Wednesday, May 11, 2011

WP7 Tombstoning

In the current version of the Windows Phone only one application a time is allowed to run inside the sandbox. While some applications have the ability to run in the background, they are specialized applications that do not run in 'the sandbox'. Our own applications have to live without background processing. 

So how do you know if your application is running or not? The answer is simple. For your application to be running, it has to be focused to the screen. In other words, if you can't see your application, it isn't running.

Switching away from your application can happen on a number of different cases including:

  • User Presses Back from the first page of your application.
  • User Presses the Home or Search button.
  • User responds to a 'toast' notification.
  • Interruption activity occurs (e.g. Phone call).

The fact that your application stops running is called 'tombstoning'. When your application is going to be tombstoned it enters a paused state where your application will be given an opportunity to temporary state. This saving of data is not to isolated storage but an in-memory store so that when your application is suspended it can be unloaded if necessary by the phone. If the user returns to your application (e.g. from the back key), your application is resumed which gives you the opportunity to take that temporary state and return your application to the same place it was when it was paused. The user should not notice this change. The whole idea here is to give you an opportunity to give the user a seamless transition from paused and resume without them knowing that the application was killed.

However this temporarily saving of the application state is not done automatically and should be implemented yourself. One library that simplifies this process is the WP7 TombStone Helper. This library adds extension methods to PhoneApplicationPage so you don't have to worry about maintaining the state of a page in your app if it gets tombstoned.

With just two lines of code the contents, checked state and scroll positions of TextBoxes, PasswordBoxes, CheckBoxes, RadioButtons, Sliders, ListBoxes and ScrollViewers will be preserved.

protected override void OnNavigatedFrom(System.Windows.Navigation.NavigationEventArgs e)
{
base.OnNavigatedFrom(e);

this.SaveState(); 
}

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
base.OnNavigatedTo(e);

this.RestoreState(); 
}

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