Windows Phone 8 : Exploring the Execution Model (part 4) - Restoring Transient State, Saving Persistent State

Restoring Transient State

When an app transitions from being tombstoned or dormant back to the running state, the PhoneApplicationService.Activated event is raised. This provides an opportunity to restore the transient and persistent state of the app.

Restoring the transient state involves taking the user to the point where she was when the Deactivated event occurred, and may involve restoring the positions of UI elements, repopulating viewmodel properties, and so on. The goal is to provide the user with a seamless experience, and to emulate a multiple application environment, in which the application appears as though it was left running in the background.

The following code demonstrates handling of the PhoneApplicationService.Activated event, to restore transient and persistent state:

void Application_Activated(object sender, ActivatedEventArgs e)
{
    /* Restore persistent state like so: */
    someObject = IsolatedStorageSettings.ApplicationSettings["Key"];

    /* Restore transient state like so: */
    DataContract = PhoneApplicationService.Current.State["DataContractKey"];
}

Saving Persistent State

Persistent state is usually stored whenever transient state is stored. In addition, your app should save its persistent state when it is closing, by subscription to the PhoneApplicationService.Closing event. Persistent state may include files or application settings, as shown in the following excerpt from the App class:

void Application_Closing(object sender, ClosingEventArgs e)
{
    System.IO.IsolatedStorage
        .IsolatedStorageSettings.ApplicationSettings["someObject Key"]
             = someObject;
}