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Handle app prelaunch

  • Artikel

Learn how to handle app prelaunch by overriding the OnLaunched method, and calling CoreApplication.EnablePrelaunch.

Introduction

When available system resources allow, the startup performance of UWP apps on desktop device family devices is improved by proactively launching the user’s most frequently used apps in the background. A prelaunched app is put into the suspended state shortly after it is launched. Then, when the user invokes the app, the app is resumed by bringing it from the suspended state to the running state—which is faster than launching the app cold. The user's experience is that the app simply launched very quickly.

Prior to Windows 10, apps didn't automatically take advantage of prelaunch. In Windows 10, version 1511, all Universal Windows Platform (UWP) apps were candidates for being prelaunched. In Windows 10, version 1607, you must opt in to prelaunch behavior by calling CoreApplication.EnablePrelaunch, and passing true. A good place to put this call is within OnLaunched, near the location that the if (e.PrelaunchActivated == false) check is made.

Whether an app is prelaunched depends on system resources. If the system is experiencing resource pressure, then apps are not prelaunched.

Some types of apps might need to change their startup behavior in order to work well with prelaunch. For example, an app that plays music when it starts up; a game that assumes that the user is present, and displays elaborate visuals when the app starts up; a messaging app that changes the user's online visibility during startup—all of these can identify when the app was prelaunched, and can change their startup behavior as described in the sections below.

The default templates for XAML Projects (C#, VB, C++) accommodate prelaunch.

Prelaunch and the app lifecycle

After an app is prelaunched, it enters the suspended state. (see Handle app suspend).

Detect and handle prelaunch

Apps receive the LaunchActivatedEventArgs.PrelaunchActivated flag during activation. Use this flag to run code that should only run when the user explicitly launches the app, as shown in the following modification to Application.OnLaunched.

protected override void OnLaunched(LaunchActivatedEventArgs e)
{
    // CoreApplication.EnablePrelaunch was introduced in Windows 10 version 1607
    bool canEnablePrelaunch = Windows.Foundation.Metadata.ApiInformation.IsMethodPresent("Windows.ApplicationModel.Core.CoreApplication", "EnablePrelaunch");

    // NOTE: Only enable this code if you are targeting a version of Windows 10 prior to version 1607,
    // and you want to opt out of prelaunch.
    // In Windows 10 version 1511, all UWP apps were candidates for prelaunch.
    // Starting in Windows 10 version 1607, the app must opt in to be prelaunched.
    //if ( !canEnablePrelaunch && e.PrelaunchActivated == true)
    //{
    //    return;
    //}

    Frame rootFrame = Window.Current.Content as Frame;

    // Do not repeat app initialization when the Window already has content,
    // just ensure that the window is active
    if (rootFrame == null)
    {
        // Create a Frame to act as the navigation context and navigate to the first page
        rootFrame = new Frame();

        rootFrame.NavigationFailed += OnNavigationFailed;

        if (e.PreviousExecutionState == ApplicationExecutionState.Terminated)
        {
            //TODO: Load state from previously suspended application
        }

        // Place the frame in the current Window
        Window.Current.Content = rootFrame;
    }

    if (e.PrelaunchActivated == false)
    {
        // On Windows 10 version 1607 or later, this code signals that this app wants to participate in prelaunch
        if (canEnablePrelaunch)
        {
            TryEnablePrelaunch();
        }

        // TODO: This is not a prelaunch activation. Perform operations which
        // assume that the user explicitly launched the app such as updating
        // the online presence of the user on a social network, updating a
        // what's new feed, etc.

        if (rootFrame.Content == null)
        {
            // When the navigation stack isn't restored navigate to the first page,
            // configuring the new page by passing required information as a navigation
            // parameter
            rootFrame.Navigate(typeof(MainPage), e.Arguments);
        }
        // Ensure the current window is active
        Window.Current.Activate();
    }
}

/// <summary>
/// This method should be called only when the caller
/// determines that we're running on a system that
/// supports CoreApplication.EnablePrelaunch.
/// </summary>
private void TryEnablePrelaunch()
{
    Windows.ApplicationModel.Core.CoreApplication.EnablePrelaunch(true);
}

Important

The TryEnablePrelaunch method in the code example above calls CoreApplication.EnablePrelaunch. And TryEnablePrelaunch is itself called only when the app is running on a version of Windows that supports CoreApplication.EnablePrelaunch. In general, if there's any doubt, then you should use a Windows API only after determining that it is supported by the platform that your code is running on. And you can do that by means of the ApiInformation class, as shown in the code example above.

