MSDN Magazine - February 2009 - (Page 24)

Deployment.Current.Dispatcher.BeginInvoke(() => { MyDataGrid.DataContext = productList; }); This code example takes the productList variable (which was filled from the data returned from a Web service call) and sets it to a UI element’s DataContext. In this case, a DataGrid will now be bound to the list of products. The Dispatcher is not needed, however, if the call was made through the WebClient class. In that case, the code could simply set the product list directly to the UI element’s DataContext. To use the SynchronizationContext class, an instance of the SynchronizationContext class must be created in a place where the UI thread is known to be available. The constructor and the load event handler are good places to create the instance of a SynchronizationContext. This code sample shows the _syncContext field being initialized in the class constructor: public Page() { _syncContext = SynchronizationContext.Current; } quest. For the sample application, e.Result will contain the XML representing the feed data. Once the feed data is gathered, it can be read into the System.ServiceModel.SyndicationFeed class using the SyndicationFeed class’s Load method. Note that when retrieving feed data and using it in a read-only manner, using LINQ to XML to retrieve the feed and loading it in a custom object may be a better option than SyndicationFeed. SyndicationFeed has more features, but if they are not being used it may not be worth the additional size added to the XAP—SyndicationFeed adds about 150KB to the XAP while LINQ to XML adds about 40KB. With the additional power of SyndicationFeed you also have some cost in size. SyndicationFeed is a special class that knows how to represent feed data (both RSS and AtomPub) as an object. It has properties that describe the feed itself, such as Title and Description, as well as an Items property that contains an IEnumerable . Figure 2 Adding a Feed private void btnAdd_Click(object sender, RoutedEventArgs e) { Uri feedUri; Uri.TryCreate(txtAddress.Text, UriKind.Absolute, out feedUri); if (feedUri == null) return; } LoadFeed(feedUri); And this code shows the SynchronizationContext instance using its Post method to make the call to the LoadProducts method. It makes sure that the LoadProducts method has access to the UI thread: if (_syncContext != null) { _syncContext.Post(delegate(object state){ LoadProducts(products); } ,null); } Besides being easier to use, WebClient requests always come back on the UI thread. This means that any results from the WebClient’s request can be easily bound to the UI elements without having to involve the Dispatcher (or, optionally, the SynchronizationContext class instead of the Dispatcher). For reading syndicated data, the WebClient class is adequate and will be used for this column’s sample application. public void LoadFeed(Uri feedUri) { WebClient request = new WebClient(); request.DownloadStringCompleted += AddFeedCompleted; request.DownloadStringAsync(feedUri); } Figure 3 AddFeedCompleted private void AddFeedCompleted(object sender, DownloadStringCompletedEventArgs e) { if (e.Error != null) return; string xml = e.Result; if (xml.Length == 0) return; StringReader stringReader = new StringReader(xml); XmlReader reader = XmlReader.Create(stringReader); SyndicationFeed feed = SyndicationFeed.Load(reader); if (_feeds.Where(f => f.Title.Text == feed.Title.Text).ToList().Count > 0) return; _feeds.Add(feed); // This also saves the feeds to isolated storage ReBindAggregatedItems(); txtAddress.Text = string.Empty; } Adding a Feed In the sample application, when the user enters a feed address and clicks the Add button, the code shown in Figure 2 executes. First the code attempts to create a Uri from the address, using the Uri.TryCreate method if possible. If it can create a Uri, the Uri is returned to the local variable feedUri. Otherwise, feedUri remains null and the code exits. Once a valid Uri is created, the LoadFeed method executes, making the HTTP request to gather the feed data using the WebClient class. The WebClient instance is created, and an event handler is assigned to the DownloadStringCompleted event. When the DownloadStringAsync method is executed and is ready to return its data, it needs to know what event handler to go to. That is why the event handler (in this case, AddFeedCompleted) must be assigned before the asynchronous event is executed. Once the request has completed, the AddFeedCompleted event handler will execute (see Figure 3). The DownloadStringCompletedEventArgs parameter contains a Result property and an Error property, both of which are important to check after each Web request. The e.Error property will be null if there were no errors during the request. The e.Result property contains the results of the Web re24 msdn magazine Figure 4 Querying Feeds with LINQ private void ReBindAggregatedItems() { //Read the feed items and bind them to the lower list var query = from f in _feeds from i in f.Items orderby i.PublishDate descending select new SyndicationItemExtra { FeedTitle = f.Title.Text, Item = i }; var items = query.ToList(); feedItemsGridLayout.DataContext = items; } Data Points

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.