CSSE 290 Web Programming

Lecture 20: Ajax

Reading: 12.1 - 12.2

Except where otherwise noted, the contents of this document are Copyright 2012 Marty Stepp, Jessica Miller, and Victoria Kirst. All rights reserved. Any redistribution, reproduction, transmission, or storage of part or all of the contents in any form is prohibited without the author's expressed written permission.

Otherwise note: Claude Anderson was given permission to modify the slides for CSSE 290 at Rose-Hulman by author Jessica Miller. The authors' original slides, based on Web Programming Step by Step, can be seen at http://webstepbook.com.

Some of the examples in some days' slides are from David Fisher at Rose-Hulman, who was kind enough to allow me to use them. My intention is to mark these examples with [DSF].

Announcements

• HW4 submission is due now
• Quiz3 resubmission is due tomorrow
• HW5 due next Monday (do a demo now) Forget the heart part
• Today's class time:
  • More of our animation example
  • Brief intro to Ajax (real work with Ajax begins next time)
  • Get started on HW5

Important note: I sent you two files yesterday. Change their names to garden.ws and garden2.ws, and copy (SFTP) them into the puzzles folder in your HW4 submission on wwwuser.csse.rose-hulman.edu, if you have not already done so.

Synchronous web communication

• synchronous: user must wait while new pages load
  • the typical communication pattern used in web pages (click, wait, refresh)

Web applications and Ajax

• web application: a dynamic web site that mimics the feel of a desktop app
  • presents a continuous user experience rather than disjoint pages
  • examples: Gmail, Google Maps, Google Docs and Spreadsheets, Flickr, A9
• Ajax: Asynchronous JavaScript and XML
  • not a programming language; a particular way of using JavaScript
  • downloads data from server in the background
  • allows dynamic updates of page without making user wait
  • avoids "click-wait-refresh" pattern
  • Concept developed gradually in late 1990's and early 2000's
  • First called "Ajax" in 2005

Asynchronous web communication

• asynchronous: user keeps interacting with page while new data loads
  • communication pattern made possible by Ajax

XMLHttpRequest

• JavaScript includes an XMLHttpRequest object that can fetch files from a web server
  • supported in IE5+, Safari, Firefox, Opera, Chrome, etc. (with minor incompatibilities)
• it can do this asynchronously (in the background, transparent to user)
• the contents of the fetched file can be put into current web page using the DOM
• sounds great!...
• ... but it is clunky to use, and has various browser incompatibilities
• Prototype provides a better wrapper for Ajax, so we will use that instead

A typical Ajax request

1. user clicks, invoking an event handler
2. handler's code creates an XMLHttpRequest object
3. XMLHttpRequest object requests info from server
4. server retrieves appropriate data, sends it back
5. XMLHttpRequest fires an event when data arrives
   • we attach a handler function to this event (callback)
6. callback event handler processes the data and displays it

An aside: Creating a new anonymous object

var name = {
	fieldName: value,
	...
	fieldName: value
};

var p = {
	x: 4,
	y: 3,
	distanceFromOrigin: function() {
		return Math.sqrt(this.x * this.x + this.y * this.y);
	}
};
alert(p.x + ", " + p.y + ", dist=" + p.distanceFromOrigin());

• in JavaScript, you can create a new object without creating a class
• the object can have methods (function properties) that refer to itself as this

Prototype's Ajax model

new Ajax.Request("url", {
	option : value,
	option : value,
	...
	option : value
});

• construct a Prototype Ajax.Request object to request a page from a server using Ajax
• constructor accepts 2 parameters:
  1. the URL to fetch, as a String,
  2. a set of options, as a set of key : value pairs in {} braces
     (an anonymous JavaScript object)
• hides messy details of the raw XMLHttpRequest; works well in all browsers

Prototype Ajax options

Prototype Ajax example

	new Ajax.Request("foo/bar/mydata.txt", {
		method: "get",
		onSuccess: myAjaxSuccessFunction
	});
	...

function myAjaxSuccessFunction(ajax) {
	do something with ajax.responseText;
}

• attach an event handler function to the request's onSuccess event
• the handler takes an Ajax response object as a parameter, which we'll name ajax

Ajax response object's properties

function myAjaxSuccessFunction(ajax) {
	alert(ajax.responseText);
}

• most commonly used property is responseText, to access the fetched text content

Handling Ajax errors

	new Ajax.Request("url", {
		method: "get",
		onSuccess: functionName,
		onFailure: ajaxFailure,
		onException: ajaxFailure
	});
	...
function ajaxFailure(ajax, exception) {
  alert("Error making Ajax request:" + 
	"\n\nServer status:\n" + ajax.status + 
    " " + ajax.statusText + 
	"\n\nServer response text:\n" + ajax.responseText);
  if (exception) {
	throw exception;
	}
}

• for user's (and developer's) benefit, show an error message if a request fails

Passing query parameters to a request

new Ajax.Request("lookup_account.php", {
	method: "get",
	parameters: {name: "Ed Smith", age: 29, password: "abcdef"},
	onFailure: ajaxFailure,
	onException: ajaxFailure
});
...

• don't concatenate the parameters onto the URL yourself with "?" + ...
  • won't properly URL-encode the parameters
  • won't work for POST requests
• query parameters are passed as a parameters object, {} braces with name : value pairs
  • (the above is equivalent to: "name=Ed+Smith&age=29&password=abcdef")

Creating a POSTrequest

new Ajax.Request("url", {
	method: "post",
	parameters: {name: value, name: value, ..., name: value},
	onSuccess: functionName,
	onFailure: functionName,
	onException: functionName
});

• method should be changed to "post" (or omitted; post is default)