Windows 8 Lifecycle and Data Management

Here is a compact yet deep technical exploration of Windows 8 Lifecycle and Data Management.

Related topics elsewhere on this same website (WilsonMar.com):

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The Windows Phone 8 App LifeCycle

Pop-up this diagram to its own window
Contrast against the Android lifecycle

The graphic and related article for Windows 8 is adapted from Rachel Appel's article on The Windows Store App Lifecycle in the MSDN Magazine Special Edition on Windows 8 and

Windows Phone 7 Basics at Pluralsight

In this graphic, boxes represent states, words in bold are events handled by functions in file App.xaml.cs:

Application_Launching () upon entry, before the app really initiates.
Application_Activated() when app resumes.
Application_Deactivated() function is called to to make the app dormant to make room for another app to be active.
Application_Closing() exiting the app.

DO THIS: Long press on the back button for a list of tombstoned apps to select.

WP8 can hold in memory page navigation history for up to 8 (WP 7 can have up to 5) dormant apps (plus a cache of their state dictionaries).

But such state information can be disgarded if the device runs short of memory. Or in Visual Studio "Tombstone upon deactivation while debugging" is checked yes.

Fast Application Resume (FAR)

Even after an app has been closed, in Windows Phone 8 app can bypass going through Launch() and to straight to Activated(), if ActivationPolicy is (manually) added to the app's AppManifest.xml, as in:

<Tasks>
	<DefaultTask Name="_default" NavigationPage="MainPage.xml" ActivationPolicy="Resume"/>
</Tasks>

Since the backstack (history from the previous instance) remains, clear it using this code:

private void IntializePhoneApplication(){
	...
	RootFrame.Navigated += CheckForResetNavigation;
	...
}
private void CheckForResetNavigation(object sender, NavigationEventArgs e){
	// If the app received a "reset" navigation, check on the next navigation
	// to see if the page stack should be reset:
	if(e.NavigationMode == NavigationMode.Reset)
		RootFrame.Navigated += ClearBackStackAfterReset;
	}
}
private void ClearBackStackAfterReset(object sender, NavigationEventArgs e){
	// Unregister the event so it doesn't get called again:
	RootFrame.Navigated += ClearBackStackAfterReset;
	// Only clear the stack for new (forward) and refresh navigations:
	if(e.NavigationMode != NavigationMode.New && e.NavigationMode != NavigationMode.Refresh )
		Return;
	}
	while( RootFrame.RemoveBackEntry() != null) {} // do nothing.
}

Resume

So app code need to determine if state has been preserved or needs to be reloaded upon resumption:

private void Application_Activated(object sender, ActivatedEventArgs e){
	if(e.IsApplicationInstancePreserved){
		// Dormant - object in memory intact
	}else{
		// Tombstoned - need to reload
	}
}

Preservation and Restoration LifeCycle C# Functions

As with other object-oriented programs, each transition to another state invokes a defined function.

Under #region preservation methods service stores page state the user changes on the screen using function PreserveControlState defined for different control data types. function PreserverFocusState saves the control where user had established focus. The functions there are called from OnNavigatedFrom.

Under #region restoration methods function RestoreControlState is defined for different control data types to re-establish page state on the screen. The funcations there are called from OnNavigatedTo. But they are called only if IsNewPageInstance and IsStatePreserved.

To establish Application state during app start-up...

Among free MS Press books

Activation From Not Running to Running State

An app is activated When the user presses the app's icon from the list of apps.

The entry point for the app is class [YourNameSpace].App. It inherits from the System class.

Silverlight apps handle Startup Event by consuming init args (InitParms properties) such as user name and permissions list.

However, there is currently no way to pass parameters into Windows Phone 7 apps.

On Launch, the Constructor from the app is invoked.

The coding to display the first screen is generated should not be touched:

App.InitializePhoneApplication()
PhoneApplicationService.Launching
App.Application_Launching
App.CompletedInitializePhoneApplication navigated event.

Deactivated to Terminated State

QUESTION: What are the differences in the lifecycle within Windows 8 Store Apps and Windows Phone 8 or Windows Phone 7 apps?

Due to the limited amount of memory on mobile devices, all mobile operating systems (including Android, etc.) can automatically and completely remove running apps from operating memory, terminating their existance without notice.

This is an important departure from Windows 7 and before (which can have programs hang forever).

If Windows Phone 7 detects that not enough memory is available, it will invoke function

PhoneApplicationService.Deactivated

App.Application_Deactivated

So transient data should be stored into a persistent media on the device.

Suspending From Running to Suspended State

When a user clicks the Home button, a phone call is received, the screen is rotated, etc. the operating system suspends the currently running app.

To keep media playing, detect idle condition with XML:

	
{Binding Path="RunUnderLockScreen".

Define a RunUnderLockScreen property.

PhoneApplicationService.Current.Disable Application Idle Detection Mode

App.CompleteInitializePhoneApplication

only have 10 seconds.

So should not save to a web service.

Resume from Suspended to Running State

As with initial activation, the system calls:

App()
App.InitializePhoneApplication()

However during reactivation:

PhoneApplicationService.Activated instead of Launching.
App.Application_Activated

Persistent data should be loaded into a transient state again.

Closing to Running State

when the user clicks a back button.

PhoneApplicationService.Closing

Appl.Application_Closing

Terminating to Terminated State

Tombstoning.

WinJS App Entry Activation Sequence

WARNING: Don't position the cursor inside function definition code or it will stop there only when the if condition returns true.

Example:

    function activated(e) {
        WinJS.UI.processAll().then(function () {
            // Navigate to either the first scenario or to the last running scenario 
            // before suspension or termination
            var url = WinJS.Application.sessionState.lastUrl || scenarios[0].url;
            WinJS.Navigation.navigate(url);
        });
    }

processAll() is not invoked until other activity occurs.

When the app is launched the activated event is raised after the DOMContentLoaded event and before the WinJS.Application.onloaded event. While the app is running, activated events may be raised at any time.

The e in activated(e) is short for an event object. If it returns true, the previous state of your app and to retrieve the activation parameters related to the contract that your app is being activated for. For a full list of the activation contracts and more details on their parameters see the ActivationKind enumeration.

This event can also be raised while your app is running if the system needs to pass the app parameters related to a new activation contract.

IMPORTANT FEATURE: Retrieve as part of activation the url from the sessionState stored before previous suspension or termination.

WinJS Activate addEventListener() to processAll

To recognize events such as when a button is clicked, controls need to be activated by invoking this code:

	document.addEventListener("DOMContentLoaded", function (e){
 	      WinJS.UI.processAll();
	}

Other apps reference an activated() function defined earlier in the source file.

    WinJS.Application.addEventListener("activated", activated, false);
    WinJS.Application.addEventListener("checkpoint", checkpoint, false);

ACRONYMN: PLM (Process Lifecycle Management)

More than just touch screens, Windows 8 brings to the desktop the behavior of Android phones in that it suspends and even terminate apps when it deems fit (to relieve memory pressure).

When an app is launched for the first time, display the app's home page.
If the app is suspended or running in the background, display the app's current page.

REFERENCE: How to activate an app

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