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The Application Execution Model in Windows Phone 7.5

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This chapter provides an overview of the application execution model in Windows Phone 7.5 and examines the various application life cycle events, which are used to coordinate state persistence and restoration. It also delves into the sample application and discusses image caching, design-time data, and consuming a simple WCF service.
This chapter is from the book

This chapter is from the book

Understanding the events within the life cycle of a Windows Phone application is critical to providing an optimal user experience on the phone. The phone’s single application process model means that your app may be interrupted and terminated at any time. It is your responsibility to maintain the appearance of continuity across interruptions, to save your app’s state whenever an interruption occurs, and, if necessary, restore the state when the user returns to your app.

While the 7.5 release of Windows Phone OS includes support for fast application switching, where your app is kept in memory and its threads suspended, you still need to preserve the state of your app because there are no guarantees that an app will not be terminated if the device memory runs low.

Like localizability, app state preservation is an aspect of development that should not be deferred and is likely one of the biggest challenges you will face as a Windows Phone developer.

Traditionally, developers of Windows desktop applications have not been overly concerned with persisting runtime state. There is usually no need to maintain state for a desktop application, since the application remains in execution and resident in memory while it is open. This is in stark contrast to Windows Phone apps, where an application may be stopped and started many times during a single session.

Seasoned ASP.NET developers who recall the days before AJAX may feel slightly more at home developing for Windows Phone, with ASP.NET’s reliance on view state to overcome the transient nature of page state in which the lifespan of a web page is limited to the period before a postback occurs. This is not too dissimilar to the state model of the phone, although Silverlight has nothing like the view state system built into ASP.NET. For that you need to roll your own, and you see how to build an automated state preservation system in Chapter 25, “Isolated Storage and State Preservation.”

There is no doubt that the single application process model of the phone presents some challenge for developers, but it may also lead to better designed and more robust applications, with an emphasis on decoupling visual elements from their state so that it can be more readily preserved.

This chapter begins with an overview of the application execution model and examines the various application life cycle events, which are used to coordinate state persistence and restoration.

You see how to enable an app to run under the lock screen. You also look at page navigation and how to optimize the user experience by using a splash screen or a loading indicator.

Finally, the chapter delves into the sample application and looks at image caching, design-time data, and consuming a simple WCF service.

Exploring the Execution Model

The execution model of Windows Phone is designed to make the phone as responsive as possible and to maximize the battery life of the device. One way that this is achieved is by limiting the phone to a single running application. Multiple applications running in the background risk slowing the foreground application and may tie up the processor and cause the phone to consume more power.

In addition to greater responsiveness and extended battery life, the execution model provides users with a consistent navigation experience between applications. On Windows Phone, users are able to launch applications from the App List screen or from a tile on the Start Experience. The hardware Back button allows users to navigate backward, through the pages of a running application or through the stack of previously running applications.

The goal of transient state preservation and restoration is to provide the user with a simulated multiple application experience, where it seems to the user that your application was left running in the background, even though it may have been terminated by the operating system.

Application State

There are two types of application state: persistent and transient. Persistent state exists when an application launches. It is saved to a private storage area called isolated storage and may include data such as configurable settings or files.

Transient state is discarded when an application is closed. It is stored at the application level in the Microsoft.Phone.Shell.PhoneApplicationService.State dictionary or at the page level in the PhoneApplicationPage.State dictionary.

There is a single PhoneApplicationService instance for the entire app, and its state dictionary should be used only by objects running in the context of the application as a whole. A unique state dictionary is created for each page in your app, and you should use it rather than the PhoneApplicationService.State dictionary whenever possible.

Transient state may include results from web service calls, or data from partially completed forms (see Figure 3.1).

Figure 3.1

Figure 3.1. Persistent state and transient state storage locations

Life Cycle Events

The Microsoft.Phone.Shell.PhoneApplicationService exposes four life cycle related CLR events, which provide an application with the opportunity to save or load state (see Figure 3.2).

Figure 3.2

Figure 3.2. Application life cycle

Launching Event

When a user selects an application from the App List screen, or from a tile on the Start Experience, or when the application is being debugged, the application moves from the stopped state, to the running state. This represents a cold start. Use the Launching event to restore any persistent state from isolated storage that is not page specific. This event occurs after the App class is instantiated, but before the main page of an application is created.

The Launching and Activated events are mutually exclusive. That is, exactly one of these two events occurs when the application is being started. Likewise, the Deactivated and Closing events are also mutually exclusive; only one of these events occurs when the application is exiting.

