如何基于Ajax技术实现无刷新用户登录功能？

// JScript 文件
function usersLogon()
{
 var userName = document.getElementById("txtuserName").value;
 var password = document.getElementById("txtpassword").value;
 var checkCode = document.getElementById("txtCheckCode").value;

 var response = userControl_logon.CheckCodeIsRight(checkCode).value;

 if(userName == "")
 {
  document.getElementById("txtuserName").focus();
  return false;
 }
 else if(password == "")
 {
  document.getElementById("txtpassword").focus();
  return false;
 }
 else if(checkCode =="")
 {
  document.getElementById("txtCheckCode").focus();
  return false;
 }
 else
 {
  if(response == true)
  {
   //判断用户是否存在
   userControl_logon.userNameAndPasswordIsExist(userName,password,userNameIsRight);
  }
  else
  {
   alert("验证码出错");
   userControl_logon.checkCodeOperaotr(refreshCheckCode);
   document.getElementById("txtpassword").value = "";
  }
 } 
}
function userNameIsRight(res)
{
 var userName = document.getElementById("txtuserName").value;
 if(res.value == true)
 {
  //用户存在,但要看此用户有没有进入管理留言版权限,
  userControl_logon.userNameIsRight(userName,CallBack);
 }
 else
 {
  alert("用户名或密码错误");
  document.getElementById("txtpassword").value = "";
  OnLoad_checkCode();
 }
}
function CallBack(res)
{
 if(res.value == true)
 {
  hideLogon();
  var url = userControl_logon.returnUrl();
  if ( url.value == 404)
  {
   showDefault();
  }
  else
  {
   document.getElementById("Url").innerHTML = '<a href="' + url.value + '">' + url.value + '</a>'
  }
 }
 else
 {
  alert("对不起你的权限不够");
  document.getElementById("txtpassword").value = "";
  OnLoad_checkCode();
 }
}
//隐藏登录框
function hideLogon()
{
 var element = document.getElementById("hideLogon")
 element.style.display = "none"
 
}
//显示返回首页
function showDefault()
{
 var element = document.getElementById("Returndefault")
 element.style.display = "block" 
}
function OnLoad_checkCode()
{
 userControl_logon.checkCodeOperaotr(refreshCheckCode);
 document.getElementById("txtuserName").focus();
 // return false;
}
///重新得到新的验证吗
function refreshCheckCode(res)
{ 
 document.getElementById("txtCheckCode").value = "";
 document.getElementById("lblNumber").innerHTML = res.value;
}
function abce()
{
 alert(document.getElementById("lblNumber").value)
}

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Drawing;
using LHB_SQL_2005;

public partial class userControl_logon : System.Web.UI.UserControl
{
 protected void Page_Load(object sender, EventArgs e)
 {
  if (!this.IsPostBack)
  {
   AjaxPro.Utility.RegisterTypeForAjax(typeof(userControl_logon));
  }
 }

 [AjaxPro.AjaxMethod]
 public static string checkCodeOperaotr()
 {
  string _checkCode = GeneralMethod.GenerateCheckCode();
  System.Web.HttpContext.Current.Session["checkCode"] = _checkCode;
  //返回验证码
  return _checkCode;
 }

 /// <summary>
 /// 判断验证是否正确
 /// </summary>
 /// <param name="checkCode"></param>
 /// <returns></returns>
 [AjaxPro.AjaxMethod]
 public static bool CheckCodeIsRight(string checkCode)
 {
  string _checkCode = (string)(System.Web.HttpContext.Current.Session["checkCode"]); 
  if (_checkCode == checkCode)
  {
   return true;
  }
  else
  {
   return false;
  }
 }
 /// <summary>
 /// 判断用户名及密码添加是否正确
 /// </summary>
 /// <param name="userName">用户名</param>
 /// <param name="_password">用户名密码</param>
 /// <returns>bool</returns>
 [AjaxPro.AjaxMethod]
 public static bool userNameAndPasswordIsExist(string userName, string _password)
 {
  string password = GeneralMethod.ToEncryptPassword(_password);
  string executeString = "SELECT COUNT(*) FROM users WHERE userName = '" + userName.ToString() + "' AND password = '" + password + "'";
  int count = int.Parse(GetCommand.ExecuteScalar(executeString));
  if (count == 1)
  {
   System.Web.HttpContext.Current.Session["userName"] = userName;
   return true;
  }
  else
  {
   return false;
  }
 }

 /// <summary>
 /// 判断用户是不是有这进入管理留言版的权限
 /// </summary>
 /// <param name="userName">用户名</param>
 /// <returns></returns>
 [AjaxPro.AjaxMethod]
 public static bool userNameIsRight(string userName)
 {
  string executeString = "SELECT [right] FROM role WHERE usersId = (select userNameId from users where userName = '" + userName + "')";
  int count = int.Parse(GetCommand.ExecuteScalar(executeString));
  if (count > 0)
  {
   return true;
  }
  else
  {
   return false;
  }
 }

 /// <summary>
 /// 返回Url值
 /// </summary>
 /// <returns></returns>
 [AjaxPro.AjaxMethod]
 public static string returnUrl()
 {
  string url = "";
  try
  {
   url = System.Web.HttpContext.Current.Session["url"].ToString();
  }
  catch
  {
   url ="404";
  }
  return url;
 }
}

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="logon.ascx.cs" Inherits="userControl_logon" %>
<script language="javascript" type="text/javascript" src="../JavaScript/logon.js">
</script>
<script language="javascript" type="text/javascript" src="JavaScript/logon.js">
</script>
<link href="../CSS/table_css.css" rel="stylesheet" type="text/css" />
<link href="CSS/table_css.css" rel="stylesheet" type="text/css" />
<body onload="OnLoad_checkCode();">
<div>
<table border="0" cellpadding="0" cellspacing="0">
  <tr>
   <td>
    <table id="hideLogon" border="0" cellpadding="0" cellspacing="0" style="display:block;">
     <tr>
      <td style="background-color: #99ccff">用户名：</td>
      <td><input type="text" id="txtuserName" style="width: 105px" /></td>
     </tr>
     <tr>
      <td style="background-color: #99ccff">密 码：</td>
      <td>
       <input id="txtpassword" type="password" style="width: 105px" /></td>
     </tr>
     <tr>
      <td style="background-color: #99ccff">验证码：</td>
      <td style="background-color: #99ccff">
       <input type= "text" id="txtCheckCode" style=" width:60px" /><label id="lblNumber"></label></td>
     </tr>
     <tr>
      <td style="background-color: #99ccff"></td>
      <td style="background-color: #99ccff">
       <input type="button" onclick="usersLogon();" value="登录" id="btnLogon" /></td>
     </tr>
    </table>
   </td>
  </tr>
  <tr>
   <td >
    <div id="Url"></div>
   </td>
  </tr>
  <tr>
   <td align="center">
    <table id="Returndefault" border="0" cellpadding="0" cellspacing="0" style="display:none;">
     <tr>
      <td>
       <asp:HyperLink ID="HyperLink1" runat="server" NavigateUrl="~/Default.aspx">返回首页</asp:HyperLink></td>
     </tr>
    </table>
   </td>
  </tr>
 </table>
</div>
</body>