MSDN Magazine - February 2009 - (Page 23)

John PAPA Data Points Syndicated Data And Isolated Storage In Silverlight Silverlight is ideal for building a syndicated news reader application. It can read RSS and AtomPub syndicated formats, communicate with Web services via HTTP requests, and handle cross-domain policies. Once the syndicated data has been received, it can be read into a class structure, parsed with LINQ, and presented to the user via XAML-based data binding. In this column I will demonstrate how to utilize these unique features to build a syndicated news reader. I will also show you how to debug problems with Web service communications from Silverlight and how to Figure 1 The SilverlightSyndication store data locally using isolated storage. All Application of the examples are available in the MSDN code download. //feedUri is of Feed syndication provides access to the RSS and AtomPub formats through Web services. Each feed is accessed through a Uri that returns XML containing the feed items using the RSS or AtomPub formats. The sample application provided with this column demonstrates techniques that read syndicated data using Web service requests into a Silverlight application. Before diving into the details, it might be helpful if I show you the finished version of the application. Then I will discuss the logic and various other aspects of the application’s code. Figure 1 shows the Silverlight application reading these two syndicated feeds: http://pipes.yahooapis.com/pipes/pipe.run?_id=957 d9624940693fb9f9644d7b12fb0e9&_render=rss http://pipes.yahooapis.com/pipes/pipe.run?_id=057559bac7aad6640b c17529f3421db0&_render=rss HTTP Web Requests and Threading Consuming syndicated feeds from Silverlight applications starts with being able to make Web requests. Both the WebClient and the HttpWebRequest classes can make HTTP Web requests from Silverlight applications. The first step in the process is deciding which technique to use to make HTTP Web requests. In most cases, the WebClient class is all that is needed since it is simpler to use (it also calls the HttpWebRequest under the covers). HttpWebRequest allows for more customization of the requests. This code snippet demonstrates how to make a request through WebClient: Getting Finished type Uri WebClient request = new WebClient(); request.DownloadStringCompleted += AddFeedCompleted; request.DownloadStringAsync(feedUri); The following code, on the other hand, shows how to make a similar call using HttpWebRequest: //feedUri is of type Uri WebRequest request = HttpWebRequest.Create(feedUri); request.BeginGetResponse(new AsyncCallback(ReadCallback), request); A Web request is made to gather the feed data when the user clicks the Add button. The feed’s Uri is stored locally on the client computer, and the title of the feed service is displayed in the upper DataGrid control. Storing the feed’s Uri on the client computer allows the Silverlight application to get the feed data for those feeds when the application starts. When the feed data is gathered, the items are placed into a list of objects and then parsed using LINQ. The results are then bound to the lower DataGrid control in sorted order. The WebClient class is easier to use because it wraps some of the functionality of the HttpWebRequest class. Since all HTTP Web requests from Silverlight are made asynchronously through both WebClient and HttpWebRequest, it is important to understand how to process the data when the calls return. When HttpWebRequest makes its asynchronous calls and it completes, the completed event handler is not guaranteed to be operating on the UI thread. If data were retrieved and meant to be displayed in a UI element, the call to the UI element must be made using a technique that transfers control back to the UI thread from the background thread. This can be done with the Dispatcher or through the SynchronizationContext class. The following code shows how to make a call using the UI thread with the Dispatcher class’s BeginInvoke method: Send your questions and comments for John to mmdata@microsoft.com. Code download at code.msdn.microsoft.com/mag200902Data. February 2009 23 http://code.msdn.microsoft.com/mag200902Data

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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.