MSDN Magazine - February 2009 - (Page 34)

Figure 3 Creating an Isolated Storage File using (IsolatedStorageFile iso = IsolatedStorageFile.GetUserStoreForApplication()) { // Open or create the low level stream IsolatedStorageFileStream fileStream; fileStream = new IsolatedStorageFileStream(fileName, FileMode.OpenOrCreate, iso); // Encapsulate the raw stream in a more convenient writer StreamWriter writer = new StreamWriter(stream); // Write some data writer.Write(DateTime.Now.ToString()); // Clean up writer.Close(); stream.Close(); } The static method GetUserStoreForApplication returns the token you use for any further access to the isolated storage. When the interface IDownloader { first call to GetUserStoreForApplication is made, the applicationvoid LoadPackage(string xapUrl, string asm, string cls); specific subtree is created, if it doesn’t exist already. event EventHandler XapDownloaded; } The Silverlight isolated storage API supplies classes to work with files and directories within the protected file system subtree. Hap- In particular, the LoadPackage method will be rewritten to incorpily, the list of classes you need to know is fairly short; you’ll find porate the logic that would check for the existence of the specified them listed in Figure 2. XAP package within the isolated storage and download it from the In the IsolatedStorageFile class, there are a number of methods Internet otherwise. Figure 6 shows a large section of the code for to create and delete files and directories, to check for the presence the Downloader class. The method first attempts to get the stream of files and directories, and to read and write new files. You employ for the XAP package from the internal cache. If this attempt fails, streams to work with files. Optionally, you can wrap up streams with the method proceeds and downloads the package from the host stream readers, which are much more comserver. (This is just what I discussed in defortable objects to work with. Figure 3 shows tail last month.) Your Silverlight Code a brief example of how to create an isolated In Silverlight, downloading is an asyn(page.xaml.cs) storage file using a stream writer. chronous process, so the internal method PackageLoaded Once you wrap a low-level stream in a StartDownload raises a “completed” event Event LoadPackage more convenient stream writer or reader, the when the package is entirely available on Method code you use to write (or read) some data is the client. The event handler first saves the nearly identical to the code you would use content of the XAP package stream to a loDownloader Class in a classic .NET application. Let’s see how cal file and then extracts resources from it. Note that in the sample code I’m only exto take advantage of the isolated storage API to save any downloaded XAP package lotracting assemblies; in a more general comIsolated Storage Cache cally and reload it later. ponent you might want to extend caching capabilities to any other type of resource Building a Permanent Cache of such as XAML for animations, images, or Figure 4 The Downloader Component and Packages other auxiliary files. the Rest of the Application In last month’s column, I used a downThe LoadPackage method in the Downloader wrapper class to hide some of the loader class is used to download Silverlightboilerplate code you need to download a user controls to insert in the current XAML XAP package and extract assemblies and tree. Because the XAP package is a multi-file other resources. The Downloader class, container, you have to specify which assemhowever, is not just a helper class. Concepbly contains the user control and the class tually, it represents a nontrivial piece of name. The code in Figure 6 simply extracts logic that you might want to isolate from the specified assembly from the package, the rest of the application code for a numloads it into the current AppDomain, and ber of reasons. then creates an instance of the specified The first reason that springs to mind is contained class. testability. By exposing the functionality What if the assembly has some dependenof a downloader component through an Figure 5 Extracting an Interface cies? The code in Figure 6 just doesn’t cover 34 msdn magazine Cutting Edge interface, you gain the possibility of quickly and effectively mocking the downloader for testing purposes. In addition, an interface represents the tool you leverage to replace a simpler downloader with a more sophisticated one that, perhaps, just happens to support package caching. Figure 4 shows the architecture of the design you should aim for. In last month’s source code, the Downloader class was a monolithic piece of code. For a more flexible design, let’s extract an interface out of it. As Figure 5 shows, Visual Studio has a context menu that, although not as rich as that of commercial refactoring tools, does provide some help with the extraction of an interface out of a class. Now that the core of your Silverlight application talks to the IDownloader interface, all the logic for package caching must go only inside the actual downloader class: http://page.xaml.cs

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

MSDN Magazine - February 2009 Contents Toolbox CLR Inside Out Data Points Cutting Edge Patterns In Practice Best Practices .Net Interop "Oslo" Basics Patterns Silverlight Under The Table Foundations Windows With C++ .NET Matters Going Places { End Bracket }

MSDN Magazine - February 2009

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