MSDN Magazine - February 2009 - (Page 26)

Figure 7 Saving Serialized Data to an Isolated Storage File private void SaveFeedsToStorage_UsingFile() { using (var isoStore = IsolatedStorageFile. GetUserStoreForApplication()) { List data = GetFeedsFromStorage_UsingFile(); if (data == null) if (txtAddress.Text.Length == 0) return; else data = new List (); using (var isoStoreFileStream = new IsolatedStorageFileStream(FEED_FILENAME, FileMode.Create, isoStore)) { data.Add(txtAddress.Text); byte[] bytes = Serialize(data); isoStoreFileStream.Write(bytes, 0, bytes.Length); } } } When the SaveFeedsToStorage_UsingSettings method is executed, it first calls the GetFeedsFromStorage_UsingSettings method, which grabs the addresses of all of the feeds from isolated storage and puts them in a single string delimited by a special character. When the app first starts, the LoadFeedsFromStorage_UsingSettings method retrieves the feeds from isolated storage: private void LoadFeedsFromStorage_UsingSettings() { string data = LoadFeedsFromStorage_UsingSettings(); string[] feedList = data.Split(new string[1] { FEED_DELIMITER }, StringSplitOptions.RemoveEmptyEntries); foreach (var address in feedList) LoadFeed(new Uri(address)); } The code first reads the list of Uri addresses for each feed from isolated storage. Then it iterates through the addresses and loads each individual feed one at a time, using the LoadFeed method. This feature allows the application to remember the feed addresses for the user and load them when the user runs the application. Packing Uri addresses into a delimited string is simple but neither elegant nor expansive. For example, if you wanted to store more than just a single scalar value, this would get complicated using this technique. Another way to store data in isolated storage is to use the IsolatedStorageFile and IsolatedStorageFileStream classes, which let you store more complex data structures, including serialized objects, per user. The data can even be segmented into different files and folders in isolated storage. This is ideal for organizing data that will be saved in isolated storage. For example, a folder could be created for all static lists of data, and a separate file for each list could be created. So within a folder in isolated storage, a file might exist for name prefixes, another for gender, and yet another for U.S. states. The sample application could create a file in isolated storage to contain the list of Uri addresses. The data must first be serialized and then sent to the file in isolated storage (as shown in Figure 7). First, an instance of the IsolatedStorageFile class for the current user is created using the GetUserStoreForApplication method. Then a file stream is created so the application can write the Uri address. The data is then serialized and written to the IsolatedStorageFileStream instance. The example for this application serializes a string, but any serializable object can be written to isolated storage as well. Reading the serialized data out of a file from isolated storage is a little more involved than the previous example. Figure 8 shows that first you must get an instance of the IsolatedStorageFile class for the user, and then check if the file exists before you read it. If the file exists, the file is opened for read access, which allows the data to be read via a stream of type IsolatedStorageFileStream. The data is read from the stream, put together, and then deserialized so it can be used to load the syndicated feeds. For simpler data structures, using serialized objects and files in isolated storage may not be necessary. However, when isolated storage is used for several types of local storage, it can help organize the data and provide easy access to read and write to it. Isolated storage should be used wisely to store data that should be cached locally. Data Points 3421db0&_render=rss. Since there is a cross-domain policy file located at http://pipes.yahooapis.com/clientaccesspolicy.xml that allows open access, this is a good alternative. A third option is to use a service such as Popfly or FeedBurner to aggregate the feeds, effectively relaying them through a service that also has an open cross-domain policy. Finally, a fourth option would be to write your own custom Web service that gathers the feeds and then relays them to the Silverlight application. Using a service such as Popfly or Yahoo Pipes offers the simplest solutions. Organized Isolated Storage Basic Isolated Storage The sample app lets a user add several feeds and view all of the items for each of those feeds. If a user enters 10 feeds and decides that he needs to close the app and come back later to read them, he would likely expect that the feeds would be remembered by the application. Otherwise, he’d have to enter the Uri for each feed every time the app is opened. Since these feeds are specific to a user, they can be stored either on the server using some token to identify the user that entered them or on the user’s computer. Silverlight allows data to be stored to a protected area of the user’s computer using the classes in the System.IO.IsolatedStorage namespace. Silverlight Isolated Storage is like cookies on steroids: it allows you to store simple scalar values or even store serialized complex object graphs on the client computer. The simplest way to save to isolated storage is to create an ApplicationSettings entry and stuff your data in it, as shown here: private void SaveFeedsToStorage_UsingSettings() { string data = GetFeedsFromStorage_UsingSettings() + FEED_DELIMITER + txtAddress.Text; if (IsolatedStorageSettings.ApplicationSettings.Contains(FEED_DATA)) IsolatedStorageSettings.ApplicationSettings[FEED_DATA] = data; else IsolatedStorageSettings.ApplicationSettings.Add(FEED_DATA, data); } This can be called every time a SyndicationFeed is added or removed from the ObservableCollection field instance called _feeds. Since the ObservableCollection exposes a CollectionChanged event, a handler can be assigned to the event that performs the save, as shown here: _feeds.CollectionChanged += ((sender, e) => { SaveFeedsToStorage_UsingSettings(); }); 26 msdn magazine

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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