异步handler两个例子

实例1 ： 在异步handler中异步访问网络 并返回结果

 public class Demo1 : IHttpAsyncHandler
    {


        #region IHttpAsyncHandler Members

        public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
        {

            context.Response.Write("begin");


            var request = (HttpWebRequest)WebRequest.Create("http://www.baidu.com");

            return request.BeginGetResponse(ar =>
            {
                var response = request.EndGetResponse(ar);

                byte[] data = new byte[response.ContentLength];

                response.GetResponseStream().Read(data, 0, data.Length);

                response.Close();

                context.Response.Write(Encoding.Default.GetString(data));
               
                cb(ar); //通知asp.net异步请求已完成
            }, context);

        }

        public void EndProcessRequest(IAsyncResult result)
        {

            var context = (HttpContext)result.AsyncState; //获取我们在BeginGetResponse中传入的HttpContext
            context.Response.Write("end");

        }

        #endregion

        #region IHttpHandler Members

        public bool IsReusable
        {
            get { return false; }
        }

        public void ProcessRequest(HttpContext context)
        {

        }

        #endregion
    }

实例2：长连接聊天室default页面发送消息。GetMessage异步handler获取最新消息。可以使用ajax轮询的调用GetMessage.消息发送时候也可以加入消息队列机制和客户端确认机制。防止消息丢失！有空在整理吧
default.aspx

    <div>
    
        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
        <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
        <asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" />
    
    </div>

default.aspx.cs

 public partial class _Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            string name = TextBox1.Text.Trim();
            string content = TextBox2.Text;


            var noticeList = GetMessage.AsyncGetMessageList.Where(ar => ar.UserName == name).ToList();

            foreach (var ar in noticeList)
            {
                GetMessage.AsyncGetMessageList.Remove(ar);
                ar.HttpContext.Response.Write(content);
                ar.Complete();
            }
        }
    }

 GetMessage

namespace AsyncHanlder
{

    public class GetMessage : IHttpAsyncHandler
    {

        public static IList<AsyncGetMessageResult> AsyncGetMessageList = new List<AsyncGetMessageResult>();
        public void ProcessRequest(HttpContext context)
        {

        }

        public bool IsReusable
        {
            get
            {
                return false;
            }
        }

        #region IHttpAsyncHandler Members

        public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
        {
            var ar = new AsyncGetMessageResult(context, cb, extraData);

            ar.UserName = "xhan";
            AsyncGetMessageList.Add(ar);

            return ar;
        }

        public void EndProcessRequest(IAsyncResult result)
        {
            //throw new NotImplementedException();
        }

        #endregion
    }
    public class AsyncGetMessageResult : IAsyncResult
    {
        public HttpContext HttpContext { get; set; }
        public AsyncCallback AsyncCallback { get; set; }
        public string UserName { get; set; }

        private object state;
        private bool isCompleted;

        #region IAsyncResult Members

        public object AsyncState
        {
            get { return state; }
        }

        public System.Threading.WaitHandle AsyncWaitHandle
        {
            get { return null; }
        }

        public bool CompletedSynchronously
        {
            get { return false; }
        }

        public bool IsCompleted
        {
            get { return isCompleted; }
        }

        #endregion

        public AsyncGetMessageResult(HttpContext ctx, AsyncCallback cb, object state)
        {
            this.HttpContext = ctx;
            this.AsyncCallback = cb;
            this.state = state;
            this.isCompleted = false;
        }

        public void Complete()
        {
            this.isCompleted = true;
            this.AsyncCallback(this);
        }
    }
}

 另外记得不再在异步操作中调用

ThreadPool.QueueUserWorkItem或者异步调用委托来实现多线程。因为两者都是使用的线程池中线程来执行方法的，所以这种方式来使用异步完全是没有意义的
。应尽量避免对CPU密集操作使用异步。那到底什么情况下使用异步的呢？主要是在做IO密集操作时候使用异步才有意义。这里指的是采用异步方式调用io操作。
一般应是调用.net提供的异步io接口来实现异步io，比如异步访问网络，异步读写文件(stream.BeiginXXX)，异步访问数据库(sqlcommand.beginxxx)等。这些异步方法
底层都是通过io完成端口来实现异步io，只需要少数的io线程处理大量io请求。从而在等待io返回时根本不用阻塞工作线程。从而使系统有更好的伸缩性和吞吐量。