Subscribing to Life Cycle Events Using the PhoneApplicationService

The PhoneApplicationService allows your app to be notified of the various life cycle events. The PhoneApplicationService is, by default, initialized in XAML by the application’s App instance, as shown in the following example:

<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>

The PhoneApplicationService is a singleton class, which exposes an instance of the class via a static property called Current. Code to handle the Launching, Closing, Activated, and Deactivated events can be placed in the App class, for which handlers are created when a new project is created, or directly in code by using the Current property of the PhoneApplicationService.

There may be times when it is tempting to promote certain kinds of transient state to persistent state. When launching your app, however, the user should feel like he is not resuming your app, but rather that it is indeed a new instance, a clean slate.

Persistent state is stored in isolated storage, while transient state is stored in the PhoneApplicationService.State dictionary, an IDictionary<string, object> that is maintained while the application is tombstoned, but abandoned when the application moves from the tombstoned state to the not running state.

Deactivation Event and Tombstoning

On entering the running state, an application must contend with being interrupted. Each interruption causes the PhoneApplicationService.Deactivated event to be raised. The app is then placed in a dormant state, where it remains in memory but its threads are suspended.

If an app is reactivated after being in the dormant state, there is no need for your app to restore its transient state, reducing its load time.

Detecting whether an app is returning from a dormant state can be done within the PhoneApplicationService.Activated event handler using the IsApplicationInstancePreserved property of the ActivatedEventArgs, as shown in the following excerpt:

void Application_Activated(object sender, ActivatedEventArgs e)

{
    if (e.IsApplicationInstancePreserved) 
    {
       /* Application was placed in the dormant state. */
    }
    else
    {
       /* Application state should be restored manually. */
    }
}

When the device’s memory usage reaches a minimum threshold, the operating system may decide to tombstone your app.

When tombstoned, the operating system is aware that the application may be reactivated. If an application moves from being tombstoned back to the running state, it will be from a cold start, and all objects must be instantiated and persistent and transient state must be restored. The only differences between the tombstoned state and the closed state are that when tombstoned, the operating system retains the transient state dictionary for the app along with an identifier for the app, so that if activated, chooser and launcher events can be resubscribed. Launchers and choosers perform common tasks, such as sending email. You learn more about choosers and launchers in Chapter 12, “Launchers and Choosers.”

An application is deactivated when it is no longer the foreground application. The following is a list of causes for deactivation:

  • The user presses the start button.
  • The phone’s lock screen is engaged without having enabled running under the lock screen. Enabling your app to run under the lock screen is discussed in the section “Running Under the Lock Screen” later in the chapter.
  • A launcher or a chooser is shown.
  • The user selects a toast notification, which launches another application.

Saving Transient State

The Deactivated event provides an application with the opportunity to save its transient and persistent state.

The goal is to enable restoration of the application to its prior state before being tombstoned. It should be assumed, however, that when the Deactivated event occurs, the application is going to be closed, moving to the closed state. The user may, after all, opt not to resume the application, or may use the Start Experience to relaunch the application, rather than using the hardware Back button to return to the application. Moreover, if the user launches many other apps, your app may get bumped off the end of the Back button application stack.

The Visual Studio new project templates place an empty handler for the Deactivated event in the App class. See the following excerpt:

void Application_Deactivated(object sender, DeactivatedEventArgs e)

{
   /* Save transient state like so:
    * PhoneApplicationService.Current.State["DataContractKey"]
    *         = DataContract;
    */
 
   /* Save persistent state like so:
    * IsolatedStorageSettings.ApplicationSettings["Key"] = someObject; */
}

You can also subscribe to the Deactivated event elsewhere, in the following manner:

PhoneApplicationService.Current.Deactivated += OnDeactivated;


void OnDeactivated(object o, DeactivatedEventArgs args)
{
...
}

Transient State Requirements

All objects to be stored in the PhoneApplicationService.State property must meet one of the following requirements:

  • It is a primitive type.
  • It is a known serializable reference type including decimal, string, or DateTime, with a matching System.Convert.ToString method signature.
  • It is capable of being serialized using a DataContractSerializer. To achieve this, it must be decorated with a System.Runtime.Serialization.DataContract attribute. Each property or field intended for serialization must be decorated with the DataMember attribute, and in turn each serializable property or field type must be decorated with the DataContract attribute.

Storing an application’s transient state can be difficult because objects containing state often have event subscriptions to or from other objects that are not serialized. Also, types from third-party libraries are usually not decorated with the DataContract attribute, preventing serialization.

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-like environment, so that to the user 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;
}
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