Windows Phone 8 : Page Navigation - Page Redirection, Hardware Back Button

1. Page Redirection

The OnNavigatingFrom method allows you to intercept a navigation event and to even cancel the navigation if needed. Additionally, there may be times when you want to redirect the user to a different URI based on some conditional logic.

The NavigationService, however, does not support overlapping navigation. That is, you are unable to cancel an existing navigation and immediately commence another.

You can, however, cancel navigation and schedule navigation to a different Uri using the page’s Dispatcher property, as shown in the following excerpt:

protected override void OnNavigatingFrom(NavigatingCancelEventArgs e)
{
    if (e.Uri.ToString().Contains("RequestedUrl"))
    {
        e.Cancel = true;
        /* Perform the redirect on the UI thread. */
        Dispatcher.BeginInvoke(() => NavigationService.Navigate(
            new Uri("RedirectUrl", UriKind.Relative)));
    }

    base.OnNavigatingFrom(e);
}

By using the Dispatcher to invoke the lambda expression, which performs the call to the NavigationService, you allow the current navigation to complete first. This works because the UI thread can be thought of as a queue of prioritized delegates, all waiting in turn to be executed. After all the Navigating event handlers have been serviced, the delegate represented by the lambda expression will be taken of the queue and performed by the UI thread. This technique, of using the Dispatcher to enqueue an action, is also useful when working with some UI controls, whose event handlers may be called before the control is finished reacting to a user action.

2. Hardware Back Button

The hardware Back button is analogous to the Back button on a web browser. However, when the user presses the Back button, past the first page of a phone app, the app is closed. This is in contrast to the phone’s hardware start button, which merely causes an app to be deactivated.

To determine whether navigation is occurring because the hardware Back button was pressed or the navigation was initiated by a call to NavigationService.GoBack, use the NavigatingEventArgs.NavigationMode property as shown:

protected override void OnNavigatedFrom(NavigationEventArgs e)
{
    base.OnNavigatedFrom(e);

    if (e.NavigationMode == NavigationMode.Back)
    {
        // Back button pressed.
    }
}

The back key button can also be canceled by overriding the PhoneApplicationPage.OnBackKeyPress, as shown in the following excerpt:

protected override void OnBackKeyPress(CancelEventArgs e)
{
    base.OnBackKeyPress(e);

    e.Cancel = true;
}

OnBackKeyPress is called before OnNavigatedFrom, and if the Back button is canceled, OnNavigatedFrom is not called at all.

Note

The Windows Phone Marketplace certification requirements forbid canceling the Back button in most cases. To maintain a consistent user experience, the Back button must only be used for backward navigation in the application. The following four certification requirements relate to use of the Back button:

- Pressing the Back button must return the application to the previous page or return to any previous page within the back stack.

- Pressing the Back button from the first screen of an application must close the application.

- If the current page displays a context menu or a dialog, the pressing of the Back button must close the menu or dialog and return the user to the screen where the context menu or dialog box was opened.

- For games, when the Back button is pressed during gameplay, the game can choose to present a pause context menu or dialog or navigate the user to the prior menu screen. Pressing the Back button again while in a paused context menu or dialog closes the menu or dialog.