AJAX in Action

This section will give you clear picture of the exact steps of AJAX operation.

Steps of AJAX Operation

A client event occurs
An XMLHttpRequest object is created
The XMLHttpRequest object is configured
The XMLHttpRequest object makes an asynchronous request to the Webserver.
Webserver returns the result containing XML document.
The XMLHttpRequest object calls the callback() function and processes the result.
The HTML DOM is updated

Lets take these steps one by one

1. A client event occurs

A JavaScript function is called as the result of an event
Example: validateUserId() JavaScript function is mapped as a event handler to a onkeyupevent on input form field whose id is set to "userid"
<input type="text" size="20" id="userid" name="id" onkeyup="validateUserId();">

2. The XMLHttpRequest object is created

var ajaxRequest;  // The variable that makes Ajax possible!
function ajaxFunction(){
 try{
   // Opera 8.0+, Firefox, Safari
   ajaxRequest = new XMLHttpRequest();
 }catch (e){
   // Internet Explorer Browsers
   try{
      ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");
   }catch (e) {
      try{
         ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");
      }catch (e){
         // Something went wrong
         alert("Your browser broke!");
         return false;
      }
   }
 }
}

3. The XMLHttpRequest object is Configured

In this step we will write a function which will be triggered by the client event and a callback function processRequest() will be registered

function validateUserId() {
   ajaxFunction();
   // Here processRequest() is the callback function.
   ajaxRequest.onreadystatechange = processRequest;
   if (!target) target = document.getElementById("userid");
   var url = "validate?id=" + escape(target.value);
   ajaxRequest.open("GET", url, true);
   ajaxRequest.send(null);
}

4. Making Asynchornous Request to the Webserver

Source code is available in the above piece of code. Code written in blue color is responsible to make a request to the web server. This is all being done using XMLHttpRequest objectajaxRequest

function validateUserId() {
   ajaxFunction();
   // Here processRequest() is the callback function.
   ajaxRequest.onreadystatechange = processRequest;

   if (!target) target = document.getElementById("userid");
   var url = "validate?id=" + escape(target.value);
   ajaxRequest.open("GET", url, true);
   ajaxRequest.send(null);

}

Assume if you enter mohammad in userid box then in the above request URL is set to validate?id=mohammad

5. Webserver returns the result containing XML document

You can implement your server side script in any language. But logic should be as follows

Get a request from the client
Parse the input from the client
Do required processing.
Send the output to the client.

If we assume that you are going to write a servlet then here is the piece of code

public void doGet(HttpServletRequest request, 
                    HttpServletResponse response)
                        throws IOException, ServletException 
{
   String targetId = request.getParameter("id");

   if ((targetId != null) && 
       !accounts.containsKey(targetId.trim())) 
   {
      response.setContentType("text/xml");
      response.setHeader("Cache-Control", "no-cache");
      response.getWriter().write("true");
   } 
   else 
   {
      response.setContentType("text/xml");
      response.setHeader("Cache-Control", "no-cache");
      response.getWriter().write("false");
   }
}

6. Callback function processRequest() is called

The XMLHttpRequest object was configured to call the processRequest() function when there is a state change to the readyState of the XMLHttpRequest object. Now this function will recieve the result from the server and will do required processing. As in the following example it sets a variable message on true or false based on retruned value from the Webserver.

function processRequest() {
   if (req.readyState == 4) {
      if (req.status == 200) {
         var message = ...;
...
}

7. The HTML DOM is updated

This is the final step and in this step your HTML page will be updated. It happens in the following way

JavaScript technology gets a reference to any element in a page using DOM API
The recommended way to gain a reference to an element is to call.

document.getElementById("userIdMessage"), 
// where "userIdMessage" is the ID attribute 
// of an element appearing in the HTML document

JavaScript technology may now be used to modify the element's attributes; modify the element's style properties; or add, remove, or modify child elements. Here is the example

<script type="text/javascript">
<!--
 function setMessageUsingDOM(message) {
    var userMessageElement = 
           document.getElementById("userIdMessage");
    var messageText;
    if (message == "false") {
       userMessageElement.style.color = "red";
       messageText = "Invalid User Id";
    } else {
       userMessageElement.style.color = "green";
       messageText = "Valid User Id";
    }
    var messageBody = document.createTextNode(messageText);


    // if the messageBody element has been created simple 
    // replace it otherwise append the new element

    if (userMessageElement.childNodes[0]) {
       userMessageElement.replaceChild(messageBody,
       userMessageElement.childNodes[0]);
    } else {
       userMessageElement.appendChild(messageBody);
    }
}
-->
</script>
<body>
<div id="userIdMessage"><div>
</body>

thats it...if you understood above mentioned seven steps then you are almost done with AJAX. In next chapter we will see XMLHttpRequest object in more detail.

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