MSDN Magazine - January 2009 - (Page 35)

dered as a sequence. This means that to avoid undesired overlapping of elements, in the case of updates you should first remove elements and then add. XAML serialization, which is performed through the XamlReader and XamlWriter classes, is based on the principle that extension references are de-referenced and runtime values are saved over design-time settings. What if you want to download XAML content and customize it before display, such as through dynamic data binding? You can’t embed bindings in the XAML source, but you can define placeholders in the downloaded XAML, retrieve them with parsing, and set them to any value you want programmatically. However, in Silverlight 2, downloading a XAP package is definitely a better solution. Figure 3 Downloading a XAP Package Dynamically public partial class Page : UserControl { private UIElement content = null; private TabItem item = null; public Page() { InitializeComponent(); } private void chkNewContent_Click(object sender, RoutedEventArgs e) { bool shouldDisplay = (sender as CheckBox).IsChecked.Value; if (shouldDisplay) { if (!IsContentAvailable()) DownloadContent(); else ShowContent(); } else { HideContent(); } Working with XAP Packages A XAP package contains an entire Silverlight application whose user interface consists essentially of a user control, which is really } just a container of XAML markup and code. private bool IsContentAvailable() As mentioned, a XAP package includes one or more assemblies { tracked by a manifest file. Processing a XAP package entails two return (content != null); } extra steps beyond download. You need to extract the main asprivate void DownloadContent() sembly and then instantiate the entry point class that starts up the { downloaded application. Needless to say, in this case you can use Downloader dl = new Downloader(); binding, styles, events, and whatever else you want in the XAML. dl.XapDownloaded += new EventHandler (OnPackageDownload); When you work with a XAP package, it’s the Silverlight runtime— dl.LoadPackage("more.xap", "more.dll", "More.ExtraTab"); not the serialization API—that processes the XAML and then } resolves references. This makes a huge difference in terms of provoid OnPackageDownload(object sender, XapEventArgs e) { gramming power. content = e.DownloadedContent; Downloading and processing a XAP package requires a bit of more ShowContent(); } work than just building an object model out of a string. Some of this private void HideContent() work—typically the content download and assembly extraction— { can be moved to a reusable downloader class (see Figure 3). this.TabList.Items.Remove(item); } The code in Figure 3 shows a sample tab-based application that loads a new tab the first time the user clicks on a checkbox. In this private void ShowContent() { case, a new XAP package is downloaded, and the user control conitem = new TabItem(); tained in its user interface is inserted in a newly created TabItem. item.Header = "Extra tab"; item.Content = content; Hiding the content from the newly downloaded package is a simthis.TabList.Items.Add(item); } ple client operation. And showing it again doesn’t require a second } round-trip for two good reasons: the content is cached in memory, and the package from which it was built is cached in the browser’s cache. Figure 4 shows the user interface of this sample application. The content inserted in the additional tab comes from a new Silverlight application project. In terms of deployment, all you have to do is store the XAP package in the ClientBin folder of the host Web site (the default location where WebClient looks for relative content) or in any other location that is safe for WebClient to reach. Now, let’s explore the code of the helper Downloader class. Note that the Downloader class used in this example is an internal class and has nothing to do with the Silverlight Downloader object available to JavaScript callers. The Figure 4 An Application Capable of Downloading Parts of UI Dynamically msdnmagazine.com January 2009 35 http://www.msdnmagazine.com

Table of Contents Feed for the Digital Edition of MSDN Magazine - January 2009

Toolbox CLR Inside Out Basic Instincts Cutting Edge Test Run First Look� Geneva Framework Silverlight Windows Mobile Service Station Security Briefs Extreme ASP.NET Foundations .NET Matters { End Bracket }

MSDN Magazine - January 2009

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