AJAX

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Definition

According to [Wikipedia]: “Ajax, shorthand for Asynchronous JavaScript and XML, is a web development technique for creating interactive web applications. The intent is to make web pages feel more responsive by exchanging small amounts of data with the server behind the scenes, so that the entire web page does not have to be reloaded each time the user requests a change. This is meant to increase the web page's interactivity, speed, and usability.”, , retrieved 15:00, 9 February 2007 (MET)

The Ajax technique uses a combination of:

  • XHTML (or HTML) and CSS, for marking up and styling information.
  • The DOM accessed with a client-side scripting language, usually ECMAScript (JavaScript)
  • The XMLHttpRequest object is used to exchange data asynchronously with the web server.
  • XML is sometimes used as the format for transferring data between the server and client.

A simple example

AJAX is really simple. The difficult part is to write really good interfaces (see toolkits in the links section) and to do something with these data on the server side ...

HTML/JavAscript
<html>
  <head>
    <title>Simple Ajax example</title>
    <script type="text/javascript" language="javascript">

var url;
var table;

function init () {
  url = "ajax1.php";
 // url = "ajax-debug.php";
  table = document.getElementById("table1");
}


function makeRequest(element) {
  // This function is called from the HTML code below
  // element is the DOM element (tag on which the user clicked)

  var http_request = false;
  
  // ---- Mozilla, Safari, etc. browsers
  if (window.XMLHttpRequest) { 
    http_request = new XMLHttpRequest();
    // This will make sure that the server response claims to be XML (in case we retrieve something else)
    if (http_request.overrideMimeType) {
      http_request.overrideMimeType('text/xml');
      
    }
    // ---- IE browsers
  } else if (window.ActiveXObject) { 
    try {
      http_request = new ActiveXObject("Msxml2.XMLHTTP");
    } catch (e) {
      try {
	http_request = new ActiveXObject("Microsoft.XMLHTTP");
      } catch (e) {}
    }
  }
  
  // ---- abort if there is no reply
  if (!http_request) {
    alert('Giving up :( Cannot create an XMLHTTP instance');
    return false;
  }
  
  // We register the function that will deal with a reply
  http_request.onreadystatechange = function() { processServerReply(http_request); };
  
  // This lines starts building the request - don't think that a GET request would work (?)
  http_request.open('POST', url, true);
  // Contents WE send from here will be urlencoded
  http_request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
  
  // here we extract the contents of tag on which the user clicked
  user_pref    = element.innerHTML;
  // This is the content of the request
  // alert(user_pref);
  user_request = "user_pref_fruit=" + user_pref;
  
  // We send the data - data are query strings
  http_request.send(user_request);
  
}



function processServerReply(http_request) {

  if (http_request.readyState == 4) {
    if (http_request.status == 200) {
      // We tell the server that we want to deal with XML as a DOM Document !!
      replyXML = http_request.responseXML;
      treatResponse (replyXML);
      // displayResponse (replyXML);
      
    } else {
      alert('There was a problem with the request.');
    }
  }
  
}

// This will change the HTML contents of the page
function treatResponse (reply) {
  // reply is a XML DOM datastructure !!
  // extract some XML - we know that it is in an "answer" tag
  // DOM HTML will not work, it's XML here 
  var answer = reply.getElementsByTagName('answer').item(0).firstChild.nodeValue;

  // new tr, td elements
  var element_tr = document.createElement("tr");
  var element_td = document.createElement("td");
  // contents for the td element
  var text    = document.createTextNode(answer);
  element_td.appendChild(text);
  element_tr.appendChild(element_td);
  table.appendChild(element_tr);
}

// just for debugging, will open up a popup window. Useful if you tell the php  to send debugging infos...
function displayResponse (reply) {
  win=window.open("","Results","width=250,height=300,status=1,resizable=1,scrollbars=1");
  win.document.open();
  win.document.write(reply);
  win.document.close();
}


 </script>


</head>

  <body onload="init()">

Simple Ajax example

    Please click on a fruit:
  • I like apples
  • I like oranges
  • I like bananas 
 Dialog history:
Server replies 
; PHP
<?php
error_reporting(E_ALL);
header ("Content-type: application/xml");   
echo '<?xml version="1.0" encoding="ISO-8859-1" ?>';

if (array_key_exists('user_pref_fruit', $_POST))  
  { $user_pref  = $_POST['user_pref_fruit']; }
 else 
   $user_pref="nothing";

echo "<answer>";
echo "Oh you like " . $user_pref . " !";
// echo "Oh you like " . $user_pref . " !" . " - Query String=" .  $_SERVER["QUERY_STRING"];
echo "</answer>";

?>

Software

JavaScript toolkits

Dojo (free software)
Aculo (free software)
Prototype.js library
  • “This amazingly well thought and well written piece of standards-compliant code takes a lot of the burden associated with creating rich, highly interactive web pages that characterize the Web 2.0 off your back.” (retrieved 15:00, 9 February 2007 (MET))
  • Unofficial Prototype documentation by Sergio Pereira
  • Amy Hoy's Cheat Sheet
  • Jonathan Snook's Prototype Dissected

Links

References

Technical how-to

  • Vlad Kofman (2007), The Web 2.0 Movement Is Here. But What Does It Mean to You?, Developper.com Atricle, HTML
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