回调实现无刷新级联下拉框

以northwind数据库为例。选择分类下拉框的一项后，无刷新更改产品下拉框的显示

前台代码：

<% ... @ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default"  %>

<! 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 > 无标题页 </ title >
     < script  type ="text/javascript" > ...
        function CallServer(args,context)
        ...{
            context.innerHTML = "Loading...";
            <%= ClientScript.GetCallbackEventReference(this,"args","GetServerData","context") %>
        }
        function GetServerData(result,context)
        ...{
            context.innerHTML = result;
        }
     </ script >
</ head >
< body >
     < form  id ="form1"  runat ="server" >
     < div >
         < asp:DropDownList  ID ="ddlCategory"  runat ="server" ></ asp:DropDownList >
         < span  id ="pnlProduct" >
         < asp:DropDownList  ID ="ddlProduct"  runat ="server" ></ asp:DropDownList >
         </ span >
     </ div >
     </ form >
</ body >
</ html >

后台代码：

using  System;
using  System.Data;
using  System.Data.SqlClient;
using  System.IO;
using  System.Configuration;
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;

public   partial   class  _Default : System.Web.UI.Page,ICallbackEventHandler
... {
    protected string callbackResult;

    protected void Page_Load(object sender, EventArgs e)
    ...{
        if (!IsPostBack)
        ...{
            BindToCategory();
            this.ddlCategory.Attributes.Add("onchange", "CallServer(this.value,pnlProduct)");
            this.ddlProduct.Items.Add(new ListItem("--产品--", "-1"));
        }
    }

    /**//// <summary>
    /// 绑定分类
    /// </summary>
    private void BindToCategory()
    ...{
        SqlConnection conn = new SqlConnection("Server=(local);database=northwind;uid=sa;pwd=123");     
        SqlDataAdapter sda = new SqlDataAdapter("SELECT categoryID,categoryName FROM categories", conn);
        DataTable dt = new DataTable();
        sda.Fill(dt);
        this.ddlCategory.DataSource = dt;
        this.ddlCategory.DataTextField = "categoryName";
        this.ddlCategory.DataValueField = "categoryID";
        this.ddlCategory.DataBind();
        this.ddlCategory.Items.Insert(0, new ListItem("--分类--", "-1"));
    }

    private void BindToProduct(int categoryID)
    ...{
        SqlConnection conn = new SqlConnection("Server=(local);database=northwind;uid=sa;pwd=123");
        SqlDataAdapter sda = new SqlDataAdapter("SELECT productID,productName FROM products WHERE categoryID=" + categoryID, conn);
        DataTable dt = new DataTable();
        sda.Fill(dt);
        this.ddlProduct.DataSource = dt;
        this.ddlProduct.DataTextField = "productName";
        this.ddlProduct.DataValueField = "productID";
        this.ddlProduct.DataBind();
        if (ddlProduct.Items.Count == 0)
        ...{
            this.ddlProduct.Items.Add(new ListItem("--产品--", "-1"));
        }
    }

    private string RenderElement(Control control)
    ...{
        StringWriter writer = new StringWriter();
        HtmlTextWriter output = new HtmlTextWriter(writer);
        control.RenderControl(output);
        output.Flush();
        output.Close();
        string htmlCode = writer.ToString();
        writer.Close();
        return htmlCode;
    }

    void ICallbackEventHandler.RaiseCallbackEvent(string str)
    ...{
        this.callbackResult = str;
    }

    string ICallbackEventHandler.GetCallbackResult()
    ...{
        int categoryID = Convert.ToInt32(callbackResult);
        BindToProduct(categoryID);
        callbackResult = RenderElement(this.ddlProduct);
        return callbackResult;
    }
}