Ajax实现无刷新树

1.建立一个aspx页面
html代码

< html  xmlns ="http://www.w3.org/1999/xhtml"   >
< head  id ="Head1"  runat ="server" >
     < title > 小山 </ title >
     < link  type ="text/css"  href ="../../Styles/tree_css/tree.css"  rel ="stylesheet" >
</ head >
< body >
     < form  id ="Form1"  runat ="server" >
     < table  width =100%  cellpadding =0  cellspacing =0  border =0 >
         < colgroup >
             < col  width =180  />
             < col  />
         </ colgroup >
         < tr >
             < td >
                 < div  class ="TreeMenu"  id ="CategoryTree"  style ="width: 100%; height: 489px" >
                 </ div >
             </ td >
             < td >
                 < iframe  id =furl  height =20  style ="height: 497px; width: 100%;" ></ iframe >
             </ td >
         </ tr >
     </ table >     
                            
             < script  language ="jscript" >
            function el(id)
            {
                return document.getElementById(id);                
            }
            function ExpandSubCategory(iCategoryID)
            {
                var li_father = el("li_" + iCategoryID);
                if (li_father.getElementsByTagName("li").length > 0) //分类已下载
                {
                    ChangeStatus(iCategoryID);
                    return;
                }
                
                li_father.className = "Opened";
                
                switchNote(iCategoryID, true);
                AjaxMethod.GetSubCategory(iCategoryID, GetSubCategory_callback);                
            }

            function GetSubCategory_callback(response)
            {
                var dt = response.value.Tables[0];
                if (dt.Rows.length > 0)
                {
                    var iCategoryID = dt.Rows[0].FatherID;
                }
                var li_father = el("li_" + iCategoryID);
                var ul = document.createElement("ul");
                for (var i = 0;i < dt.Rows.length;i++)
                {
                    if (dt.Rows[i].IsChild == 1) //叶子节点
                    {
                        var li = document.createElement("li");
                        li.className = "Child";
                        li.id = "li_" + dt.Rows[i].CategoryID;
                        
                        var img = document.createElement("img");
                        img.id = dt.Rows[i].CategoryID;
                        img.className = "s";
                        img.src = "../../Styles/tree_css/s.gif";
                        
                        var a = document.createElement("a");
                        var id = dt.Rows[i].CategoryID;
                        a.onmouseover = function()
                        {
                            PreviewImage(id);
                        };
                        a.href = "javascript:OpenDocument('" + dt.Rows[i].CategoryID + "');";
                        a.innerHTML = dt.Rows[i].CategoryName;
                    }
                    else
                    {
                        var li = document.createElement("li");
                        li.className = "Closed";
                        li.id = "li_" + dt.Rows[i].CategoryID;
                        
                        var img = document.createElement("img");
                        img.id = dt.Rows[i].CategoryID;
                        img.className = "s";
                        img.src = "../../Styles/tree_css/s.gif";
                        img.onclick = function () {
                            ExpandSubCategory(this.id);
                        };
                        img.alt = "展开/折叠";
                        
                        var a = document.createElement("a");
                        a.href = "javascript:ExpandSubCategory(" +
                            dt.Rows[i].CategoryID + ");";
                        a.innerHTML = dt.Rows[i].CategoryName;
                    }
                    li.appendChild(img);
                    li.appendChild(a);
                    ul.appendChild(li);    
                }
                li_father.appendChild(ul);
                
                switchNote(iCategoryID, false);
            }
            
            // 叶子节点的单击响应函数
            function OpenDocument(iCategoryID)
            {
                // 预加载信息
                PreloadFormUrl(iCategoryID);
            }
            
            function PreviewImage(iCategoryID)
            {
                
            }

            function ChangeStatus(iCategoryID)