Creating a device background task in Windows 8.1 (UWP device apps)

Article
03/17/2023

In Windows 8.1, your UWP app can synchronize data on your peripheral device. If your app is associated with device metadata, that UWP device app can also perform device updates, such as firmware updates. This topic describes how to create a device background task that uses the DeviceUseTrigger or DeviceServicingTrigger. Device background agents that use these triggers are subject to policies that ensure user consent and help preserve battery life while devices are being synced and updated. For more info about device background tasks, see Device sync and update for UWP device apps.

Note

This topic corresponds to the Custom USB device sample. The Custom USB device sample demonstrates a background task that performs device sync with the DeviceUseTrigger.

Although the device background task in the Custom USB device sample features a DeviceUseTrigger, everything discussed in this topic can also be applied to device background tasks that use DeviceServicingTrigger. The only difference between using the two triggers are the policy checks made by Windows.

The app manifest

To use a device background task, your app must declare it in the app manifest file of your foreground app, like is done for system-triggered background tasks. For more info, see Device sync and update for UWP device apps.

In this example from an app package manifest file, DeviceLibrary.SyncContent is an entry points from the foreground app. DeviceLibrary.SyncContent is the entry point for the background task that uses the DeviceUseTrigger.

<Extensions>
  <Extension Category="windows.backgroundTasks" EntryPoint="DeviceLibrary.SyncContent">
    <BackgroundTasks>
      <m2:Task Type="deviceUse" /> 
    </BackgroundTasks>
  </Extension>
</Extensions>

The device background task

The device background task class implements the IBackgroundTask interface and contains the actual code you create to either sync or update your peripheral device. The background task class is executed when the background task is triggered and from the entry point provided in your app's application manifest.

The device background class in the Custom USB device sample contains the code to perform a sync to a USB device using the DeviceUseTrigger background task. For complete details, download the sample. For more info about implementing IBackgroundTask and the background task infrastructure of Windows see Supporting your app with background tasks.

Key portions of the device background task in Custom USB device sample include:

The IoSyncBackgroundTask class implements the IBackgroundTask interface required by the Windows background task infrastructure.
The IoSyncBackgroundTask class obtains the DeviceUseDetails instance passed to the class in the IoSyncBackgroundTask class's Run method and uses this instance to report progress back to the Microsoft Store app and to register for cancellation events.
The IoSyncBackgroundTask class's Run method also calls the private OpenDevice and WriteToDeviceAsync methods that implement the background device sync code.

The foreground app

The foreground app in the Custom USB device sample registers and triggers a device background task that uses DeviceUseTrigger. This section provides an overview of the steps your foreground app will take to register, trigger and handle progress for a device background task.

The foreground app in the Custom USB device sample performs the following steps to use a device background task:

Creates new DeviceUseTrigger and BackgroundTaskRegistration objects.
Checks to see if any background tasks were previously registered by this app and cancels them by calling the BackgroundTaskRegistration.Unregister method on the task.
The private SetupBackgroundTask method registers the background task that will sync with the device. The SetupBackgroundTask method is called from the SyncWithDeviceAsync method in the next step.
1. Initializes the DeviceUseTrigger and saves it for later use.
2. Creates a new BackgroundTaskBuilder object and uses its Name, TaskEntryPoint and SetTrigger properties and method to register the app's DeviceUseTrigger object and background task name. The BackgroundTaskBuilder object's TaskEntryPoint property is set to the full name of the background task class that will be run when the background task is triggered.
3. Registers for completion and progress events from the background task so the foreground app can provide completion and progress updates to the user.
The private SyncWithDeviceAsync method registers the background task that will sync with the device and starts the background sync.
1. Calls the SetupBackgroundTask method from the previous step and registers the background task that will sync with the device.
2. Calls the private StartSyncBackgroundTaskAsync method which starts the background task. That method closes the app's handle to the device to ensure that the background task is able to open the device when it starts.
  
  Important
  
  The background task will need to open the device to perform the update so the foreground app must close its connections to the device before calling RequestAsync.
Next, the StartSyncBackgroundTaskAsync method calls the DeviceUseTrigger object's RequestAsync method which starts triggers the background task and returns the DeviceTriggerResults object from RequestAsync used to determine if the background task started successfully.

Important

Windows checks to ensure that all necessary task initiation policy checks have been completed. If all policy checks are completed the update operation is now running as a background task outside of the foreground app, allowing the app to be safely suspended while the operation is in progress. Windows will also enforce any runtime requirements and cancel the background task if those requirements are no longer met.
Finally, the SyncWithDeviceAsync method uses the DeviceTriggerResults object returned from StartSyncBackgroundTaskAsync to determine if the background task started successfully. A switch statement is used to inspect the result from DeviceTriggerResults
The foreground app implements a private OnSyncWithDeviceProgress event handler that will update the app UI with progress from the device background task.
The foreground app implements a private OnSyncWithDeviceCompleted event handler to handle the transition from background tasks to foreground app when the background task has completed.
1. Uses the CheckResults method of the BackgroundTaskCompletedEventArgs object to determine if any exceptions were thrown by the background task.
2. The foreground app reopens the device for use by the app now that the background task is complete and updates the UI to notify the user.
The foreground app implements private button click event handlers from the UI to start and cancel the background task.
1. The private Sync_Click event handler calls the SyncWithDeviceAsync method described in the previous steps.
2. The private CancelSync_Click event handler calls the private CancelSyncWithDevice method to cancel the background task.
The private CancelSyncWithDevice method unregisters and cancels any active device syncs so the device can be reopened by using the BackgroundTaskRegistration.Unregister method.