Windows Phone 8 : Background File Transfer Sample Code - Using URL Rerouting with a WCF Service

Within the BackgroundAgents solution in the downloadable sample code is a project named WPUnleashed.BackgroundAgents.Web. This project exposes a WCF service named BackupService, which allows the phone app to save files to the Backups directory on the server by way of its SaveFile method (see Listing 1). The SaveFile method accepts a Stream, which is written to a file on the server. The unique ID of the user is also sent to the server to allow correct retrieval of the file at a later time.

LISTING 1. BackupService Class

[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)]
[AspNetCompatibilityRequirements(
    RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
public class BackupService : IBackupService
{
    public void SaveFile(string userId, string fileName, Stream fileStream)
    {
        string location = string.Format(
                             @"~\Backups\{0}_{1}", userId, fileName);
        var filepath = HttpContext.Current.Server.MapPath(location);
        using (Stream outputStream = File.OpenWrite(filepath))
        {
            CopyStream(fileStream, outputStream);
        }
    }

    static void CopyStream(Stream input, Stream output)
    {
        var buffer = new byte[8 * 1024];
        int streamLength;

        while ((streamLength = input.Read(buffer, 0, buffer.Length)) > 0)
        {
            output.Write(buffer, 0, streamLength);
        }
    }
}

There is no retrieval method in the WCF service because an ordinary HTTP GET request is used to download the file.

The WPUnleashed.BackgroundAgents contains a web reference to the WPUnleashed.BackgroundAgents.Web project, and the service is consumed within the TodoListViewModel class.

1. Using URL Rerouting with a WCF Service

The Web Application project uses URL rerouting to allow the BackgroundTransferRequest to pass the user ID and filename to the service via the URL.

The routing system on the server is initialized in the RegisterRoutes method of the Global class in the Web Application project. The URL routing APIs reside in the System.Web.Routing namespace. A new ServiceRoute is added to the RouteTable, so that when a request for the URL BackupService arrives, the built-in WebServiceHostFactory creates an instance of the BackupService class to service the request. See the following excerpt:

public class Global : System.Web.HttpApplication
{
    protected void Application_Start(object sender, EventArgs e)
    {
        RegisterRoutes();
    }

    void RegisterRoutes()
    {
        RouteTable.Routes.Add(new ServiceRoute(
            "BackupService", new WebServiceHostFactory(),
             typeof(BackupService)));
    }
...
}

The service interface for the backup service defines a UriTemplate that maps the incoming request URL, which includes the user ID and filename, to the SaveFile method parameters. This means that we are able to translate the incoming URL and forward the call to the service method. See the following:

[ServiceContract(Namespace = "http://danielvaughan.org")]
public interface IBackupService
{
    [OperationContract, WebInvoke(
        Method = "POST", UriTemplate = "UploadFile/{userId}/{fileName}")]
    void SaveFile(string userId, string fileName, Stream fileStream);
}

URL rerouting is an elegant way of providing a WCF service with extra information, such as the user’s ID, while still remaining compatible with the BackgroundTransferRequest and its simple Uri UploadLocation property.