MSDN Magazine - January 2009 - (Page 36)

Figure 5 Download Complete void OnCompleted(object sender, OpenReadCompletedEventArgs e) { if (PackageDownloaded == null) return; if (e.Error != null) return; // Load a particular assembly from the XAP package Assembly a = GetAssemblyFromPackage(assemblyName, e.Result); // Get an instance of the XAML object object page = a.CreateInstance(className); // Fire the event XapEventArgs args = new XapEventArgs(); args.DownloadedContent = page as UserControl; XapDownloaded(this, args); Figure 6 Extracting an Assembly from a XAP Package Assembly GetAssemblyFromPackage(string assemblyName, Stream xap) { // Local variables Uri assemblyUri = null; StreamResourceInfo resPackage = null; StreamResourceInfo resAssembly = null; AssemblyPart part = null; // Initialize assemblyUri = new Uri(assemblyName, UriKind.Relative); resPackage = new StreamResourceInfo(xap, null); resAssembly = Application.GetResourceStream( resPackage, assemblyUri); // Extract an assembly part = new AssemblyPart(); Assembly a = part.Load(assemblySri.Stream); return a; } } JavaScript downloader object is essentially a script-callable wrapper for WebClient. Processing XAP Content The Downloader class uses WebClient to download a XAP package and then extracts the assembly that contains the application from the zipped stream and instantiates the specified root class. All this logic is hidden behind a relatively simple programming facade made of a LoadPackage method and a XapDownloaded event. The method has the following signature: public void LoadPackage( string xapURL, string assemblyName, string className); sembly from the package and gets an instance of the user control class in the codebehind of the XAML block to add. Next, a custom event is fired to the caller so that the XAML subtree can be processed. At this point, one thing remains to expand on: the details of how an assembly is extracted from the zipped stream downloaded from the URL. This code is shown in Figure 6. This is boilerplate code you use to extract an assembly from a XAP package. StreamResourceInfo and Application.GetResourceStream are also common tools for extracting resource packages such as images or media content from the current XAP. It receives the URL to the package, the name of the assembly in the package to extract, and the name of the class to instantiate. As mentioned, this class is the codebehind of the XAML interface file. The XapDownloaded event is fired when the downloader class has finished with the XAP processing and has the user control ready for the client application. Here’s the definition of the event: public event EventHandler XapDownloaded; Summing It Up The event argument class returns the following information: public class XapEventArgs : EventArgs { public UserControl DownloadedContent; } Let’s dig out the logic of the LoadPackage method. The method employs the binary interface of WebClient to point to the XAP package: void LoadPackage(string package, string asm, string cls) { assemblyName = asm; className = cls; Uri address = new Uri(package, UriKind.RelativeOrAbsolute); WebClient client = new WebClient(); client.OpenReadCompleted += new OpenReadCompletedEventHandler(OnCompleted); client.OpenReadAsync(address); Silverlight applications are expected to download and install quickly and run fast on the user’s machine. Managed code outperforms interpreted JavaScript, so performance will indeed be snappy. But because downloading large Silverlight applications, or applications that require large external graphic and media content, can create undesired latency, breaking the download into steps helps. It is even better if you program these steps to occur on demand. The WebClient class offers an effective and asynchronous programming API to download any sort of resources you can access over the Internet. The extensible model for the Silverlight object model allows you to incorporate any changes in the existing structure quickly. Finally, the stream-based API centered around the StreamResourceInfo class makes it easy to extract content from the resources of assemblies and XAP packages. Any downloaded resource is cached by the browser. This sort of caching is not permanent, though. In Part 2 of this column, I’ll use isolated storage to make any downloaded content persist longer on the user’s machine, thus saving many potential round-trips. For more Silverlight goodies, see Jeff Prosise’s Wicked Code column “Silverlight Tips, Tricks, and Best Practices” at msdn.microsoft.com/ magazine/cc164249.  } The download proceeds asynchronously and fires the OpenReadCompleted event when it is finished. Figure 5 shows the implementation for the handler of the event. If no error occurred, a helper function extracts the specified as36 msdn magazine Dino Esposito is an architect at IDesign and the coauthor of Microsoft .NET: Architecting Applications for the Enterprise (Microsoft Press, 2008). Based in Italy, Dino is a frequent speaker at industry events worldwide. You can join his blog at weblogs.asp.net/despos. Cutting Edge http://msdn.microsoft.com/magazine/cc164249 http://msdn.microsoft.com/magazine/cc164249 http://weblogs.asp.net/despos

Table of Contents Feed for the Digital Edition of MSDN Magazine - January 2009

Toolbox CLR Inside Out Basic Instincts Cutting Edge Test Run First Look� Geneva Framework Silverlight Windows Mobile Service Station Security Briefs Extreme ASP.NET Foundations .NET Matters { End Bracket }

MSDN Magazine - January 2009

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