, the code in default templates which bootstraps your application and creates the frame where your pages are hosted is by default in App.xaml.cs. When the app launches, it checks if the root content is a Frame and creates a new one if not.

protected override async void OnLaunched(LaunchActivatedEventArgs args)
{
	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();

		//code to restore navigation state
		//this will cause navigation for "a" page if there was anything on navigation stack

		// Place the frame in the current Window
		Window.Current.Content = rootFrame;
	}
	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
		if (!rootFrame.Navigate(typeof(GroupedItemsPage), "AllGroups"))
		{
			throw new Exception("Failed to create initial page");
		}
	}
}

Once the frame is created, it navigates to the first page. If you are implementing an extended splash screen, this code moves there but we still have a frame and all navigation happens in that frame. Here are the reasons why I think that the default navigation is not optimal. Your mileage may vary.

1. Entrance Animations for Pages: If we want to use entrance animations for pages, that doesn’t come by default. It’s doable with the default frame created in bootstrap code as you can just add theme transitions to the frame and it would work as you’d expect.

//
if (rootFrame.ContentTransitions == null) rootFrame.ContentTransitions = new TransitionCollection();
rootFrame.ContentTransitions.Clear();
rootFrame.ContentTransitions.Add(new EdgeUIThemeTransition { Edge = EdgeTransitionLocation.Right });
//

If you want to use different entrance animations for different pages, that’s when it gets ugly. Let’s say you are using one consistent entrance animation for all pages but for one special page, you want a unique entrance animation. It’s doable but you’ll have to keep changing frame content transitions in the code every time you need different transitions from what is assigned to frame right now. That’s a big “no”. A different approach is to use separate frames as we will see later in this post. I used this approach in Khan Academy app to slide in the player from bottom of the screen whereas all other pages use standard animation.

2. Maintaining User Actions Across Page Navigations: Every time you navigate away from a page, the instance of the page gets disposed. Navigating to that page again will recreate the instance and have the root frame navigate to it. If the main page of the app is complex and has lot of content with options for users to perform different actions, maintaining the state where user was when he/she navigated away and restoring that state on navigating back to the main page is tricky. For example, let’s say user performs following actions in Khan Academy app.

• Launch the app
• Open the subject popup
• Navigate within the subject popup
• Tap on a video to play

At this point, if user navigates to player screen on the root frame, navigating back to main page will cause recreation of main page and we’d lose the state for user interactions. This is because I am showing subject details in popup rather than navigating to a separate page due to deep content hierarchy. If I’d do a full page navigation every time user wants to open subject content, this problem wouldn’t exist but the experience would be annoying as the users will have to keep going back and forth all the time.

This can be solved by enabling NavigationCacheMode on the page which makes Xaml runtime use one instance of the page across navigations. This comes with additional state maintenance though. Alternatively, you can use separate frames as will see below.

3. Performance due to Page Navigations: As the page instances are getting destroyed every time you navigate from a page and getting recreated when you navigate to it, this obviously comes with performance overhead. And it’s very noticeable on Windows RT devices which use low powered ARM chips. You can use NavigationCacheMode to cache pages but that comes with additional handling to refresh them when needed. This still doesn’t avoid the work needed for “navigating” to the cached page. In some cases, this might be good enough, you should know your options and pick what works for you.

Using multiple Frames to mitigate these issues

First, we would replace the root frame which was provided by the template with a page with multiple frames in it, lets call it HostPage. So, instead of creating a root frame and assigning that to current window content, we create an instance of HostPage and assign that to window content. At this point, HostPage becomes our one stop shop for all things navigation. For Khan Academy app, here is how the HostPage Xaml looks like.

<Grid x:Name="rootGrid" Style="{StaticResource LayoutRootStyle}">
	<Image Source="../Assets/Images/footer-leaves.png" VerticalAlignment="Bottom" HorizontalAlignment="Center" RenderTransformOrigin="0.5,0.5" >
		<Image.RenderTransform>
			<CompositeTransform ScaleY="-1"/>
		</Image.RenderTransform>
	</Image>

	<Frame x:Name="hostFrame" Foreground="{x:Null}">
		<Frame.ContentTransitions>
			<TransitionCollection>
				<ContentThemeTransition HorizontalOffset="300" />
			</TransitionCollection>
		</Frame.ContentTransitions>
	</Frame>

	<Grid Visibility="{Binding IsOffline, Converter={StaticResource VisibilityConverter}}"
		  HorizontalAlignment="Center" VerticalAlignment="Top" Margin="0,0,0,10">
		<Border Background="{StaticResource ThemeContrastColorBrush}" CornerRadius="5" Margin="-10" />
		<TextBlock Text="Offline Mode" Style="{StaticResource TitleTextStyle}" Foreground="{StaticResource LightForegroundThemeBrush}" />
	</Grid>

