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Microsoft Silverlight 3: A Beginner s Guide
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The WebClient Class
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Silverlight includes a class called the WebClient class that is used to directly download resources from a web server The WebClient class was formerly called the HTTP Downloader The WebClient class is best suited to directly downloading larger resources from a web server that may take time to download, including images and media files All networking calls made in Silverlight are asynchronous calls; hence, all calls made by the WebClient class are asynchronous A few steps are involved in using the WebClient class Walking through an example is the best way to illustrate the WebClient class The example shown here starts with a simple piece of XAML that includes a single Image control that will display dynamically downloaded images:
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<UserControl x:Class="L1001Page" xmlns="http://schemasmicrosoftcom/winfx/2006/xaml/presentation" xmlns:x="http://schemasmicrosoftcom/winfx/2006/xaml" Width="400" Height="300" Loaded="UserControl_Loaded"> <Grid x:Name="LayoutRoot" Background="White"> <Image Height="59" HorizontalAlignment="Left" Margin="26,27,0,0" VerticalAlignment="Top" Width="56" x:Name="imgWeather"/> </Grid> </UserControl>
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The starting Silverlight project includes four small JPG files, as shown in Figure 10-1
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Figure 10-1 The starting Silverlight project with included image files
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10: Retrieving Server Resources
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Notice in Figure 10-1 that the images are all shown as separate files The WebClient class is capable of downloading a single file at a time from a server but is also capable of downloading a collection of files from the server that has been compressed into a zip file The first step necessary in using the WebClient class is to create an instance of the class Generally, an instance of the WebClient class will be created as a class-level member The syntax to create an instance is shown in the following code snippet:
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// create an instance of the WebClient class WebClient downloader = new WebClient();
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The next step in the process of using the WebClient class is to assign a callback handler to the OpenReadCompleted event A callback handler is normally assigned in the constructor of the class in the code-behind file for the Silverlight control The following code snippet illustrates assigning a callback handler:
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// assign a callback handler downloaderOpenReadCompleted += new OpenReadCompletedEventHandler(downloader_OpenReadCompleted);
The WebClient class is now ready for use In order to make a call to retrieve resources from a server, use the OpenReadAsync method The next code snippet illustrates making a call to the server to retrieve a file named Imageszip An instance of the Uri class is created to indicate where the request resource resides in relation to the Silverlight application Finally, the third parameter that can be passed to the OpenReadAsync is used to identify the file to extract from the zip file package if one is requested In the following code snippet, the file sunnyjpg is extracted If a single file is to be downloaded instead of a package, the name of the file is used as the first parameter instead of the package name and then the third parameter is omitted
// download images downloaderOpenReadAsync(new Uri("imageszip", UriKindRelative), sunnyjpg);
The next step in using the WebClient class is to write code in the callback handler to determine what to do with the requested resource once it arrives The following code snippet displays the image that is returned in the WebClient request:
// display the image StreamResourceInfo photosDownloaded = new StreamResourceInfo(eResult as Stream, null); string photoToGrab = eUserStateToString(); StreamResourceInfo photoStream =
Microsoft Silverlight 3: A Beginner s Guide
ApplicationGetResourceStream(photosDownloaded, new Uri(photoToGrab, UriKindRelative)); BitmapImage bitmap = new BitmapImage(); bitmapSetSource(photoStreamStream); imgWeatherSource = bitmap;
The result of the code in the callback method is shown in Figure 10-2 Refer to the entire code listing, shown next, to gain a full understanding of using the WebClient class:
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; SystemWindowsResources; SystemIO; SystemWindowsMediaImaging;
Figure 10-2 The dynamically downloaded image displayed
10: Retrieving Server Resources
namespace L1001 { public partial class Page : UserControl { // create an instance of the WebClient class WebClient downloader = new WebClient ( ); public Page ( ) { InitializeComponent ( ); // assign a callback handler downloaderOpenReadCompleted += new penReadCompletedEventHandler ( downloader_OpenReadCompleted ); } void downloader_OpenReadCompleted ( objectsender, OpenReadCompletedEventArgs e ) { // display the image StreamResourceInfo photosDownloaded = new StreamResourceInfo ( eResult as Stream, null ); string photoToGrab = eUserStateToString ( ); StreamResourceInfo photoStream = ApplicationGetResourceStream ( photosDownloaded, new Uri ( photoToGrab, UriKindRelative ) ); BitmapImage bitmap = new BitmapImage ( ); bitmapSetSource ( photoStreamStream ); imgWeatherSource = bitmap; } private void UserControl_Loaded ( object sender, RoutedEventArgs e ) { // download images downloaderOpenReadAsync(new Uri("imageszip",UriKindRelative), "sunnyjpg"); } } }
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