Âåðíóòüñÿ â áèáëèîòåêó
Mastering Ajax, Part 1: Introduction to Ajax
Àâòîð: McLaughlin Brett
Èñòî÷íèê: http://www.ibm.com/developerworks/web/library/wa-ajaxintro1/
Summary
Ajax, which consists of HTML, JavaScript™ technology, DHTML, and DOM, is an outstanding approach that helps you transform clunky Web interfaces into interactive Ajax applications. The author, an Ajax expert, demonstrates how these technologies work together – from an overview to a detailed look – to make extremely efficient Web development an easy reality. He also unveils the central concepts of Ajax, including the XMLHttpRequest object.
Five years ago, if you didn't know XML, you were the ugly duckling whom nobody talked to. Eighteen months ago, Ruby came into the limelight and programmers who didn't know what was going on with Ruby weren't welcome at the water cooler. Today, if you want to get into the latest technology rage, Ajax is where it's at.
However, Ajax is far more than just a fad; it's a powerful approach to building Web sites and it's not nearly as hard to learn as an entire new language.
Before I dig into what Ajax is, though, let's spend just a few moments understanding what Ajax does. When you write an application today, you have two basic choices:
These are both familiar; desktop applications usually come on a CD (or sometimes are downloaded from a Web site) and install completely on your computer. They might use the Internet to download updates, but the code that runs these applications resides on your desktop. Web applications – and there's no surprise here – run on a Web server somewhere and you access the application with your Web browser.
More important than where the code for these applications runs, though, is how the applications behave and how you interact with them. Desktop applications are usually pretty fast (they're running on your computer; you're not waiting on an Internet connection), have great user interfaces (usually interacting with your operating system), and are incredibly dynamic. You can click, point, type, pull up menus and sub-menus, and cruise around, with almost no waiting around.
On the other hand, Web applications are usually up-to-the-second current and they provide services you could never get on your desktop (think about Amazon.com and eBay). However, with the power of the Web comes waiting – waiting for a server to respond, waiting for a screen to refresh, waiting for a request to come back and generate a new page.
Obviously this is a bit of an oversimplification, but you get the basic idea. As you might already be suspecting, Ajax attempts to bridge the gap between the functionality and interactivity of a desktop application and the always-updated Web application. You can use dynamic user interfaces and fancier controls like you'd find on a desktop application, but it's available to you on a Web application.
So what are you waiting for? Start looking at Ajax and how to turn your clunky Web interfaces into responsive Ajax applications.
When it comes to Ajax, the reality is that it involves a lot of technologies – to get beyond the basics, you need to drill down into several different technologies (which is why I'll spend the first several articles in this series breaking apart each one of them). The good news is that you might already know a decent bit about many of these technologies – better yet, most of these individual technologies are easy to learn – certainly not as difficult as an entire programming language like Java or Ruby.
By the way, Ajax is shorthand for Asynchronous JavaScript and XML (and DHTML, and so on). The phrase was coined by Jesse James Garrett of Adaptive Path (see the Resources section) and is, according to Jesse, not meant to be an acronym.
Here are the basic technologies involved in Ajax applications:
div, span, and other dynamic HTML elements to
mark up your HTML.
Let's break these down and get a better idea of what each does. I'll delve into each of these more in future articles; for now focus on becoming familiar with these components and technologies. The more familiar you are with this code, the easier it will be to move from casual knowledge about these technologies to mastering each (and really blowing the doors off of your Web application development).
The first object you want to understand is probably the one that's newest to you; it's called XMLHttpRequest. This is a JavaScript object and
is created as simply as shown in Listing 1.
<script language="javascript" type="text/javascript"> var xmlHttp = new XMLHttpRequest(); </script> |
I'll talk more about this object in the next article, but for now realize that this is the object that handles all your server communication. Before you go
forward, stop and think about that – it's the JavaScript technology through the XMLHttpRequestobject that talks to the server.
That's not the normal application flow and it's where Ajax gets much of its magic.
In a normal Web application, users fill out form fields and click a Submit button. Then, the entire form is sent to the server, the server passes on processing to a script (usually PHP or Java or maybe a CGI process or something similar), and when the script is done, it sends back a completely new page. That page might be HTML with a new form with some data filled in or it might be a confirmation or perhaps a page with certain options selected based on data entered in the original form. Of course, while the script or program on the server is processing and returning a new form, users have to wait. Their screen will go blank and then be redrawn as data comes back from the server. This is where low interactivity comes into play – users don't get instant feedback and they certainly don't feel like they're working on a desktop application.
Ajax essentially puts JavaScript technology and the XMLHttpRequest object between your Web form and the server. When users fill out
forms, that data is sent to some JavaScript code and not directly to the server. Instead, the JavaScript code grabs the form data and sends a
request to the server. While this is happening, the form on the users screen doesn't flash, blink, disappear, or stall. In other words, the JavaScript code
sends the request behind the scenes; the user doesn't even realize that the request is being made. Even better, the request is sent asynchronously, which
means that your JavaScript code (and the user) doesn't wait around on the server to respond. So users can continue entering data, scrolling around, and
using the application.
Then, the server sends data back to your JavaScript code (still standing in for the Web form) which decides what to do with that data. It can update form
fields on the fly, giving that immediate feeling to your application – users are getting new data without their form being submitted or refreshed. The
JavaScript code could even get the data, perform some calculations, and send another request, all without user intervention! This is the power of XMLHttpRequest. It can talk back and forth with a server all it wants, without the user ever knowing about what's really going on. The result
is a dynamic, responsive, highly-interactive experience like a desktop application, but with all the power of the Internet behind it.
Once you get a handle on XMLHttpRequest, the rest of your JavaScript code turns out to be pretty mundane. In fact, you'll use JavaScript code
for just a few basic tasks:
For those first two items, you want to be very familiar with the getElementById() method as shown in Listing 2.
// