对asp.net实现DropDownList、ListBox无刷新三级联动(javascript)的改进

上篇文章说明了如何实现三级联动，但如果数据量较大，加载时会慢一些，因此经过测试，将需要读取的数据先写入一个文本文件中，然后在加载页面时直接调用此文本文件的内容，省去了读取数据库浪费的时间，数据量大时速度比先前快了许多。
数据库请见上一篇文章“asp.net实现DropDownList、ListBox无刷新三级联动(javascript)”

生成数据文本文件的代码

         // 将地区信息写入文本文件area.js
         // 添加二级地区
         string  strAreaData, strSQL;
        strAreaData  =   " <script language = 'JavaScript' type='text/javascript'>var onecount;onecount=0;subcat = new Array(); " ;
        strSQL  =   " select * from nts_area where oneid<>0 and twoid<>0 and threeid=0 order by sort " ;
        IDataReader dr  =  DataClass.GetDataReader(strSQL);
         int  i1  =   0 ;
         while  (dr.Read())
         {
            strAreaData += "subcat[" + i1.ToString() + "] = new Array('" + dr["areaname"].ToString() + "','" + dr["oneid"].ToString() + "','" + dr["twoid"].ToString() + "');";
            i1++;

        }
        strAreaData  +=   " onecount= "   +  i1.ToString()  +   " ; " ;

         // 添加三级地区
        strAreaData  +=   " var onecount1;onecount1=0;subcat1 = new Array(); " ;

        strSQL  =   " select * from nts_area where oneid<>0 and twoid<>0 and threeid<>0 order by sort " ;
        dr  =  DataClass.GetDataReader(strSQL);
         int  n1  =   0 ;
         while  (dr.Read())
         {
            strAreaData += "subcat1[" + n1.ToString() + "] = new Array('" + dr["areaname"].ToString() + "','" + dr["oneid"].ToString() + "','" + dr["twoid"].ToString() + "','" + dr["threeid"].ToString() + "');"; n1++;

        }
        dr.Close();
        strAreaData  +=   " onecount1= "   +  n1.ToString()  +   " ;</script> " ;

        StreamWriter str;
        File.Delete(Server.MapPath( " ~/js/area.js " ));

        str  =  File.CreateText(Server.MapPath( " ~/js/area.js " ));
        str.WriteLine(strAreaData);
        str.Close();

aspx页面代码

<%-- 地区无刷新显示    --%>              
<%  Response.WriteFile(Server.MapPath( " ~/js/area.js " ));  %>
     < script language  =   " JavaScript "  type = " text/javascript " >

    function changearea1()
     {
    document.all("<% =Ddl_AreaTwo.ClientID %>").length = 0; 
    document.all("<% =Ddl_AreaTwo.ClientID %>").options[0] = new Option('选择城市','0');
    document.all("<% =Ddl_AreaThree.ClientID %>").length = 0; 
    document.all("<% =Ddl_AreaThree.ClientID %>").options[0] = new Option('选择城市','0');
    var oneid=document.all("<% =Ddl_AreaOne.ClientID %>").value;
    var i;
    for (i=0;i < onecount; i++)
        {
            if (subcat[i][1] == oneid)
            { 
                document.all("<% =Ddl_AreaTwo.ClientID %>").options[document.all("<% =Ddl_AreaTwo.ClientID %>").length] = new Option(subcat[i][0], subcat[i][2]);
            }        
        }
        
    }
    
        function changearea2()
     {
    document.all("<% =Ddl_AreaThree.ClientID %>").length = 0; 
    document.all("<% =Ddl_AreaThree.ClientID %>").options[0] = new Option('选择城市','0');
    var twoid=document.all("<% =Ddl_AreaTwo.ClientID %>").value;
    var oneid=document.all("<% =Ddl_AreaOne.ClientID %>").value;
    var i;
    for (i=0;i < onecount1; i++)
        {
            if (subcat1[i][2] == twoid)
            { 
                if (subcat1[i][1] == oneid)
                {
                    document.all("<% =Ddl_AreaThree.ClientID %>").options[document.all("<% =Ddl_AreaThree.ClientID %>").length] = new Option(subcat1[i][0], subcat1[i][3]);
                }        
            }
        }
    }       
    
     </ script >                 
                     < asp:DropDownList ID = " Ddl_AreaOne "  runat = " server "   AppendDataBoundItems = " True " >
                         < asp:ListItem Value = " 0 " > 请选择 </ asp:ListItem >
                     </ asp:DropDownList >
                     < asp:DropDownList ID = " Ddl_AreaTwo "  runat = " server "   >
                         < asp:ListItem Value = " 0 " > 请选择 </ asp:ListItem >
                     </ asp:DropDownList >
                     < asp:DropDownList ID = " Ddl_AreaThree "  runat = " server "   >
                         < asp:ListItem Value = " 0 " > 请选择 </ asp:ListItem >
                     </ asp:DropDownList >

cs页面代码

     protected   void  Page_Load( object  sender, EventArgs e)
     {
        Ddl_AreaOne.Attributes.Add("onchange", "changearea1()");
        Ddl_AreaTwo.Attributes.Add("onchange", "changearea2()");
        string strSQL;
        strSQL = "select * from nts_area where oneid<>0 and twoid=0 and threeid=0 order by sort";
        Ddl_AreaOne.DataSource = DataClass.GetDataReader(strSQL);    //此为我写的读取DataReader对象的类
        Ddl_AreaOne.DataTextField = "areaname";
        Ddl_AreaOne.DataValueField = "oneid";
        Ddl_AreaOne.DataBind();
    }