How to suspend an app (XAML)

發行項
08/31/2015

Learn how to save important application data when the system suspends your app. The example registers an event handler for the Suspending event and saves a string to a file.

Roadmap: How does this topic relate to others? See:

Instructions

Step 1: Register the suspending event handler

Register to handle the Suspending event, which indicates that your app should save its application data before the system suspends it.

using System;
using Windows.ApplicationModel;
using Windows.ApplicationModel.Activation;
using Windows.UI.Xaml;

partial class MainPage
{
   public MainPage()
   {
      InitializeComponent();
      Application.Current.Suspending += new SuspendingEventHandler(App_Suspending);
   }
}

Public NotInheritable Class MainPage

   Public Sub New()
      InitializeComponent() 
      AddHandler Application.Current.Suspending, AddressOf App_Suspending
   End Sub

End Class

using namespace Windows::ApplicationModel;
using namespace Windows::ApplicationModel::Activation;
using namespace Windows::Foundation;
using namespace Windows::UI::Xaml;
using namespace AppName;

MainPage::MainPage()
{
   InitializeComponent();
   Application::Current->Suspending += 
       ref new SuspendingEventHandler(this, &MainPage::App_Suspending);
}

Step 2: Save application data before suspension

When your app handles the Suspending event, it has the opportunity to save its important application data in the handler function. The app should use the LocalSettings storage API to save simple application data synchronously.

partial class MainPage
{
    async void App_Suspending(
        Object sender, 
        Windows.ApplicationModel.SuspendingEventArgs e)
    {
        // TODO: This is the time to save app data in case the process is terminated
    }
}

Public NonInheritable Class MainPage

    Private Sub App_Suspending(
        sender As Object, 
        e As Windows.ApplicationModel.SuspendingEventArgs) Handles OnSuspendEvent.Suspending

        ' TODO: This is the time to save app data in case the process is terminated
    End Sub

End Class

void MainPage::App_Suspending(Object^ sender, SuspendingEventArgs^ e)
{
    // TODO: This is the time to save app data in case the process is terminated
}

Step 3: Release exclusive resources and file handles

When your app handles the Suspending event, it also has the opportunity to release exclusive resources and file handles. Examples of exclusive resources are cameras, I/O devices, external devices, and network resources. Explicitly releasing exclusive resources and file handles helps to ensure that other apps can access them while your app isn't using them. When the app is activated after termination, it should open its exclusive resources and file handles.

Remarks

The system suspends your app whenever the user switches to another app or to the desktop or Start screen. The system resumes your app whenever the user switches back to it. When the system resumes your app, the content of your variables and data structures is the same as it was before the system suspended the app. The system restores the app exactly where it left off, so that it appears to the user as if it's been running in the background.

The system attempts to keep your app and its data in memory while it's suspended. However, if the system does not have the resources to keep your app in memory, the system will terminate your app. When the user switches back to a suspended app that has been terminated, the system sends an Activated event and should restore its application data in its OnLaunched method.

The system doesn't notify an app when it's terminated, so your app must save its application data and release exclusive resources and file handles when it's suspended, and restore them when the app is activated after termination.

Note If you need to do asynchronous work when your app is being suspended you will need to defer completion of suspend until after your work completes. You can use the GetDeferral method on the SuspendingOperation object (available via the event args) to delay completion of suspend until after you call the Complete method on the returned SuspendingDeferral object.

Note To improve system responsiveness in Windows 8.1, apps are given low priority access to resources after they are suspended. To support this new priority, the suspend operation timeout is extended so that the app has the equivalent of the 5-second timeout for normal priority on Windows or between 1 and 10 seconds on Windows Phone. You cannot extend or alter this timeout window.

A note about debugging using Visual Studio: Visual Studio prevents Windows from suspending an app that is attached to the debugger. This is to allow the user to view the Visual Studio debug UI while the app is running. When you're debugging an app, you can send it a suspend event using Visual Studio. Make sure the Debug Location toolbar is being shown, then click the Suspend icon.

Tasks

How to activate an app

How to resume an app

Conceptual

Application lifecycle

Guidelines

Guidelines for app suspend and resume

Reference

Windows.UI.Xaml.Application