	<Frame x:Name="playerFrame" Foreground="{x:Null}">
		<Frame.ContentTransitions>
			<TransitionCollection>
				<PaneThemeTransition Edge="Bottom" />
			</TransitionCollection>
		</Frame.ContentTransitions>
	</Frame>

	<ProgressBar x:Name="busyProgressBar" HorizontalAlignment="Stretch" Height="10" Margin="0,5" VerticalAlignment="Top"
				 IsIndeterminate="{Binding IsLoading}" Visibility="{Binding IsLoading, Converter={StaticResource VisibilityConverter}}"/>
</Grid>

It has two frames, one for hosting all pages and a separate one for player. When user plays a video (from featured videos, subject popup, search or download page), the player page is loaded in player frame with a unique sliding entrance animation from bottom of the screen. When the player is closed, user gets back to where he/she was without causing navigation as we just remove player page from player frame . The order in which you put the frames is important.

An additional benefit of this approach is that we can put additional content in HostPage which we need to display regardless of which page user is on. For Khan Academy app, I have added a background image, a global progress indicator and a offline indicator to show status about network activity and availability. I use a separate class to monitor long running operations and network connectivity changes to control visibility for these items.

As the app can be launched due to user starting the app or initiating search, the host page would have to be smart enough to handle that. This is something we do for root frame in default implementation which moves to HostPage. In my case, I created a few properties to handle this which HostPage used for navigation. Here is the code for HostPage.

    public sealed partial class HostPage : Page
    {
        static HostPage _instance;

        public bool LoadNavigationState { get; set; }
        public Type StartupScreen { get; set; }
        public string Parameter { get; set; }
        public static HostPage Current
        {
            get
            {
                return _instance;
            }
        }

        public HostPage()
        {
            _instance = this;

            this.InitializeComponent();
            this.Loaded += OnHostPageLoaded;
            this.Unloaded += HostPageUnloaded;

            this.DataContext = GlobalStatus.Current;
        }

        async void OnHostPageLoaded(object sender, RoutedEventArgs e)
        {
            //Create a Frame to act as the navigation context and associate it with a SuspensionManager key
            SuspensionManager.RegisterFrame(hostFrame, "AppFrame");

            if (LoadNavigationState)
            {
                // Restore the saved session state only when appropriate
                await SuspensionManager.RestoreAsync();
            }

            if (hostFrame.Content == null)
            {
                // When the navigation stack isn't restored navigate to the main page,
                if (!hostFrame.Navigate(StartupScreen, Parameter))
                {
                    throw new Exception("Failed to create initial page");
                }
            }
        }

        #region navigation methods
        public async Task Navigate(Type sourcePageType)
        {
            //navigate on the host frame
        }

        public async Task Navigate(Type sourcePageType, object parameter)
        {
            //navigate on the host frame
        }

        public Page GetCurrentPage()
        {
            return hostFrame.Content as Page;
        }
        #endregion

        #region player methods
        // Launches player for the passed video id with first playlist which contains the video. Used from search suggestion selection in search charm
        public async Task ShowPlayer(string youTubeId)
        {
            //navigate on the player frame
        }

        // Launches player for passed playlist and video ids. Used from search and downloads screens
        public async Task ShowPlayer(string playlistId, string youTubeId)
        {
            //navigate on the host frame
        }
        #endregion
    }

To wire this up, we assign a HostPage instance to current window content instead of a frame which we saw in first code snippet above. Notice that we also handle different scenarios in which app can be activated.

	Type pageType = null; string parameter = null;

	if (_args.Kind == ActivationKind.Launch)
	{
		pageType = typeof(MainPage);
	}
	else if (_args.Kind == ActivationKind.Search)
	{
		pageType = typeof(SearchPage);

		var searchArgs = _args as SearchActivatedEventArgs;

		if (searchArgs != null) parameter = searchArgs.QueryText;
	}

	Window.Current.Content = new HostPage
	{
		LoadNavigationState = _args.PreviousExecutionState == ApplicationExecutionState.Terminated,
		StartupScreen = pageType,
		Parameter = parameter
	};

And that should be it. With this approach, I avoided a number of navigations and customized the ones which I kept to distinguish the app behavior. This approach helped with improving navigation experience and performance in Khan Academy app. Hope this helps you in developing great apps. Chime away in comments if you have any feedback or questions.