/// <summary>
/// Creates a new NavigationService for a Frame, registering the frame and NavigationService
/// for future retrieval and registering the frame with the NavigationContextService.
/// </summary>
/// <param name="frame">The frame to register.</param>
/// <param name="defaultPage">The default page to navigate to if there is no state to restore.</param>
/// <param name="defaultNavigationContext">The navigation context to provide on navigation to defaultPage.</param>
/// <param name="restoreState">True to attempt to restore previously saved state, false to just navigate to defaultPage
/// without attempting to restore state.</param>
/// <returns>The NavigationService for the registered frame.</returns>
public static INavigationService RegisterFrame(Frame frame, Type defaultPage, NavigationContextBase defaultNavigationContext = null, bool restoreState = true)
{
if (frame == null)
{
throw new ArgumentNullException(nameof(frame));
}
if (defaultPage == null)
{
throw new ArgumentNullException(nameof(defaultPage));
}
if (GetNavigationService(frame) != null)
{
throw new InvalidOperationException("Frame already associated with a NavigationService.");
}
// Clean up references before adding a new one.
RemoveFrame();
NavigationService navigationService = new NavigationService(frame, new NavigationContextService());
// Ensure navigationService is fully registered before restoring state
// or navigating so that it can be accessed by PageBase.
NavigationServices.Add(navigationService);
if (restoreState)
{
// This triggers NavigatedTo on PageBase if there is state restored.
navigationService.RestoreState();
}
// If state wasn't restored, navigate to a default page to populate the Frame.
if (navigationService.RootFrame.Content == null)
{
navigationService.Navigate(defaultPage, defaultNavigationContext);
}
return navigationService;
}
/// <summary>
/// Called when the page is navigated to. Initializes properties, such as
/// the navigation service and view model, as well as restores context
/// from the suspension manager.
/// </summary>
/// <param name="e">The event args.</param>
protected override async void OnNavigatedTo(NavigationEventArgs e)
{
base.OnNavigatedTo(e);
if (e == null)
{
return;
}
// Get NavigationService
if (this.navigationService == null)
{
this.navigationService = NavigationService.GetNavigationService(this.Frame) as NavigationService;
if (this.navigationService == null)
{
throw new InvalidOperationException("Frame not registered before page navigation.");
}
this.contextService = this.navigationService.ContextService;
Debug.Assert(this.contextService != null, "A navigation service's context service should never be null.");
}
// Set ViewModel
if (this.ViewModel == null)
{
INavigatableViewModel viewModel = this.DataContext as INavigatableViewModel;
if (viewModel == null)
{
throw new InvalidOperationException("ViewModel must implement INavigatableViewModel interface.");
}
this.ViewModel = viewModel;
}
// Load page state if this page wasn't already cached by the frame
IDictionary<string, PageState> pageStates = this.navigationService.PageStates;
this.pageKey = "Page-" + this.Frame.BackStackDepth;
// Get the NavigationContext
NavigationContextBase context = this.GetContextFromParameter(e.Parameter);
PageState pageState = null;
if (e.NavigationMode == NavigationMode.New)
{
// Clear existing state for forward navigation when adding a new page to the
// navigation stack
var nextPageKey = this.pageKey;
int nextPageIndex = this.Frame.BackStackDepth;
while (pageStates.Remove(nextPageKey))
{
nextPageIndex++;
nextPageKey = "Page-" + nextPageIndex;
}
// TODO: Also clean up stored navigation contexts when removing forward stack.
}
else
{
// Load page state using the same strategy for loading suspended state and
// recreating pages discarded from cache
pageStates.TryGetValue(this.pageKey, out pageState);
}
// Activate the ViewModel
await this.ViewModel.Activate(this.navigationService, context, pageState);
}