Passing Page Arguments Using Query Strings

Query strings allow for key value pairs to be embedded in a URL and passed to a page. Just like HTML web applications, Windows Phone apps use query string parameters for inter-page communication.

The PhoneApplicationPage.NavigationContext property, which is initialized after the page is created, is used to retrieve the query string. Its QueryString property is an IDictionary of string key and value pairs. The following excerpt from the WebBrowserView.xaml.cs, in the downloadable sample code, demonstrates how to retrieve a query string value:

Click here to view code image

protected overide void OnNavigatedTo(NavigationEventArgs e)
{
    base.OnNavigatedTo(e);

    string url;
    if (NavigationContext.QueryString.TryGetValue("url", out url)
        && url != null && url.ToLower() != "webbrowserview.xaml")
    {
        ViewModel.LoadPage(url);
    }
}

FIGURE 3.6 Pages are placed on the history stack.

While on the history stack, the page remains in memory unless the application is tombstoned or closed. This means that subscribing to the PhoneApplicationService.Deactivated event provides the page with the opportunity to save its transient state. It is, however, preferable to use the page’s OnNavigatedFrom method for the saving of page state.

Using the Deactivate event to save page state runs the risk of slowing down deactivation when all pages on the history stack are saving their state simultaneously.

Restoration of transient state should occur with the page’s OnNavigatedTo method. The OnNavigatedTo method is called when the PhoneApplicationFrame navigates to the page. This is triggered by the following actions:

Navigation to a specified page URI occurs using one of various navigation methods such as the PhoneApplicationFrame.Navigate method.

The NavigationService.GoBack method is called.

The user presses the hardware Back button.

The page is the current page when the app moves from the tombstoned or dormant state to the running state.

The page is the app’s start page and the app is launched.