MSDN Magazine - February 2009 - (Page 25)

Each SyndicationItem class instance represents a feed item for the feed. For example, the feeds are represented by instances of the SyndicationFeed class, and their Items collections contain the individual posts from the feeds. Once the SyndicationFeed class is loaded with the feed and its items, the code shown in Figure 3 checks to see if the same feed has already been gathered. If so, the code exits immediately. Otherwise, the feed is added to the local ObservableCollection called _feeds. Through the ReBindAggregatedItems method, the feed items from all of the loaded feeds are then filtered, sorted, and bound to the lower DataGrid. Since the WebClient class made the HTTP Web request, the AddFeedCompleted event handler will have access to the UI thread. This is why the code inside the ReBindAggregatedItems method can bind the data to the DataGrid UI element without the Dispatcher’s help. Silverlight 2 Client Hosted on some domain Same Domain HTTP Request some domain Web Domain Cross Domain HTTP Request some feeds domain Web Domain Figure 5 Cross-Domain Call for a Feed Parsing Feeds When the ReBindAggregatedItems method executes, the feed data is stored in a collection of SyndicatedFeed instances and their respective collections of SyndicatedItem instances. LINQ is ideal for querying the feed data since it is now in an object structure. The data did not need to be loaded into SyndicatedFeed objects. Instead it could have been kept in its native XML format (as RSS or AtomPub), and it could have been parsed using an XmlReader or LINQ to XML. However, the SyndicatedFeed class makes it easier to manage, and LINQ can still be used to query the data. Displaying the feed items for several feeds requires that the feed items are all mashed together. The LINQ query shown in Figure 4 demonstrates how to grab all of the feed items (SyndicationItem instances) for all of the feeds (SyndicationFeed instances) and sort them by their publication date. Notice in Figure 4 that the query returns a list of SyndicationItemExtra classes. The SyndicationItemExtra class is a custom class that has FeedTitle property of type string and an Item property of type SyndicationItem. The application displays the feed items in the DataGrid, and most of the data for this can be found in the SyndicationItem class. However, since the application mashes together items from several feeds, displaying the title of the feed for each feed item makes it clear which feed each item is from. The title for the feed is not accessible from the SyndicationItem class, so the application uses a custom class called SyndicationItemExtra, which will store the SyndicationItem and the feed’s title. The feed items are then bound to the Grid panel feedItemsGridLayout in the Silverlight application. The Grid panel contains the DataGrid as well as other UI elements (such as the number of items displayed in a TextBlock) that are involved in data-binding operations to display information about the feed items. Figure 6 Debugging Cross-Domain Feed Calls Cross-Domain Requests for Feeds Requests to gather feeds are HTTP Web requests that generally make requests to a different Web domain. Any Web request from Silverlight that communicates with a different domain than the one that hosts the Silverlight application must comply with the msdnmagazine.com remote domain’s cross-domain policy. The diagram in Figure 5 demonstrates this situation. For more on cross-domain policies, please refer to my September 2008 Data Points column (msdn.microsoft.com/magazine/cc794260). In that column I discuss the file formats and how the policies work. When an HTTP Web request is made across domains, Silverlight preempts the request by first requesting the cross-domain policy file from the remote Web server. Silverlight first looks for the clientaccesspolicy.xml file (the Silverlight cross-domain policy file), and if it is not found, it then looks for the crossdomain.xml file (the Flash cross-domain policy file). If neither file is found, the request fails and an error is thrown. This error can be caught in the DownloadStringCompleted event handler and presented to the user, if desired. For example, if the Uri http://johnpapa.net/feed/default.aspx is entered into the sample application, Silverlight will first look for one of the cross-domain policy files on the johnpapa.net Web server’s root. If neither of the files are found, then an error is returned to the application, at which point the application can notify the user if desired. Figure 6 shows the FireBug plug-in, which is tracking all requests from the browser. It shows the browser looking for the cross-domain policy files, not finding them, and returning without actually making the request for the RSS feed. FireBug is a great tool for watching HTTP requests in Firefox, and Web Development Helper is a great tool when using Internet Explorer. Another option is Fiddler2, which is a standalone application that can watch all traffic on your computer. One solution to this problem is to ask the Web administrator for the feed to put a clientaccesspolicy.xml file in the Web server’s root. This may not be realistic, since you most likely do not control the remote Web server nor do you know who does. Another option is to see if the feed uses an intermediary service such as Yahoo Pipes. For example, the main feed at johnpapa.net can be retrieved through Yahoo Pipes using the Uri http://pipes.yahooapis.com/pipes/pipe.run?_id=057559bac7aad6640bc17529fFebruary 2009 25 http://msdn.microsoft.com/magazine/cc794260 http://www.msdnmagazine.com

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.