Windows Phone 7.1 - Things to know about Dormant and Tombstone states

In WP 7.1, if we click on the start button when an application is running, the application will have the Deactivated event got fired. Usually developers will save any application state in this event handler into the application level dictionary object represented through the PhoneApplicationService.State property and make use of it in the Activated state again.

After the Deactivated event got invoked, the application which is in running state is moved to the Dormant state. Most developers get this state unnoticed. In this Dormant state, application still remain in memory but without processing happen and without application getting terminated. Based on the memory resource requirements for other applications, the application in the Dormant state has the possibility to get into the Tombstone state. The Tombstone state for an application represents the application as terminated, in the meantime, it holds the state info of the application. By state info we mean, the application state which is represented through the PhoneApplicationService.State property (as specified above) and the page state which is represented through PhoneApplicationPage.State property.

The main thing we need to notice is the reactivation scenario of an application. The application might get  reactivated from both the states directly. In both the cases, it raises the Activated event, where we need to identify whether the application is activated from the Tombstone state. If so, we can get values from the application level state dictionary (PhoneApplicationService.State) and make use of it. If it was from Dormant state, we don't need to do anything as the OS automatically preserves the state.

Since both scenarios raises the Activated event, there needs to be a mechanism to identify whether the immediate previous state is Dormant or Tombstone state. It is the IsApplicationInstancePreserved property of the ActivatedEventArgs which helps us to achieve this. As you might guess, if the IsApplicationInstancePreserved is true then it was from Dormant state and if it is false, then it was form Tombstone state.

While debugging your application you can validate both these Dormant and Tombstone state scenarios by clicking on the start button of your emulator when your application is running . When you click on the start button of the emulator, by default, the application will move to the Dormant state. If you want to validate the Tombstone state, check the "Tombstone upon deactivation while debugging" check box in the Debug tab of the project properties.

Following snippet helps you to understand handling of Dormant and Tombstone states:

Snippet 1 (App.xaml.cs)

private void Application_Launching(object sender, LaunchingEventArgs e)
{
    Debug.WriteLine("Application launching event");
}

private void Application_Activated(object sender, ActivatedEventArgs e)
{
    Debug.WriteLine("Application activated event");
    
    if (e.IsApplicationInstancePreserved)
    {
        Debug.WriteLine("Application activated from Dormant state....");
    }
    else
    {
        Debug.WriteLine("Application activated from Tombstone state.....");
    }
}

private void Application_Deactivated(object sender, DeactivatedEventArgs e)
{
    Debug.WriteLine("Application deactivated event");
}

private void Application_Closing(object sender, ClosingEventArgs e)
{
    Debug.WriteLine("Application closing event");
}