在asp.net页面中使用异步读取

刚亲自撰写一篇技术文章，请大家多指教。哈哈。
原文首发：http://bbs.5inet.net/topic.aspx?topicid=181

有的时候我们需要在网页里读取论坛的信息，在传统ASP的时候我们使用的是JS或者是IFRAME，这两种方式都不是很方便，而且对搜索引擎不友好。现在有了.Net，我们有了另一种方式。

要求：论坛需要提供RSS支持。

代码如下：

    #region task class
    //这是一个任务类，执行具体的任务
    public class RssAsyncTask
    {
        private String _rssContent;
        private AsyncTaskDelegate _dlgt;
        private string rssUrl;
        private bool _success;

        public bool IsSuccess
        {
            get
            {
                return _success;
            }
        }

        public RssAsyncTask(string rssUrl)
        {
            this.rssUrl = rssUrl;
        }

        // Create delegate.
        protected delegate void AsyncTaskDelegate();

        public String GetRssContent()
        {
            return _rssContent;
        }
        public void DoTheAsyncTask()
        {
            // Introduce an artificial delay to simulate a delayed
            // asynchronous task. Make this greater than the
            // AsyncTimeout property.
            WebClient wc = new WebClient();
            try
            {
                _rssContent = wc.DownloadString(rssUrl);
                _success = true;
            }
            catch (Exception e)
            {
                _rssContent = e.Message;
            }
            finally
            {
                wc.Dispose();
            }
            //Thread.Sleep(TimeSpan.FromSeconds(5.0));
        }

        // Define the method that will get called to
        // start the asynchronous task.
        public IAsyncResult OnBegin(object sender, EventArgs e,
            AsyncCallback cb, object extraData)
        {
            //_rssContent = "Beginning async task.";

            _dlgt = new AsyncTaskDelegate(DoTheAsyncTask);
            IAsyncResult result = _dlgt.BeginInvoke(cb, extraData);

            return result;
        }

        // Define the method that will get called when
        // the asynchronous task is ended.
        public void OnEnd(IAsyncResult ar)
        {
            //_rssContent = "Asynchronous task completed.";
            _dlgt.EndInvoke(ar);
        }

        // Define the method that will get called if the task
        // is not completed within the asynchronous timeout interval.
        public void OnTimeout(IAsyncResult ar)
        {
            _rssContent = "Ansynchronous task failed to complete " +
                "because it exceeded the AsyncTimeout parameter.";
        }
    }
    #endregion

    //一个自定义的控件，继承自另一个自定义控件。
    public class RArticle
        : LPanel
    {
        #region properties
        string rssUrl;

        public string RssUrl
        {
            get { return rssUrl; }
            set { rssUrl = value; }
        }

        int maxRecordNumber = 6;

        public int MaxRecordNumber
        {
            get { return maxRecordNumber; }
            set { maxRecordNumber = value; }
        }
        #endregion

        RssAsyncTask task;
        protected override void OnInit(EventArgs e)
        {
            base.OnInit(e);
            task = new RssAsyncTask(this.rssUrl);
            PageAsyncTask asyncTask = new PageAsyncTask(task.OnBegin, task.OnEnd, task.OnTimeout, null);

            Page.RegisterAsyncTask(asyncTask);
            Page.ExecuteRegisteredAsyncTasks();
        }

        static Random r = new Random();
        protected override void Render(System.Web.UI.HtmlTextWriter writer)
        {
            string rssContent = task.GetRssContent();
            XmlDocument doc = null;
            if (task.IsSuccess)
            {
                doc = new XmlDocument();
                doc.LoadXml(rssContent);

                this.Title = doc.SelectSingleNode("rss/channel/title").InnerText;
                this.TitleNavigateUrl = doc.SelectSingleNode("rss/channel/link").InnerText;
                this.ShowTitle = true;
            }
            base.RenderBegin(writer);

            writer.WriteBeginTag("div");
            writer.WriteAttribute("class", "child2");
            Right(writer);
            writer.WriteBeginTag("ul");
            Right(writer);

            if (doc != null)
            {
                #region success

                XmlNodeList items = doc.SelectNodes("rss/channel/item");
                List<XmlNode> nodes = new List<XmlNode>();
                foreach (XmlNode node in items)
                    nodes.Add(node);

                //使用范型进行日期的倒序排列
                nodes.Sort(new Comparison<XmlNode>(delegate(XmlNode n1, XmlNode n2)
                {
                    DateTime d1 = DateTime.Parse(n1.SelectSingleNode("pubDate").InnerText);
                    DateTime d2 = DateTime.Parse(n2.SelectSingleNode("pubDate").InnerText);
                    TimeSpan ts = d2 - d1;
                    return (int)ts.TotalSeconds;
                }));

                for (int i = 0; i < maxRecordNumber; i++)
                {
                    XmlNode node = nodes[i];
                    writer.WriteBeginTag("li");
                    Right(writer);
                    writer.WriteBeginTag("a");
                    writer.WriteAttribute("target", "_blank");
                    writer.WriteAttribute("href", node.SelectSingleNode("link").InnerText);
                    Right(writer);
                    writer.Write(node.SelectSingleNode("title").InnerText);
                    writer.WriteEndTag("a");
                    writer.WriteEndTag("li");
                }

                #endregion
            }
            else
            {
                writer.WriteBeginTag("pre");
                Right(writer);
                writer.Write(task.GetRssContent());
                writer.WriteEndTag("pre");
            }

            writer.WriteEndTag("ul");
            writer.WriteEndTag("div");

            RenderChildren(writer);

            base.RenderEnd(writer);
        }
    }
使用方法：
一、注册控件

<%@ Register Assembly="Controls" Namespace="Limited.Controls" TagPrefix="lm" %>
二、调用

<lm:RArticle ID="RArticle1" runat="server" MaxRecordNumber="10" RssUrl="http://bbs.5inet.net/rss.aspx" />
为了简便起见，本程序就没有使用缓存之类的技术了，如有必要，请自行添加。