I love short stories to get started in a new technology. So here is one about ajax - the current hype on web platforms.
What is it for
Send requests asynchronous to your webserver and process the results within the current client-page-DOM without re-loading the whole page!
If you need special data in your webpage as response to a user action then use ajax. It is not meant for a whole Web GUI. If you use ajax for too much user navigation you will explore the two major drawbacks soon: The user can’t use the browser’s back button and search engines will ignore you (well, there are workarounds but those are bad browser dependent hacks).
The typical situation where ajax is useful is when you have two drop-down lists and the content of the second list depends on the selected entry of the first. I.e. “choose your cpu:” - first list contains manufacturers “Intel”, “AMD”, “IBM”, “Motorola”, second list contains processor model.
Technology, Syntax, Objects
On client side your users need a javascript enabled webbrowser of a “newer” generation. On server side you need nothing special, just use your favorite technique (there’s more importance on the selected protocol format - like just string or xml or json). On webdesigner’s side you need some DHTML and javascript.
A typical javascript function that initiates an ajax call with an url as parameter looks like this:
var request = false; // you need this variable globally
function initiateAjaxCall(url)
{
request = new XMLHttpRequest();
request.open("get", url, true);
request.onreadystatechange = async_event;
request.send(NULL);
}
Where the XMLHttpRequest is the javascript-ajax object to focus on (for IE6 you need some workarounds well known in the web). ajax is “Asynchronous Javascript and XML” so it’s asynchronous and so you need a callback function (a function that gets called when an event occurs). Above “async_event” is declared to be your event handling function to handle the webserver’s asynchronous response.
There are four events your callback will be called with:
ajax state table:
0 = uninitialized
1 = loading
2 = loaded
3 = interactiv
4 = complete
For this quick intro, only state 4 is of interest. Maybe I’ll get back on this later in another post (can program nice progress bars with this infos).
Your event handler could look like this:
function async_event() {
switch(request.readyState) {
case 4:
if (request.status != 200) {
alert("HTTP Response not OK : Code:"+request.status);
} else {
myProcessing(request.responseText);
}
break;
default: break;
}
}
In the “myProcessing” function the final response processing is done (usualy it’s any DHTML manipulation of the current DOM).
Full Code Example
It’s not that easy to invent a simple example that makes at least little sense, so don’t be to critical with this.
Initial view of the example
To show up the technology in short code we build a website that displays the current client time (javascript) and a button to request the current server time. When the button is pressed, the current server time get’s fetched from the server by magical AJAX and is finally display via a DOM manipulation in - again - javascript.
We start with the server side (in php - to keep it simple)
File getTime.php
<?php echo date("H:i:s"); ?>
That was pretty simple, now we do the html page that uses ajax:
File getTime.html
<html>
<head><title>AJAX Test 1</title>
<script type="text/javascript">
<!--
// our global request variable
var request = false;
// function to initiate the ajax call
function initiateAjaxCall(url)
{
request = new XMLHttpRequest();
request.open("get", url, true);
request.onreadystatechange = async_event;
request.send(null);
}
// ajax callback function
function async_event() {
switch(request.readyState) {
case 4:
if (request.status != 200) {
myProc("Error Response Code: "+request.status);
} else {
myProc(request.responseText);
}
break;
default: break;
}
}
// function to print the ajax result
function myProc(text) {
var now = new Date();
var ct = document.getElementById("CTIME");
var st = document.getElementById("STIME");
ct.innerHTML = ""+now.getHours()+":"+
now.getMinutes()+":"+
now.getSeconds();
st.innerHTML = text;
}
//-->
</script>
</head>
<body onLoad="myProc('---');">
<h1>Ajax Test 1</h1>
Client Time: <span id="CTIME"></span><br>
Server Time: <span id="STIME"></span>
<br><br>
<input type="submit" value="fetch server time via ajax now"
onClick="initiateAjaxCall('getTime.php');">
</body>
</html>
After pressing the button, server’s time is displayed
As promised yesterday, I’ll try the OpenMP features of Intel C++ 10.0 for Linux today. Cause I’m lazy, I use a wide spread “Hello World” for OpenMP example, it is very simple and it’s only purpose is to demonstrate the basics of how to parallelize a code with OpenMP.
