Microsoft Silverlight 3: A Beginner s Guide in .NET framework

Generate QR Code JIS X 0510 in .NET framework Microsoft Silverlight 3: A Beginner s Guide

To create an RSS reader using Silverlight 3, create a new Silverlight project using Visual Studio 2010 Next, right-click the Silverlight project in the Solution Explorer and select Add Reference Scroll to and select the SystemServiceModelSyndication namespace Design the Pagexaml file to include a method of collecting the feed URL and a button to trigger the request Additionally, include a TextBlock or other control for displaying the feed results The sample XAML is shown in the following markup:

<UserControl x:Class="L0602Page" xmlns="http://schemasmicrosoftcom/winfx/2006/xaml/presentation" xmlns:x="http://schemasmicrosoftcom/winfx/2006/xaml" Width="573" Height="343"> <Canvas x:Name="LayoutRoot"> <CanvasBackground> <LinearGradientBrush EndPoint="0282999992370605,0155000001192093" StartPoint="0717000007629395,0845000028610229"> <GradientStop Color="#FFAA9563"/> <GradientStop Color="#FFFFFFFF" Offset="0527"/> </LinearGradientBrush> </CanvasBackground> <TextBlock Height="Auto" Width="Auto" CanvasLeft="8" CanvasTop="8" Text="Enter the URL for the RSS feed to acquire below" TextWrapping="Wrap" x:Name="tbHeading"/> <TextBox Height="Auto" Width="428" CanvasLeft="8" CanvasTop="34537" Text="" TextWrapping="Wrap" x:Name="txtURL"/> <Button Height="20772" Width="125" CanvasLeft="440" CanvasTop="34537" Content="Acquire Feed" x:Name="btnAcquire" Click="btnAcquire_Click" /> <ScrollViewer Height="275691" Width="557" CanvasLeft="8" CanvasTop="59309" x:Name="svResults" HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto"> <TextBlock Height="Auto" Width="Auto" Text="no feed results to display" TextWrapping="Wrap" x:Name="tbResults"/> </ScrollViewer> </Canvas> </UserControl>

The results of the preceding markup, as displayed in Internet Explorer 8, are shown in Figure 6-5

6:

Figure 6-5

Silverlight 3 ships with a class designed for working with syndication feeds the SystemServiceModelSyndicationSyndicationFeed class However, prior to you working with a feed using the SyndicationFeed class, the feed contents must first be requested, read into memory, and parsed into the correct format Furthermore, when a syndication feed is requested, a cross-domain call is being made, so the server where the feed resides must have a CrossDomainxml file in place to allow a cross-domain request The upcoming code listing illustrates asynchronously requesting the contents of a feed, parsing the feed contents into XML, and loading the contents into a SyndicationFeed class An additional step must be taken to display the results When the request is made for the feed contents and the callback method is invoked, the callback method is functioning in the context of a background worker thread The background worker thread is used when networking requests are being handled The Silverlight user interface is managed using a separate thread of execution, commonly referred to as the UI thread Silverlight prevents attempts in the background worker thread from directly interacting with the UI thread Hence, it will be difficult or impossible to directly update the Silverlight UI from the callback method that handles the loading and parsing of the syndication feed contents

Silverlight 3 includes a class called the Dispatcher class that is used to communicate between Silverlight threads If the Dispatcher class is not used and an attempt is made to directly update the UI thread from the background worker thread, Silverlight will throw an exception, stating that unauthorized cross-thread access has been attempted The following well-documented code listing illustrates a request for syndication feed contents:

using using using using using using using using using using using using using using System; SystemCollectionsGeneric; SystemLinq; SystemNet; SystemWindows; SystemWindowsControls; SystemWindowsDocuments; SystemWindowsInput; SystemWindowsMedia; SystemWindowsMediaAnimation; SystemWindowsShapes; SystemXml; SystemServiceModel; SystemServiceModelSyndication;

namespace L0602 { public partial class Page : UserControl { // create an instance to represent the uri Uri uri; public Page() { InitializeComponent(); } /// <summary> /// this method makes the request /// </summary> /// <param name="sender"></param> /// <param name="e"></param> private void btnAcquire_Click(object sender, RoutedEventArgs e) { // store the URL of the feed uri = new Uri(txtURLText); // create a request

6:

HttpWebRequest request = (HttpWebRequest)HttpWebRequest Create(uri); // make the request requestBeginGetResponse(new AsyncCallback(displayFeed), request); } /// <summary> /// this method handles the callback /// </summary> /// <param name="asyncResult">the result of the request /// </param> void displayFeed(IAsyncResult asyncResult) { // nab the request HttpWebRequest request = (HttpWebRequest) asyncResult AsyncState; // nab the response HttpWebResponse response = (HttpWebResponse)requestEndGetResponse(asyncResult); // parse the response to xml XmlReader reader = XmlReaderCreate(response GetResponseStream()); // load the feed SyndicationFeed feed = SyndicationFeedLoad(reader); // call to the UI thread to update the UI DispatcherBeginInvoke(delegate { // clear the results textblock tbResultsText = ""; // display the feed items foreach (SyndicationItem item in feedItems) { tbResultsText += itemTitleText + Environment NewLine; } }); } } }