Process media files in the background

Článek
10/20/2022

This article shows you how to use the MediaProcessingTrigger and a background task to process media files in the background.

The example app described in this article allows the user to select an input media file to transcode and specify an output file for the transcoding result. Then, a background task is launched to perform the transcoding operation. The MediaProcessingTrigger is intended to support many different media processing scenarios besides transcoding, including rendering media compositions to disk and uploading processed media files after processing is complete.

For more detailed information on the different Universal Windows app features utilized in this sample, see:

Create a media processing background task

To add a background task to your existing solution in Microsoft Visual Studio, Enter a name for your comp

From the File menu, select Add and then New Project....
Select the project type Windows Runtime Component (Universal Windows).
Enter a name for your new component project. This example uses the project name MediaProcessingBackgroundTask.
Click OK.

In Solution Explorer, right-click the icon for the "Class1.cs" file that is created by default and select Rename. Rename the file to "MediaProcessingTask.cs". When Visual Studio asks if you want to rename all of the references to this class, click Yes.

In the renamed class file, add the following using directives to include these namespaces in your project.

using Windows.ApplicationModel.Background;
using Windows.Storage;
using Windows.UI.Notifications;
using Windows.Data.Xml.Dom;
using Windows.Media.MediaProperties;
using Windows.Media.Transcoding;
using System.Threading;

Update your class declaration to make your class inherit from IBackgroundTask.

public sealed class MediaProcessingTask : IBackgroundTask
{

Add the following member variables to your class:

An IBackgroundTaskInstance that will be used to update the foreground app with the progress of the background task.
A BackgroundTaskDeferral that keeps the system from shutting down your background task while media transcoding is being performed asynchronously.
A CancellationTokenSource object that can be used to cancel the asynchronous transcoding operation.
The MediaTranscoder object that will be used to transcode media files.

IBackgroundTaskInstance backgroundTaskInstance;
BackgroundTaskDeferral deferral;
CancellationTokenSource cancelTokenSource = new CancellationTokenSource();
MediaTranscoder transcoder;

The system calls Run method of a background task when the task is launched. Set the IBackgroundTask object passed into the method to the corresponding member variable. Register a handler for the Canceled event, which will be raised if the system needs to shut down the background task. Then, set the Progress property to zero.

Next, call the background task object's GetDeferral method to obtain a deferral. This tells the system not to shut down your task because you are performing asynchronous operations.

Next, call the helper method TranscodeFileAsync, which is defined in the next section. If that completes successfully, a helper method is called to launch a toast notification to alert the user that transcoding is complete.

At the end of the Run method, call Complete on the deferral object to let the system know that your background task is complete and can be terminated.

public async void Run(IBackgroundTaskInstance taskInstance)
{
    Debug.WriteLine("In background task Run method");

    backgroundTaskInstance = taskInstance;
    taskInstance.Canceled += new BackgroundTaskCanceledEventHandler(OnCanceled);
    taskInstance.Progress = 0;

    deferral = taskInstance.GetDeferral();
    Debug.WriteLine("Background " + taskInstance.Task.Name + " is called @ " + (DateTime.Now).ToString());

    try
    {
        await TranscodeFileAsync();
        ApplicationData.Current.LocalSettings.Values["TranscodingStatus"] = "Completed Successfully";
        SendToastNotification("File transcoding complete.");

    }
    catch (Exception e)
    {
        Debug.WriteLine("Exception type: {0}", e.ToString());
        ApplicationData.Current.LocalSettings.Values["TranscodingStatus"] = "Error ocurred: " + e.ToString();
    }


    deferral.Complete();
}

In the TranscodeFileAsync helper method, the file names for the input and output files for the transcoding operations are retrieved from the LocalSettings for your app. These values will be set by your foreground app. Create a StorageFile object for the input and output files and then create an encoding profile to use for transcoding.

Call PrepareFileTranscodeAsync, passing in the input file, output file, and encoding profile. The PrepareTranscodeResult object returned from this call lets you know if transcoding can be performed. If the CanTranscode property is true, call TranscodeAsync to perform the transcoding operation.

