Introduction to Ajax programming

Web development

Client-side programming

Rich Internet Applications
and AJAX

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Rich Internet Application
Rich Internet applications (RIA) are web
applications that have the features and
functionality of traditional desktop
applications.
RIAs typically
transfer the processing necessary for the user
interface to the web client
keep the bulk of the data (i.e., maintaining the
state of the program, the data etc) back on the
application server.

Main goals of RIAs
Most sophisticated RIAs exhibit a look and
feel approaching a desktop environment.
Richer. User-interface behaviors not obtainable
using only the HTML widgets available to
standard browser-based Web applications: drag
and drop, using a slider to change data,
calculations performed by the client and which
do not need to be sent back to the server, ...
More responsive. The interface behaviors are
typically much more responsive than those of a
standard Web browser that must always interact
with a remote server.

Performance of RIAs
Client/Server balance. The demand for
client and server computing resources is
better balanced. This frees server resources,
allowing the same server hardware to handle
more client sessions concurrently.

Performance of RIAs
Asynchronous communication. The client
engine can interact with the server without
waiting for the user to perform an interface
action such as clicking on a button or link.
This allows the user to view and interact with
the page asynchronously from the client
engine's communication with the server.
Example: prefetching (an application anticipates
a future need for certain data, and downloads it
to the client before the user requests it)

Performance or RIAs
Network efficiency. Network traffic may be
significantly reduced because an application-
specific client engine can be more intelligent
than a Web browser when deciding what
data needs to be exchanged with servers.
Less data is being transferred for each
interaction, and overall network load is reduced.
However, use of asynchronous prefetching
techniques can neutralize or even reverse this
potential benefit.

AJAX definition
Asynchronous JavaScript And XML.
AJAX is a type of programming made
popular in 2005 by Google (with Google
Suggest).
AJAX is not a new programming language,
but a new way to use existing standards.
With AJAX you can create better, faster, and
more user-friendly web applications.
AJAX is based on JavaScript and HTTP
requests.

Key enabling technology
With AJAX, your JavaScript can
communicate directly with the server, using
the JavaScript XMLHttpRequest object.
By using the XMLHttpRequest object, a web
developer can update a page with data from
the server -- after the page has loaded!
The XMLHttpRequest object is supported in
Internet Explorer 5.0+, Safari 1.2, Mozilla
1.0 / Firefox, Opera 8+, and Netscape 7.
http://www.w3.org/TR/XMLHttpRequest/

XMLHttpRequest – the name
The name of the object is wrong, but
maintained for historical reasons:
May receive any text-based content, not just
XML
May use also HTTPS, not just HTTP protocol
May handle both Requests and Responses, of
all HTTP methods

Standard definition
interface XMLHttpRequest {
// event handler
attribute EventListener onreadystatechange;
// state
const unsigned short UNSENT = 0;
const unsigned short OPENED = 1;
const unsigned short HEADERS_RECEIVED = 2;
const unsigned short LOADING = 3;
const unsigned short DONE = 4;
readonly attribute unsigned short readyState;

Standard definition
// request
void open(in DOMString method, in DOMString url);
void open(in DOMString method, in DOMString url, in boolean async);
void open(in DOMString method, in DOMString url, in boolean async, in
DOMString user);
void open(in DOMString method, in DOMString url, in boolean async, in
DOMString user, in DOMString password);
void setRequestHeader(in DOMString header, in DOMString value);
void send();
void send(in DOMString data);
void send(in Document data);
void abort();

Standard definition
// response
DOMString getAllResponseHeaders();
DOMString getResponseHeader(in DOMString header);
readonly attribute DOMString responseText;
readonly attribute Document responseXML;
readonly attribute unsigned short status;
readonly attribute DOMString statusText;
};

Request states
UNSENT = 0
The request is not initialized
OPENED = 1
The request has been set up
HEADERS_RECEIVED = 2
The request has been sent
LOADING = 3
The request is in process
DONE = 4
The request is complete

State transition diagram
.setRequestHeader()
new
OPENED OPENED
UNSENT .open()
not sent sent
.send()
.send()

Connection closed All headers
received
DONE DONE
LOADING
error not error
HEADERS
(partial) body _RECEIVED
Connection closed received

