MSDN Magazine - February 2009 - (Page 37)

in some custom and persistent dictionary that tracks when the package was downloaded. Needless to say, this information has to be persisted in some way to the isolated storage. Figure 10 illustrates this concept. CacheIndex is a helper class that uses the Silverlight native application settings API to persist a dictionary of XAP names and download times to the isolated storage. The m_ItemIndex member is a plain Dictionary object instantiated in CacheIndex constructor, as shown in Figure 11. ApplicationSettings is a very nice feature of Silverlight 2. It basically consists of a string/object dictionary and is automatically read from storage upon application loading and saved back to the storage upon shutdown. Any (serializable) object you add to the dictionary is automatically persisted. By creating a XAPCACHENAME entry in the dictionary, you arrange to persist the content of the XAP dictionary. The XAP dictionary contains one entry for each downloaded package paired with the time of the download, as you see in Figure 12. Note that the ApplicationSettings API also offers you a Save method to force persistence before application shutdown. Figure 11 The CacheIndex Class public class CacheIndex { private const string XAPCACHENAME = "XapCache"; private Dictionary m_ItemsIndex; public CacheIndex() { IsolatedStorageSettings iss; iss = IsolatedStorageSettings.ApplicationSettings; if (iss.Contains(XAPCACHENAME)) m_ItemsIndex = iss[XAPCACHENAME] as Dictionary ; else { m_ItemsIndex = new Dictionary (); iss[XAPCACHENAME] = m_ItemsIndex; iss.Save(); } } } Figure 12 Adding Download Information public void Add(string xapFile) { m_ItemsIndex[xapFile] = DateTime.Now; IsolatedStorageSettings iss; iss = IsolatedStorageSettings.ApplicationSettings; iss.Save(); } public void Remove(string xapFile) { m_ItemsIndex.Remove(xapFile); IsolatedStorageSettings iss; iss = IsolatedStorageSettings.ApplicationSettings; iss.Save(); } Putting It All Together All the changes and details discussed so far occur behind the curtain of a public class—the Downloader class. By incorporating this class in your Silverlight application, you gain multiple levels of caching in a single shot. With proper coding you can cache downloaded user controls for the duration of the application session. If you download content that goes to, say, a tab item, you can then hide and show the tab item repeatedly without the need to download the package over and over again. If you download through the WebClient class (as in last month’s column), you pass through the browser engine and get browser-level caching. Any downloaded XAP package lives on the user’s machine until the user clears the browser cache. Finally, the Downloader class you get with this column supports a permanent cache through isolated storage. This means that any time you download via WebClient, the XAP package is also saved to the local storage. The Downloader class, however, also offers to pick up the XAP file from the storage if a valid package can be found. Note that this works across application sessions. You download the package once, you work and then you shut down the application. When you resume, the package is reloaded from the storage if it has not expired. What if the package expires? In this case, the downloader passes through WebClient. But at this point, WebClient might just return a previously browser-cached copy of the same package. This is by design. To bypass the browser-level caching, you have to disable it in the original HTTP request, as discussed last month. You must cache properties at the page level or get the package through an HTTP handler where you can set expiration policies more precisely without affecting other resources that go with the page. In the current implementation, resources are extracted from the XAP package every time they are used. However, this is an aspect you can further improve on with the Downloader class. Likewise, the package delivers a user control but doesn’t track changes the user can force on its user interface. Tracking dynamic changes to the XAML tree is another topic and deserves an article of its own. There are two ways to access a Silverlight private file system on the local user’s machine. In all the code snippets for this column, I used the method GetUserStoreForApplication on the IsolatedStorageFile class. This method returns a token to access a section of the file system that is isolated per application, meaning that all and only the assemblies associated with an application will use the same store. You can also select a store and share it across all applications hosted on the same site. In this case, you get the token through the method GetUserStoreForSite. Note too that local storage can be administered through the Silverlight configuration dialog box (right-click on a Silverlight app) and even disabled completely. In this case, when attempting to get the token, an exception will be raised. A disk quota also applies to local storage on a per-domain basis; its default value is 1MB. Remember this when planning for a permanent Silverlight cache.  Dino Esposito is an architect at IDesign and the co-author of Microsoft .NET: Architecting Applications for the Enterprise (Microsoft Press, 2008). Based in Italy, Dino is a frequent speaker at industry events worldwide. You can join his blog at weblogs.asp.net/despos. February 2009 37 Some Final Notes msdnmagazine.com Caching the XAP package doesn’t mean caching individual resources such as DLLs, XAML animations, or multimedia content. http://weblogs.asp.net/despos 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

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