JAVASCRIPT

Reading Data from an XML File for a Dynamic AJAX-based Banner System

Undeniably, AJAX has introduced a noticeable impact into the development of rich web applications, and consequently there are many enthusiastic web developers that continue discovering more and more creative uses for this powerful technology. Particularly, in this series of articles this "creativity" is focused entirely upon building an AJAX-driven banner application for displaying, on the browser, a certain number of banners via a few HTTP requests made in the background.

Author Info:

By: Alejandro Gervasio

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July 31, 2007

TABLE OF CONTENTS:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-
8859-1" />
<title>AJAX-Driven Dynamic Banner System</title>
<script language="javascript" type="text/javascript">
// send http requests
function sendHttpRequest(url,callbackFunc,respXml){
   var xmlobj=null;
   try{
     xmlobj=new XMLHttpRequest();
   }
   catch(e){
     try{
       xmlobj=new ActiveXObject("Microsoft.XMLHTTP");
     }
     catch(e){
       alert('AJAX is not supported by your browser!');
       return false;
     }
   }
   xmlobj.onreadystatechange=function(){
     if(xmlobj.readyState==4){
       if(xmlobj.status==200){
         respXml?eval(callbackFunc+'(xmlobj.responseXML)'):eval
(callbackFunc+'(xmlobj.responseText)');
       }
     }
   }
   // open socket connection
   xmlobj.open('GET',url,true);
   // send http header
   xmlobj.setRequestHeader('Content-Type','text/plain;
charset=UTF-8');
   // send http request
   xmlobj.send(null);
}
// store banners into JavaScript array
function storeBanners(banners){
   var banners=banners.getElementsByTagName('banner');
   if(!banners){return};
   // save banner data to JS arrays
   for(var i=0;i<banners.length;i++){
     images[i]=banners[i].getElementsByTagName('image')
[0].firstChild.nodeValue;
     urls[i]=banners[i].getElementsByTagName('url')
[0].firstChild.nodeValue;
   }
   // display first banner
   displayBanner();
}
// display banners
function displayBanner(){
   var bannercont=document.getElementById('bannercontainer');
   if(!bannercont){return};
   // clean up headlines container
   bannercont.innerHTML='';
   // create <img> element
   var img=document.createElement('img');
   img.setAttribute('src',images[index]);
   // create <a> element
   var a=document.createElement('a');
   a.setAttribute('href',urls[index]);
   img.setAttribute('width',180);
   img.setAttribute('height',400);
   a.appendChild(img);
   // append banner to banner container
   bannercont.appendChild(a);
   index++;
   if(index>urls.length-1){index=0};
   setTimeout("displayBanner()",15*1000);
}
// initialize data arrays and index
var images=new Array();
var urls=new Array();
var index=0;
window.onload=function(){
   if(document.getElementById&&document.
getElementsByTagName&&document.createElement){
     // fetch the first banner after loading the web page
     sendHttpRequest('banners.xml','storeBanners',true);
   }
}
</script>
<style type="text/css">
body{
   margin: 0;
   padding: 0;
   background: #eee;
}

h1{
   text-align: center;
   font: bold 24px Arial, Helvetica, sans-serif;
   color: #000;
}

#bannercontainer{
   text-align: center;
   width: 180px;
   height: 400px;
   margin-left: auto;
   margin-right: auto;
   background: #fff;
   border: 1px solid #000;
}

#bannercontainer img{
border: none;
}
</style>
</head>
<body>
<h1>AJAX-Driven Dynamic Banner System</h1>
<div id="bannercontainer"></div>
</body>
</html>

(definition of 'banners.xml' file)

<?xml version="1.0" encoding="iso-8859-1"?>
<banners>
<banner>
    <image>banner1.gif</image>
    <url>http://www.myhosting.com</url>
</banner>
<banner>
    <image>banner2.gif</image>
    <url>http://www.myphp.com</url>
</banner>
<banner>
    <image>banner3.gif</image>
    <url>http://www.myjs.com</url>
</banner>
</banners>

And wrapping up, the full source code corresponding to this application wouldn't be complete if I don't show you once again the sample banner images that I created in the previous article of the series, so here they are:

Final thoughts

In this second installment of the series, I introduced a major improvement into the original AJAX-based banner application by modifying one of its principal JavaScript functions. As you saw, the direct consequence in doing this was the relocation of the corresponding banner data into a new compact XML file, and the elimination of a PHP file that reads this data from the web server.

Nonetheless, the application still relies on a global JavaScript variable to control the sequence in which the banners are displayed on the browser. This can be considered a bad programming habit. Thus, in the last part of the series, I'm going to fix this issue, in addition to tweaking the source code a bit, so all the banner data can be fetched from MySQL.

Now that you've been warned, are you going to miss it? I hope not!

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