There's also code in the example above that you can uncomment if your app needs to opt out of prelaunch when running on Windows 10, version 1511. In version 1511, all UWP apps were automatically opted in to prelaunch, which might not be appropriate for your app.

Use the VisibilityChanged event

Apps activated by prelaunch are not visible to the user. They become visible when the user switches to them. You may want to delay certain operations until your app's main window becomes visible. For example, if your app displays a list of what's new items from a feed, you could update the list during the VisibilityChanged event rather than use the list that was built when the app was prelaunched because it may become stale by the time the user activates the app. The following code handles the VisibilityChanged event for MainPage:

public sealed partial class MainPage : Page
{
    public MainPage()
    {
        this.InitializeComponent();

        Window.Current.VisibilityChanged += WindowVisibilityChangedEventHandler;
    }

    void WindowVisibilityChangedEventHandler(System.Object sender, Windows.UI.Core.VisibilityChangedEventArgs e)
    {
        // Perform operations that should take place when the application becomes visible rather than
        // when it is prelaunched, such as building a what's new feed
    }
}

DirectX games guidance

DirectX games should generally not enable prelaunch because many DirectX games do their initialization before prelaunch can be detected. Starting with Windows 1607, Anniversary edition, your game will not be prelaunched by default. If you do want your game to take advantage of prelaunch, call CoreApplication.EnablePrelaunch(true).

If your game targets an earlier version of Windows 10, you can handle the prelaunch condition to exit the application:

void ViewProvider::OnActivated(CoreApplicationView const& /* appView */, Windows::ApplicationModel::Activation::IActivatedEventArgs const& args)
{
    if (args.Kind() == Windows::ApplicationModel::Activation::ActivationKind::Launch)
    {
        auto launchArgs{ args.as<Windows::ApplicationModel::Activation::LaunchActivatedEventArgs>()};
        if (launchArgs.PrelaunchActivated())
        {
            // Opt-out of Prelaunch.
            CoreApplication::Exit();
        }
    }
}

void ViewProvider::Initialize(CoreApplicationView const & appView)
{
    appView.Activated({ this, &App::OnActivated });
}

void ViewProvider::OnActivated(CoreApplicationView^ appView,IActivatedEventArgs^ args)
{
    if (args->Kind == ActivationKind::Launch)
    {
        auto launchArgs = static_cast<LaunchActivatedEventArgs^>(args);
        if (launchArgs->PrelaunchActivated)
        {
            // Opt-out of Prelaunch
            CoreApplication::Exit();
            return;
        }
    }
}

General guidance

  • Apps should not perform long running operations during prelaunch because the app will terminate if it can't be suspended quickly.
  • Apps should not initiate audio playback from Application.OnLaunched when the app is prelaunched because the app won't be visible and it won't be apparent why there is audio playing.
  • Apps should not perform any operations during launch which assume that the app is visible to the user, or assume that the app was explicitly launched by the user. Because an app can now be launched in the background without explicit user action, developers should consider the privacy, user experience and performance implications.
    • An example privacy consideration is when a social app should change the user state to online. It should wait until the user switches to the app instead of changing the status when the app is prelaunched.
    • An example user experience consideration is that if you have an app, such as a game, that displays an introductory sequence when it is launched, you might delay the introductory sequence until the user switches to the app.
    • An example performance implication is that you might wait until the user switches to the app to retrieve the current weather information instead of loading it when the app is prelaunched and then need to load it again when the app becomes visible to ensure that the information is current.
  • If your app clears its Live Tile when launched, defer doing this until the visibility changed event.
  • Telemetry for your app should distinguish between normal tile activations and prelaunch activations to make it easier to narrow down the scenario if problems occur.
  • If you have Microsoft Visual Studio 2015 Update 1, and Windows 10, Version 1511, then you can simulate prelaunch for App your app in Visual Studio 2015 by choosing Debug > Other Debug Targets > Debug Windows Universal App PreLaunch.