XMLHttpRequest properties
onreadystatechange
stores the function that will process the
response from a server
xmlHttp.onreadystatechange =
function() { ... }
readyState
holds the status of the server’s response. Each
time readyState changes, the
onreadystatechange function will be executed.
responseText
the data sent back from the server can be
retrieved with the responseText property

Methods
open(method, url, async, user, password)
method = “GET”, “POST”
url = complete URL to request
async = true/false (optional, default=true)
user, password (optional)
Interrupts any on-going send()
setRequestHeader(header, value)
Adds a new header to the HTTP Request
Content-Type is one common header to send
Examples: text/xml, application/xml

Methods
send(data)
Initiates the request
data = HTTP request body (optional)
May be a Document or DOMString
The URL was already given in open()
send() terminates immediately if async==true,
but transfer continues in the background
Generates readystatechange events
send() transfers data synchronously if
async==false

Methods
getAllResponseHeaders()
Return all response headers as a single string,
with headers separated by CR+LF
Invalid if UNSENT or OPENED
getResponseHeader(header)
Returns the value of a single header
Invalid if UNSENT or OPENED

Receiving the response body
responseText of type DOMString
If LOADING (partial body) or DONE
Allow access to a “raw string” of the response
body
responseXML of type Document
Only if DONE
For text/xml (or application/xml or *+xml) content
types, otherwise null
Allows access to the DOM of the XML document

Example
Create a standard HTML form with two text
fields: username and time.
The username field will be filled in by the
user and the time field will be filled in using
AJAX.
No submit button is needed.

Example

<html>
<body> <form name="myForm">
Name: <input type="text" name="username" />
Time: <input type="text" name="time" />
</form> </body>
</html>

Creating an XMLHttpRequest
object
<script type="text/javascript">
function ajaxFunction()
{
var xmlHttp;
xmlHttp=new XMLHttpRequest();

...
}
</script>

Supporting all browsers
{
var xmlHttp;
try {
// Firefox, Opera 8.0+, Safari
xmlHttp=new XMLHttpRequest();
}
catch (e) {
// Internet Explorer
try { // Internet Explorer 6.0+
xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
}
catch (e) {
try { // Internet Explorer 5.5+
xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
}
catch (e) {
alert("Your browser does not support AJAX!");
return false;
}
}
}
}
</script>

Calling the server
xmlHttp.open("GET","time.jsp",true);
xmlHttp.send(null);

Processing the response

xmlHttp.onreadystatechange=function()
{
if(xmlHttp.readyState==4)
{
// Get the data from the server's response
document.myForm.time.value=xmlHttp.responseText;
}
}

Attaching to an event

<form name="myForm">
Name: <input type="text"
onkeyup="ajaxFunction();" name="username" /
>
</form>

Complete example
<html>
<body>
{
var xmlHttp=new XMLHttpRequest();

xmlHttp.onreadystatechange=function()
{
if(xmlHttp.readyState==4)
{
document.myForm.time.value=xmlHttp.responseText;
}
}

xmlHttp.open("GET","time.asp",true);
xmlHttp.send(null);
}
</script>
<form name="myForm">
Name: <input type="text"
onkeyup="ajaxFunction();" name="username" />
</form> </body>
</html>

Exercise 1
Create an auto-complete feature for entering
the name in a FORM
For every typed letter, an associated text
must be updated, reflecting the list of all
possible names with those initial(s)
Once submitted, the name adds up to the list
Clicking on the suggestion auto-fills the box

Name Jo Suggestions: Joe, Joseph, John

SUBMIT

Exercise 2
Create a FORM for entering the name of a
city, based on two drop-down menus
(<select> tags).
The first <select> contains the list of all
provinces (AO, BO, CN, MI, TO, ...)
The second <select> contains the list of all cities
in the province
Every time the user changes the province,
then the list of cities MUST be updated
The form may be submitted only if
information is complete

References
http://en.wikipedia.org/wiki/Rich_Internet_Ap
plications
http://en.wikipedia.org/wiki/AJAX
http://www.w3schools.com/ajax/
http://www.w3.org/TR/XMLHttpRequest/

Introduction to Ajax programming

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