Silverlight how to: RSS feed stored as JSON in Isolated Storage

joel — Fri, 28 Mar 2008 22:07:51 +0000

This week I was reading two of my favorite Microsoft Evangelist blogs; Kirk Allen Evans’ and Tim Heuer and was inspired to build upon two of their posts. Kirk wrote a great entry on Creating a JSON Service with WebGet and WCF 3.5 and Tim on
Calling web services with Silverlight 2. I thought it would be interesting to take the concepts covered in both of these posts and put them together into my own how to. In my sample I read an RSS feed into a JSON string, stores it in Isolated storage, and displays it in a ListBox. Once displayed, I will check every 30 seconds to see if the RSS feed has changed. Out-of-the-box my solution is not practical, but illustrates a flexible technique for caching a serialized collection of data in the event the service is unavailable.

In the first part of my sample I check to see if I have a cache of RSS in Isolated Storage. If so, I use the “DataContractJsonSerializer” class to de-serialize the JSON array. Once de-serialized, I can bind it to my ListBox control in “Page.xaml”

void Page_Loaded(object sender, RoutedEventArgs e)
{
    RssService.RssItem[] items;
    if (_isf.FileExists("blog.json"))
    {
        //Read from IsolatedStorage the last set of blog entires
        using (IsolatedStorageFileStream isfs = 
         new IsolatedStorageFileStream("blog.json", System.IO.FileMode.Open, _isf))
        {
            //Deserialize the JSON array that was stored in Isolated storage.
            DataContractJsonSerializer djson = 
             new DataContractJsonSerializer(typeof(RssService.RssItem[]));
 
            items = djson.ReadObject(isfs) as RssService.RssItem[];
 
            isfs.Position = 0;
            using (StreamReader sr = new StreamReader(isfs))
            {
                this.txtBlock.Text = sr.ReadToEnd();
            }
            isfs.Close();
        }
        listBox.ItemsSource = items;
    }
 
    . . . . . . 
}

After we update our presentation of RSS, I will insert a animating UserControl to be use when we go to retrieve additional RSS items. This retrieval from our WCF 3.5 service will be executed every 10 seconds when our “DispatcherTimer” fires.

    //insert a UserControl that animates a spinner when we are retrieving 
    //an update of RSS data
    _indicator.HorizontalAlignment = HorizontalAlignment.Center;
    _indicator.VerticalAlignment = VerticalAlignment.Center;
    this.LayoutRoot.Children.Add(_indicator);
    _indicator.Visibility = Visibility.Collapsed;
 
    //we will use a timer here to simulate a 5 second delay in grabbing 
    //the next rss update
    _dt.Interval = new TimeSpan(0, 0, 10);
    _dt.Tick += new EventHandler(_dt_Tick);
    _dt.Start();

Upon successful retrieval from our WCF service, we will update our isolated storage by serializing our array of RssItems[] into JSON, and writing that text to a file.

void wcf_GetFeedsCompleted(object sender, 
    IsolatedStorage.RssService.GetFeedsCompletedEventArgs e)
{
    try
    {
        RssService.RssItem[] items = e.Result as RssService.RssItem[];
        //if we were able to grab a new set of Rss items then update Isolated storage
        if (items.Length > 0)
        {
            if (_isf.FileExists("blog.json"))
            {
                _isf.DeleteFile("blog.json");
            }
            using (IsolatedStorageFileStream isfs = 
             new IsolatedStorageFileStream("blog.json", System.IO.FileMode.Create, _isf))
            {
                //Take the array of RssItem[] objects and convert it to a JSON array.
                DataContractJsonSerializer djson = new DataContractJsonSerializer(items.GetType());
                MemoryStream ms = new MemoryStream();
                djson.WriteObject(ms, items);
 
                this.txtBlock.Text = System.Text.Encoding.UTF8.GetString
                   (ms.GetBuffer(), 0, Convert.ToInt16(ms.Length));
 
                isfs.Write(ms.GetBuffer(), 0, Convert.ToInt16(ms.Length));
                isfs.Close();
            }
            listBox.ItemsSource = items;
        }
        this.listBox.Opacity = 1;
        _indicator.Animation.Stop();
        _indicator.Visibility = Visibility.Collapsed;
    }
    catch (Exception ex)
    {
        this.txtBlock.Text = ex.Message;
    }
}

Thanks again to Tim and Kirk for the majority of my example.

Code: IsolatedStorage.zip

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Silverlight how to: RSS feed stored as JSON in Isolated Storage