Navigation and Passing Data Between WP7 Pages

Unlike other popular mobile platforms, Windows Phone
7 (WP7) uses a different scheme for navigation between screens.
The first thing to point out is that WP7 screens are actually called
Pages. Think of WP7 Pages as HTML pages within a web browser. While this may
seem like a stretch, in actuality it isn’t too far off. Similar to a web
browser viewing a web site, you have a default page and you click on links to
navigate to other pages, and you can back up to a previous page by clicking on
the back button.

Since all WP7 devices come with a back button, the WP7 operating system
needs a hierarchy of navigation to allow the user to go back to a previous
page. HTML pages within a web browser support a back button because of the
history created by the user following links and through redirects. Thus if the
user clicks the back button the web browser returns to the previous page in
history. While WP7 does not have the equivalent of a link, it does, however,
have the equivalent of a redirect provided by the NavigationService.Navigate
method. This method provides another similarity to a redirect in that it also
uses an equivalent URL. Known as a URI (Universal Resource Identifier) it
serves the same purpose of a path to the Page as well as a Query String. In
fact the form is the same as you would expect in a traditional web site.

Next, we can dig into an example for using navigation within WP7 such as the
list and detail screen. For instance, when we have a ListBox control on a Page
and would like to send the user to a details page. The following code snippet
shows the usage of the Navigate method to direct the user to the DetailsPage
with a single query string parameter called selectedItem.

NavigationService.Navigate(new Uri("/DetailsPage.xaml?selectedItem=" + MainListBox.SelectedIndex, UriKind.Relative));

Thus when this method is called the user is sent directly to the
DetailsPage. When the DetailsPage is displayed, you will then need to extract
the Query String parameter to direct the user to the correct record. The
following snippet shows the OnNavigateTo method, which is called when you use the
NavigationService to navigate to a Page.

protected override void OnNavigatedTo(NavigationEventArgs e)
{
string selectedIndex = "";

if (NavigationContext.QueryString.TryGetValue("selectedItem", out selectedIndex))
{
int index = int.Parse(selectedIndex);

//Display the data for the selected index ...
}
}

As you can see, it is quite easy to pull parameters out from the Query
String using the TryGetValue method from the NavigationContext.QueryString
class.

Conclusion

With the above code snippets it should be easy to see the similarities between
HTML Pages with regard to navigation and passing data between Pages. If you are
familiar with creating sites using ASP.Net you
should feel right at home with regard to navigation. While the above snippets
were pulled from the Windows Phone DataBound Application template to support
the List/Detail pages, you should be able to see how you can expand these
snippets to support the navigation needs of your application.

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