LoadState中的Frame.Navigate

Question

I'm having trouble trying to navigate automatically between pages in my Windows 8.1 app based on a little check. It just doesn't want to navigate to another page when doing this in LoadState , as if something isn't loaded yet, but it doesn't give an error either. When I insert a delay using (for example) await Task.Delay(2000) before doing Frame.Navigate , then my app will redirect without any problem.

protected async override void LoadState(Object navigationParameter, Dictionary<String, Object> pageState)
    {
        MyData oData = await getData();

        if (oData != null)
        {
             this.Frame.Navigate(typeof(newPage), oData);
        }
        else
        {
             // do something else
        }

    }

Do I have to put this code in another load- or navigated-event? Or how can I make this work?

Answer 1

In LoadState and SaveState you should only save and restore the page state (called when suspending and reactivating the app). Do nothing else (like navigating).

Put your logic into the OnNavigatedTo method instead...

Answer 2

If you want to navigate from method that called when page is loads, you should place your navigation code to OnNavigatedTo(...). But do not forget to wrap your code in Dispatcher.RunAsync(...) - Frame navigation in xaml return false

Answer 3

I tried calling Frame.Navigate(...) from the OnNavigatedTo method but still the navigation didn't occur.

There are other answers which say use Dispatcher.RunAsync , but that feels like it's making assumptions about the threading model of Windows Phone.

Here's what I do: attach a handler to the Loaded event of the page instead, and put my "redirect" logic in there. Loaded fires after OnNavigateTo and after NavigationHelper_LoadState, but before the page has become visible.

    public LaunchPadPage() {
        this.InitializeComponent();

        this.navigationHelper = new NavigationHelper(this);
        this.navigationHelper.LoadState += this.NavigationHelper_LoadState;
        this.navigationHelper.SaveState += this.NavigationHelper_SaveState;

        this.Loaded += LaunchPadPage_Loaded;

        this.app = (App)App.Current;
    }

    private void NavigationHelper_LoadState(object sender, LoadStateEventArgs e) {
        // Let's show the root zone items
        // NB: In case we don't have this data yet, do nothing
        if (app.Hierarchy != null)
        {
            DefaultViewModel["Items"] = app.Hierarchy.RootItems;
        }
    }

    private void LaunchPadPage_Loaded(object sender, RoutedEventArgs e) {
        // No data? Go to the downloads page instead.
        if (app.Hierarchy == null)
        {
            Frame.Navigate(typeof(DownloadingPage));
        }
    }