Category Archives: 46582643-e8d0-43cd-bd72-526409e4f111

WP7 Tombstoning Pattern Tip

Customizing App.xaml.cs

If you are familiar with tombstoning on Windows Phone, you know there are four events in the Windows Phone application lifecycle that can be handled: PhoneApplicationService.Launching, PhoneApplicationService.Closing, PhoneApplicationService.Activated and PhoneApplicationService.Deactivated.  The first two are for normal app startup and shutdown, while the latter two are for tombstoning scenarios.

Along with these events are two different property bags that application data can be saved to. 

IsolatedStorageSettings.ApplicationSettings  is typically used for backing up persistent data that you want available every time the application starts up. 

PhoneApplicationService.Current.State is used for transient data that you only want persisted if your Silverlight application gets tombstoned.  It is the equivalent of session data in an ASP.NET web application.

In the Launching and Closing event handlers you would typically save to Isolated Storage, while in handlers for Activated and Deactivated you would want to use Current.State.  The former coding artifacts are for persistent data while the latter are for transient data.

These categories are imperfect, however.  It is possible to go through the Deactivated event but never have Activated called.  This occurs if for memory management reasons your application is permanently terminated (a permanent death scenario rather than the temporary death that tombstoning is supposed to trigger).  This can also happen, of course, if the user for whatever reason just decides not to return to your app after it gets tombstoned.

Another way to look at this is that your application is permanently terminated without ever invoking the PhoneApplication.Closing event.

In order to handle this common scenario, you must save any persistent data in the Deactivated event as well as the Closing event.

A simple pattern for structuring your code to cover all these possibilities is to make sure the code fragment for saving persistent data is called in both the Dectivated and Closing event handlers.  Similarly, the fragment for restoring persistent data should be called in both the Activated and Launching handlers.  We do this by taking care of persistent data backup and restores in two new methods: BackupPersistentData and RestorePersistentData. Those who remember the IDisposible pattern in C# will recognize some of the same coding idioms being used here.

The following code goes in App.xaml.cs:

private void BackupPersistentData()
{
    var store = IsolatedStorageSettings.ApplicationSettings;
    //save persistent data
    store.Save();
}
private void RestorePersistentData()
{
    var store = IsolatedStorageSettings.ApplicationSettings;
    //restore persistent data
}

private void Application_Closing(object sender
    , ClosingEventArgs e)
{
    BackupPersistentData();
}

private void Application_Launching(object sender
    , LaunchingEventArgs e)
{
    RestorePersistentData();
}

private void Application_Deactivated(object sender
    , DeactivatedEventArgs e)
{
    BackupPersistentData();
    var store = PhoneApplicationService.Current.State;
    //save transient data
}

private void Application_Activated(object sender
    , ActivatedEventArgs e)
{
    RestorePersistentData();
    var store = PhoneApplicationService.Current.State;
    //restore transient data
}

Sticking to this pattern has helped me to avoid a lot of unforeseen mistakes in my own phone apps.

 



Getting the current theme in WP7 for Silverlight

Customizing App.xaml.cs

Windows Phone devices allow users two theme options, either dark or light.

In the WP7 beta dev tools, however, there no straight-forward way for Windows Phone Silverlight developers to find out what the current theme is, though there is a hack.  The developer must interrogate the app resources to find the color of the PhoneBackGroundColor.  Under the dark theme it is black, while under the light theme it is white.

The following code can be thrown into App.xaml.cs to add a little elegance to this hack. 

First, a brief enum is needed.  It can be added in App.xaml.cs above the App class, or simply placed in its own cs file in the project:

public enum Theme{Light,Dark}

A CurrentTheme property is added to the App class:

private static Theme _currentTheme;

public static Theme CurrentTheme
{
    get { return _currentTheme; }
    private set { _currentTheme = value; }
}

And then we assign a value to CurrentTheme in the App class constructor by interrogating Resources:

public App()
{
    var bgc = Resources["PhoneBackgroundColor"].ToString();
    if (bgc == "#FF000000")
        CurrentTheme = Theme.Dark;
    else
        CurrentTheme = Theme.Light;

    //etc.

The CurrentTheme is now retrievable from anywhere in the application like so:

    switch(App.CurrentTheme)
    {
        case Theme.Dark:
            MessageBox.Show("Dark");
            break;
        case Theme.Light:
            MessageBox.Show("Light");
            break;
    }

Moreover, if a user presses the Windows button at lower center on a device and resets the theme, the new theme will be assigned to CurrentTheme when the application returns from tombstoning.

There is a rumor that in the RTM of the WP7 dev tools, two new properties will be available for picking up the current theme: DarkThemeVisibility and LightThemeVisibility.

For ease of migration to the RTM, you can add the following two properties to App.xaml.cs for now as placeholders – you should be able to simply switch them out later should the new methods become available:

public static bool DarkThemeVisibility
{
    get { return CurrentTheme == Theme.Dark; }
}

public static bool LightThemeVisibility
{
    get { return CurrentTheme == Theme.Light; }
}

How to Quit a WP7 Silverlight Application

Customizing App.xaml.cs

In the current Beta dev tools for Windows Phone 7, there is no way to exit a Silverlight application (though there is a method to do this for XNA apps).

Currently the best way – and I use the term “best” loosely – to do this is to throw an exception.  An unhandled exception always manages to kill your application.

If we must use a hack, however, we might as well  do it with a bit of finesse.  Below is a simple implementation that can be placed in the App class of a WP7 Silverlight project that clarifies the behavior of the app when we intentionally throw an exception in order to exit rather than simply throwing an exception exceptionally.

This code can be called from anywhere in your app in order to Exit predictably.

Here is the call:
App.Quit();
And here is the implementation in App.xaml.cs:
private class QuitException : Exception { }

public static void Quit()
{
    throw new QuitException();
}

private void Application_UnhandledException(object sender, 
    ApplicationUnhandledExceptionEventArgs e)
{
    if (e.ExceptionObject is QuitException)
        return;

    if (System.Diagnostics.Debugger.IsAttached)
    {
        System.Diagnostics.Debugger.Break();
    }

    //etc.
}
Windows Phone 8 Update

If you are doing Windows Phone 8 development, things get easier.  After having so many apps going through the store using the above method to exit out, the WP Dev\Design team decided to just implement their own escape method.  Sometimes the doctor is right and sometimes the patient just has a better understanding of what the pain feels like.  To exit out of a Windows Phone 8 app, call Application.Terminate .