The AsTask method enables you to track the progress the asynchronous operation or cancel it. Create a new Progress object, specifying the units of progress you desire and the name of the method that will be called to notify you of the current progress of the task. Pass the Progress object into the AsTask method along with the cancellation token that allows you to cancel the task.

  private async Task TranscodeFileAsync()
  {
      transcoder = new MediaTranscoder();

      try
      {
          var settings = ApplicationData.Current.LocalSettings;

          settings.Values["TranscodingStatus"] = "Started";

          var inputFileName = ApplicationData.Current.LocalSettings.Values["InputFileName"] as string;
          var outputFileName = ApplicationData.Current.LocalSettings.Values["OutputFileName"] as string;

          if (inputFileName == null || outputFileName == null)
          {
              return;
          }


          // retrieve the transcoding information
          var inputFile = await Windows.Storage.StorageFile.GetFileFromPathAsync(inputFileName);
          var outputFile = await Windows.Storage.StorageFile.GetFileFromPathAsync(outputFileName);

          // create video encoding profile                
          MediaEncodingProfile encodingProfile = MediaEncodingProfile.CreateMp4(VideoEncodingQuality.HD720p);

          Debug.WriteLine("PrepareFileTranscodeAsync");
          settings.Values["TranscodingStatus"] = "Preparing to transcode ";
          PrepareTranscodeResult preparedTranscodeResult = await transcoder.PrepareFileTranscodeAsync(
              inputFile, 
              outputFile, 
              encodingProfile);

          if (preparedTranscodeResult.CanTranscode)
          {
              var startTime = TimeSpan.FromMilliseconds(DateTime.Now.Millisecond);
              Debug.WriteLine("Starting transcoding @" + startTime);

              var progress = new Progress<double>(TranscodeProgress);
              settings.Values["TranscodingStatus"] = "Transcoding ";
              settings.Values["ProcessingFileName"] = inputFileName;
              await preparedTranscodeResult.TranscodeAsync().AsTask(cancelTokenSource.Token, progress);

          }
          else
          {
              Debug.WriteLine("Source content could not be transcoded.");
              Debug.WriteLine("Transcode status: " + preparedTranscodeResult.FailureReason.ToString());
              var endTime = TimeSpan.FromMilliseconds(DateTime.Now.Millisecond);
              Debug.WriteLine("End time = " + endTime);
          }
      }
      catch (Exception e)
      {
          Debug.WriteLine("Exception type: {0}", e.ToString());
          throw;
      }
  }

In the method you used to create the Progress object in the previous step, Progress, set the progress of the background task instance. This will pass the progress to the foreground app, if it is running.

void TranscodeProgress(double percent)
{
    Debug.WriteLine("Transcoding progress:  " + percent.ToString().Split('.')[0] + "%");
    backgroundTaskInstance.Progress = (uint)percent;
}

The SendToastNotification helper method creates a new toast notification by getting a template XML document for a toast that only has text content. The text element of the toast XML is set and then a new ToastNotification object is created from the XML document. Finally, the toast is shown to the user by calling ToastNotifier.Show.

private void SendToastNotification(string toastMessage)
{
    ToastTemplateType toastTemplate = ToastTemplateType.ToastText01;
    XmlDocument toastXml = ToastNotificationManager.GetTemplateContent(toastTemplate);

    //Supply text content for your notification
    XmlNodeList toastTextElements = toastXml.GetElementsByTagName("text");
    toastTextElements[0].AppendChild(toastXml.CreateTextNode(toastMessage));

    //Create the toast notification based on the XML content you've specified.
    ToastNotification toast = new ToastNotification(toastXml);

    //Send your toast notification.
    ToastNotificationManager.CreateToastNotifier().Show(toast);
}

In the handler for the Canceled event, which is called when the system cancels your background task, you can log the error for telemetry purposes.

private void OnCanceled(IBackgroundTaskInstance sender, BackgroundTaskCancellationReason reason)
{
    Debug.WriteLine("Background " + sender.Task.Name + " Cancel Requested..." + reason.ToString());
}

Register and launch the background task

Before you can launch the background task from your foreground app, you must update your foreground app's Package.appmanifest file to let the system know that your app uses a background task.

In Solution Explorer, double-click the Package.appmanifest file icon to open the manifest editor.
Select the Declarations tab.
From Available Declarations, select Background Tasks and click Add.
Under Supported Declarations make sure that the Background Tasks item is selected. Under Properties, select the checkbox for Media processing.
In the Entry Point text box, specify the namespace and class name for your background test, separated by a period. For this example, the entry is:

MediaProcessingBackgroundTask.MediaProcessingTask

Next, you need to add a reference to your background task to your foreground app.

In Solution Explorer, under your foreground app project, right-click the References folder and select Add Reference....
Expand the Projects node and select Solution.
Check the box next to your background task project and click OK.

The rest of the code in this example should be added to your foreground app. First, you will need to add the following namespaces to your project.

using Windows.ApplicationModel.Background;
using Windows.Storage;

Next, add the following member variables that are needed to register the background task.

MediaProcessingTrigger mediaProcessingTrigger;
string backgroundTaskBuilderName = "TranscodingBackgroundTask";
BackgroundTaskRegistration taskRegistration;

The PickFilesToTranscode helper method uses a FileOpenPicker and a FileSavePicker to open the input and output files for transcoding. The user may select files in a location that your app does not have access to. To make sure your background task can open the files, add them to the FutureAccessList for your app.

