Problem dynamically adding XML items to listbox, C#?

I'开发者_JAVA技巧m pulling feeds for my RSS project and I am running into a problem of not knowing how to allow the user to load more items into the collection. At the current moment, everything loads at once. While this is in some cases all right, I would like the user to be able to choose how things get loaded in case they have a slow mobile connection.

This is borrowed code and thus it only adds to my confusion.

Where could i be able to inject code into this sample to allow a dynamic loading of items, say, 30 at a time?

Rss Class:

namespace MyRSSService
{ 

public class RssService
{
    /// Gets the RSS items.
    /// <param name="rssFeed">The RSS feed.</param>
    /// <param name="onGetRssItemsCompleted">The on get RSS items completed.</param>
    /// <param name="onError">The on error.</param>
    public static void GetRssItems(string rssFeed, Action<IList<RssItem>> onGetRssItemsCompleted = null, Action<Exception> onError = null, Action onFinally = null)
    {
        WebClient webClient = new WebClient();

        // register on download complete event
        webClient.OpenReadCompleted += delegate(object sender, OpenReadCompletedEventArgs e)
        {
            try
            {
                // report error
                if (e.Error != null)
                {
                    if (onError != null)
                    {
                        onError(e.Error);
                    }
                    return;
                }

                // convert rss result to model
                IList<RssItem> rssItems = new List<RssItem>();                   
                Stream stream = e.Result;
                XmlReader response = XmlReader.Create(stream);
                {
                    SyndicationFeed feeds = SyndicationFeed.Load(response);

                    foreach (SyndicationItem f in feeds.Items)
                    {
                        RssItem rssItem = new RssItem(f.Title.Text, f.Summary.Text, f.PublishDate.ToString(), f.Links[0].Uri.AbsoluteUri);
                        rssItems.Add(rssItem);
                    }
                }    

                // notify completed callback
                if (onGetRssItemsCompleted != null)
                {
                    onGetRssItemsCompleted(rssItems);
                }
            }
            finally
            {
                // notify finally callback
                if (onFinally != null)
                {
                    onFinally();
                }
            }
        };
        webClient.OpenReadAsync(new Uri(rssFeed));
      }
    }
  }

items setting class:

namespace MyRSSService
{
public class RssItem
{
    /// <summary>
    /// Initializes a new instance of the <see cref="RssItem"/> class.
    /// </summary>
    /// <param name="title">The title.</param>
    /// <param name="summary">The summary.</param>
    /// <param name="publishedDate">The published date.</param>
    /// <param name="url">The URL.</param>
    public RssItem(string title, string summary, string publishedDate, string url)
    {
        Title = title;
        Summary = summary;
        PublishedDate = publishedDate;
        Url = url;

        // Get plain text from html
        PlainSummary = HttpUtility.HtmlDecode(Regex.Replace(summary, "<[^>]+?>", ""));
    }
    public string Title { get; set; }
    public string Summary { get; set; }
    public string PublishedDate { get; set; }
    public string Url { get; set; }
    public string PlainSummary { get; set; }
   }
 }

the binding C# to the page to display the feeds

public partial class FeedPage : PhoneApplicationPage
{
    private const string WindowsPhoneBlogPosts = "http://feeds.bbci.co.uk/news/rss.xml";

    public FeedPage()
    {
        InitializeComponent();
        RssService.GetRssItems(WindowsPhoneBlogPosts, (items) => { listbox.ItemsSource = items; }, (exception) => { MessageBox.Show(exception.Message); }, null);
    }
}

---

Unless the server where the feed is hosted provides an API to limit the number of returned items (for example, this practice is used for the Xbox Marketplace), you will be downloading the entire feed, even if you decide to only show a part of it.