ASP.NET and Ajax - using XmlHttpRequest

1. 使用AJAX

If you are not interested in using the ASP.NET AJAX library offered for ASP.NET, but would like to feature small amounts of AJAX functionality on your pages, you can do this easily with some javascript and a receptive page.

 

UPDATE: A much simpler way to add small doses of AJAX fuctionality to ASP.NET Web Forms can be achieved by using jQuery. Please see this article: http://www.mikesdotnetting.com/Article/104/Many-ways-to-communicate-with-your-database-using-jQuery-AJAX-and-ASP.NET

I only use AJAX in small doses on web sites - mainly to improve the user-experience. Typical instances will involve retrieving a full record based on a users sleection in a drop down list. I don't want to perform a full page postback, so I get the javascript xmlhttpserver object to do it for me behind the scenes. Here's an example that fetches the address details for a Northwind Traders customer.

 

First, a generic function to instantiate an instance of xmlhttprequest or xmlhttp, depending on the browser:

  function GetXmlHttpObject(handler)
  { 
  var objXmlHttp=null
  if (navigator.userAgent.indexOf("MSIE")>=0)
  { 
  var strName="Msxml2.XMLHTTP"
  if (navigator.appVersion.indexOf("MSIE 5.5")>=0)
  {
  strName="Microsoft.XMLHTTP"
  } 
  try
  { 
  objXmlHttp=new ActiveXObject(strName)
  objXmlHttp.onreadystatechange=handler 
  return objXmlHttp
  } 
  catch(e)
  { 
  alert("Error. Scripting for ActiveX might be disabled") 
  return 
  } 
  } 
  if (navigator.userAgent.indexOf("Mozilla")>=0)
  {
  objXmlHttp=new XMLHttpRequest()
  objXmlHttp.onload=handler
  objXmlHttp.onerror=handler 
  return objXmlHttp
  }
  } 

Now, a simple dropdownlist populated by a SqlDataSource control that fetches the CustomerID and Customer Name. Note the empty div, CustomerDetails below the dropdownlist:

<div>
<asp:DropDownList ID="DropDownList1" 
runat="server" 
DataSourceID="SqlDataSource1"
DataTextField="CompanyName" 
DataValueField="CustomerID">
</asp:DropDownList>

<asp:SqlDataSource 
ID="SqlDataSource1" 
runat="server" 
ConnectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=|DataDirectory|Northwind.mdb"
DataSourceMode="DataReader" 
ProviderName="System.Data.OleDb"
SelectCommand="SELECT [CustomerID], [CompanyName] FROM [Customers]">
</asp:SqlDataSource>

</div>
<div id="CustomerDetails"></div>

Two more javascript functions are needed. One to fire the GetXmlHttpObject method, and denote the page to send a request to, and one to handle the response by checking the readtState property of the object for a value of 4 or complete, then to write the reponse to the empty div:

  function GetCustomer(id)
  { 
  var url="FetchCustomer.aspx?CustomerID=" + id ;
  xmlHttp=GetXmlHttpObject(stateChanged);
  xmlHttp.open("GET", url , true);
  xmlHttp.send(null);
  } 
  
  
  function stateChanged() 
  { 
  if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
  {
  document.getElementById('CustomerDetails').innerHTML=xmlHttp.responseText;
  }
  } 

So now we have a page that gets called by xmlhttp (FetchCustomer.aspx) and we have a value to pass to the querystring for the page. So the next thing to do is to add an event handler to the DropDownList that will fire the GetCustomer() event and pass a CustomerID value:

  if (!Page.IsPostBack)
  {
  DropDownList1.DataBind();
  DropDownList1.Items.Insert(0, "");
  }

  DropDownList1.Attributes["onChange"] = "GetCustomer(this.value);";
  HttpResponse myHttpResponse = Response;
  HtmlTextWriter myHtmlTextWriter = new HtmlTextWriter(myHttpResponse.Output);
  DropDownList1.Attributes.AddAttributes(myHtmlTextWriter);

And finally, the code for the FetchCustomer.aspx page. Everyting has been removed from the .aspx file except the first line. We don't want DocTypes or default <form> tags disturbing the piece of html to be emitted in the response:

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

And the code-behind makes doubly sure by calling Response.Clear() before querying the database and emitting the resulting data as html:

using System;
using System.Data;
using System.Data.OleDb;
using System.Text;

public partial class FetchCustomer : System.Web.UI.Page
{
  protected void Page_Load(object sender, EventArgs e)
  {
    Response.Clear();
    StringBuilder sb = new StringBuilder();
    sb.Append("<br />");
    string provider = "Provider=Microsoft.Jet.OLEDB.4.0;";
    string db = "Data Source=|DataDirectory|Northwind.mdb";
    string connstr = provider + db;
    string query = "SELECT * FROM Customers WHERE CustomerID = ?";
    OleDbConnection conn = new OleDbConnection(connstr);
    OleDbCommand cmd = new OleDbCommand(query, conn);
    cmd.Parameters.AddWithValue("", Request.QueryString["CustomerID"]);
    conn.Open();
    OleDbDataReader dr = cmd.ExecuteReader();
    while(dr.Read())
    {
      sb.Append(dr[1].ToString() + "<br />");
      sb.Append(dr[4].ToString() + "<br />");
      sb.Append(dr[5].ToString() + "<br />");
      sb.Append(dr[6].ToString() + "<br />");
      sb.Append(dr[7].ToString() + "<br />");
      sb.Append(dr[8].ToString() + "<br />");
      sb.Append("Tel: " + dr[9].ToString() + "<br />");
    } 
    dr.Close();
    dr.Dispose();
    conn.Close();
    conn.Dispose();
    Response.Write(sb.ToString())
    Response.End();
  }
}

2. ASP.Net集成的方案
http://www.asp.net/web-forms/tutorials/aspnet-ajax/understanding-partial-page-updates-with-asp-net-ajax