Finally, set entries for the input and output file names in the LocalSettings for your app. The background task retrieves the file names from this location.

private async void PickFilesToTranscode()
{
    var openPicker = new Windows.Storage.Pickers.FileOpenPicker();

    openPicker.SuggestedStartLocation = Windows.Storage.Pickers.PickerLocationId.VideosLibrary;
    openPicker.FileTypeFilter.Add(".wmv");
    openPicker.FileTypeFilter.Add(".mp4");

    StorageFile source = await openPicker.PickSingleFileAsync();

    var savePicker = new Windows.Storage.Pickers.FileSavePicker();

    savePicker.SuggestedStartLocation =
        Windows.Storage.Pickers.PickerLocationId.VideosLibrary;

    savePicker.DefaultFileExtension = ".mp4";
    savePicker.SuggestedFileName = "New Video";

    savePicker.FileTypeChoices.Add("MPEG4", new string[] { ".mp4" });

    StorageFile destination = await savePicker.PickSaveFileAsync();

    if(source == null || destination == null)
    {
        return;
    }

    var storageItemAccessList = Windows.Storage.AccessCache.StorageApplicationPermissions.FutureAccessList;
    storageItemAccessList.Add(source);
    storageItemAccessList.Add(destination);

    ApplicationData.Current.LocalSettings.Values["InputFileName"] = source.Path;
    ApplicationData.Current.LocalSettings.Values["OutputFileName"] = destination.Path;
}

To register the background task, create a new MediaProcessingTrigger and a new BackgroundTaskBuilder. Set the name of the background task builder so that you can identify it later. Set the TaskEntryPoint to the same namespace and class name string you used in the manifest file. Set the Trigger property to the MediaProcessingTrigger instance.

Before registering the task, make sure you unregister any previously registered tasks by looping through the AllTasks collection and calling Unregister on any tasks that have the name you specified in the BackgroundTaskBuilder.Name property.

private void RegisterBackgroundTask()
{
    // New a MediaProcessingTrigger
    mediaProcessingTrigger = new MediaProcessingTrigger();

    var builder = new BackgroundTaskBuilder();

    builder.Name = backgroundTaskBuilderName;
    builder.TaskEntryPoint = "MediaProcessingBackgroundTask.MediaProcessingTask";
    builder.SetTrigger(mediaProcessingTrigger);

    // unregister old ones
    foreach (var cur in BackgroundTaskRegistration.AllTasks)
    {
        if (cur.Value.Name == backgroundTaskBuilderName)
        {
            cur.Value.Unregister(true);
        }
    }

    taskRegistration = builder.Register();
    taskRegistration.Progress += new BackgroundTaskProgressEventHandler(OnProgress);
    taskRegistration.Completed += new BackgroundTaskCompletedEventHandler(OnCompleted);

    return;
}

A typical app will register their background task when the app is initially launched, such as in the OnNavigatedTo event.

Launch the background task by calling the MediaProcessingTrigger object's RequestAsync method. The MediaProcessingTriggerResult object returned by this method lets you know whether the background task was started successfully, and if not, lets you know why the background task wasn't launched.

private async void LaunchBackgroundTask()
{
    var success = true;

    if (mediaProcessingTrigger != null)
    {
        MediaProcessingTriggerResult activationResult;
        activationResult = await mediaProcessingTrigger.RequestAsync();

        switch (activationResult)
        {
            case MediaProcessingTriggerResult.Allowed:
                // Task starting successfully
                break;

            case MediaProcessingTriggerResult.CurrentlyRunning:
            // Already Triggered

            case MediaProcessingTriggerResult.DisabledByPolicy:
            // Disabled by system policy

            case MediaProcessingTriggerResult.UnknownError:
                // All other failures
                success = false;
                break;
        }

        if (!success)
        {
            // Unregister the media processing trigger background task
            taskRegistration.Unregister(true);
        }
    }

}

A typical app will launch the background task in response to user interaction, such as in the Click event of a UI control.

The OnProgress event handler is called when the background task updates the progress of the operation. You can use this opportunity to update your UI with progress information.

private void OnProgress(IBackgroundTaskRegistration task, BackgroundTaskProgressEventArgs args)
{
    string progress = "Progress: " + args.Progress + "%";
    Debug.WriteLine(progress);
}

The OnCompleted event handler is called when the background task has finished running. This is another opportunity to update your UI to give status information to the user.

private void OnCompleted(IBackgroundTaskRegistration task, BackgroundTaskCompletedEventArgs args)
{
    Debug.WriteLine(" background task complete");
}

Sdílet prostřednictvím

Process media files in the background

Create a media processing background task

Register and launch the background task

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