Issues in Application Architecture - Retaining Data across Instances

If we want SecondPage to “remember” the last color it was set to, something outside of SecondPage must be responsible for saving that data. That could be MainPage.

Or, SecondPage could save its state in isolated storage. Isolated storage is much like regular disk storage. To access it, you use classes in the System.IO.IsolatedStorage namespace. Every Windows Phone 7 application has access to isolated storage but only to files that the application itself has created. Isolated storage allows an application to save data between multiple executions, and is ideal for saving application settings.

A third solution is provided by a class named PhoneApplicationService, defined in the Microsoft.Phone.Shell namespace. An instance of PhoneApplicationService is created in the standard App.xaml file:

<Application.ApplicationLifetimeObjects>
    <!--Required object that handles lifetime events for the application-->
    <shell:PhoneApplicationService
        Launching="Application_Launching" Closing="Application_Closing"
        Activated="Application_Activated" Deactivated="Application_Deactivated"/>
</Application.ApplicationLifetimeObjects>

					  

Following the PhoneApplicationService tag are four events being associated with handlers. Don’t create a new PhoneApplicationService. You can obtain this existing PhoneApplicationService with the static PhoneApplicationService.Current property.

PhoneApplicationService contains a property named State, which is a dictionary that lets you save and restore data. This State property is of type IDictionary<string, object>. You store objects in this dictionary using text keys. This data is only retained while the application is running, so it’s not suitable for application settings that must be preserved between multiple executions of a program. Data retained by the applicaton only when it’s running is sometimes known as “transient” data.

Any object you store in this State dictionary must be serializable, that is, it must be possible to convert the object into XML, and recreate the object from XML. It must have a public parameterless constructor, and all its public properties must either be serializable or be of types that have Parse methods to convert the strings back to objects.

It’s not always obvious what objects are serializable and which ones are not. When I first started experimenting, I tried to store SolidColorBrush objects in the StateBrush has a property named Transform of type Transform, an abstract class. I had to serialize the Color instead. dictionary. The program raised an exception that said “Type ‘System.Windows.Media.Transform’ cannot be serialized.” It took awhile to remember that

Let’s modify the previous program so that SecondPage uses this State property. In the SilverlightRetainData project, everything is the same except for a usingMicrosoft.Phone.Shell namespace and two overrides in SecondPage. Here they are: directive for the

Example 1. Silverlight Project: SilverlightRetainData File: SecondPage.xaml.cs (excerpt)

protected override void OnNavigatedFrom(NavigationEventArgs args)
{
    if (ContentPanel.Background is SolidColorBrush)
    {
        Color clr = (ContentPanel.Background as SolidColorBrush).Color;

        if (args.Content is MainPage)
            (args.Content as MainPage).ReturnedColor = clr;

        // Save color
        PhoneApplicationService.Current.State["Color"] = clr;
    }

    base.OnNavigatedFrom(args);
}

protected override void OnNavigatedTo(NavigationEventArgs args)
{
    // Retrieve color
    if (PhoneApplicationService.Current.State.ContainsKey("Color"))
    {
        Color clr = (Color)PhoneApplicationService.Current.State["Color"];
        ContentPanel.Background = new SolidColorBrush(clr);
    }

    base.OnNavigatedTo(args);
}				  

During the OnNavigatedFrom call, if there’s a valid Color object available, then it’s saved in the State dictionary with a key of “Color”:

PhoneApplicationService.Current.State["Color"] = clr;

During the OnNavigatedTo override, if the key exists, then the Color value is loaded from the dictionary and SolidColorBrush is made from the Color. The key will not exist if you’ve just started running the program and you’ve navigated to SecondPage for the first time. But on subsequent navigations to SecondPage, the page is restored to the color you last set.

Every time you exit the program by pressing the Back button on the main page, the State dictionary is discarded with the rest of the PhoneApplicationService. This State dictionary is only suitable for saving transient data that a program needs to retain while it’s running. If you need to save data between multiple executions of a program, use isolated storage.

Now try this: Navigate to SecondPage. Touch the screen to change the color. Now press the phone’s hardware Start button. You’ve left the SilverlightRetainData program. From the phone’s start screen, you can navigate to other programs, but eventually you’ll want to press the phone’s Back button to return to the SilverlightRetainData program and SecondPage. The color is still there.

Now go back to MainPage. The color you set in SecondPage is displayed. From MainPage, press the phone’s hardware Start button, leaving the program. Navigate around a bit if you want but eventually start pressing the Back button to come back to SilverlightRetainData and MainPage.