MuellerDesigns.net - Client to server requests using XMLHttpRequest

Monday, February 28, 2005

Client to server requests using XMLHttpRequest

I was reading an article on Script Callbacks in ASP.NET 2.0 when I discovered a similar solution for ASP.NET 1.x. This article also displays their work around for ASP.NET 1.x. The solution below provides an example of how we can have our current page retrieve information from the server without having to refresh the page. Users complain about too many postbacks to the server, page refreshing, et cetera. If you have ever wanted to be able to send an HttpRequest to the server, process some information and return that response to the client, you can. It is actually quite simple and only requires a few steps.

My solution, like a number of examples on the web, involves using the XMLHttpRequest object in my javascript client code, an IHttpHandler, and a modification to the web.config's settings.

From your webpage, you will need two client-script functions, and means to interact with them. I have decided to display my entire page's HTML.

<html>
    <body>
        <form id="TestForm">
            <p></p>
            Try this:
            <input id="TestButton" type="button" onclick="ServerRequest();" value="Test Me"/>
            <p></p>
            <input id="TestBox" type="text" NAME="TestBox"/>
        </form>
    </body>
</html>
<script language="javascript">

    function ServerRequest( ){
        var term = "This is a test";
        var url = "TestHandler.axd";

        if (window.XMLHttpRequest){
            // branch for native XMLHttpRequest object
            request = new XMLHttpRequest();
            request.onreadystatechange = processRequestChange;

            //false = the script waits for the request to be sent and for a response to arrive from the server
            request.open("POST", url, false);
            request.send(term);
        }
        else if (window.ActiveXObject){
            // branch for IE/Windows ActiveX version
            request = new ActiveXObject("Microsoft.XMLHTTP");
            if (request){
                request.onreadystatechange = processRequestChange;
                //false = the script waits for the request to be sent and for a response to arrive from the server
                request.open("POST", url, false);
                request.send(term);
            }
        }
        document.getElementById("TestBox").value = request.responseText;
    }

    function processRequestChange(){
        // only if request shows "loaded"
        if (request.readyState == 4){
            // only if "OK"
            if (request.status == 200) {
                // Any processing you want goes here.
            }
            else {
                alert("There was a problem retrieving the XML data:\n" + request.statusText);
            }
        }
    }
</script>

Your project will then need an IHttpHandler created to accept and process HTTP requests.

using System;
using System.IO;
using System.Xml;
using System.Web;
using System.Web.SessionState;

namespace MyTestRequest
{
    public class TestHandler : IHttpHandler, IReadOnlySessionState
    {
        public void ProcessRequest(HttpContext context)
        {
            // Ingest the http request
            StreamReader reader = new StreamReader(context.Request.InputStream);
            string searchTerm = context.Server.UrlDecode( reader.ReadToEnd() );

            // Do your processing here...

            // Set the context response's properties (optional)
            context.Response.ContentEncoding = System.Text.Encoding.UTF8;
            context.Response.ContentType = "text/xml";

            // Send the response
            context.Response.Write( searchTerm );
        }

        public bool IsReusable
        {
            get { return false; }
        }
    }
}

Finally, the web.config will need the following settings supplied in the system.web section.

That is it for this example. We are able to do some client processing, send an HttpRequest to the server and receive a response while not refreshing the web browser. The functionality in this example is trivial, but the possibilities seem endless. ASP.NET 2.0 will make this example for ASP.NET 1.x easier with built-in script callbacks.

Monday, February 28, 2005 1:54:53 PM (Mountain Standard Time, UTC-07:00) | Comments [0] |

Comments are closed.