ajax处理耗时事件，显示loading的方法

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm5.aspx.cs" Inherits="WebApplication1.WebForm5" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <script language="javascript" type="text/javascript">
        var xmlHttp;
        //xmlHttpRequest
        function createXMLHttpRequest()
        {
            if (window.ActiveXObject)
                xmlHttp = new ActiveXObject("Microsoft.XMLHTTP"); //IE
            else if (window.XMLHttpRequest)
                xmlHttp = new XMLHttpRequest(); //Firefox
        }

        function htmer_loading()
        {
            createXMLHttpRequest();
            xmlHttp.onreadystatechange = function ()
            {
                if (xmlHttp.readyState == 4 && xmlHttp.status == 200)
                //若响应完成，则显示htmer.asp中的内容
                    document.getElementById("show").innerHTML = xmlHttp.responseText;
                else
                //若响应未完成，则显示Loading
                    document.getElementById("show").innerHTML = "Loading......";
            }
            xmlHttp.open("GET", "Handler1.ashx", true);
            xmlHttp.send(null);
        }
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <input type="button" value="check ajax" onclick="htmer_loading()" />
    </div>
    <div id="show">
    </div>
    </form>
</body>
</html>

耗时请求页面处理Handler1.ashx

public class Handler1 : IHttpHandler
{

        public void ProcessRequest(HttpContext context)
        {
            context.Response.ContentType = "text/plain";

Thread.Sleep(3000);

context.Response.Write("<Br/> test result");

context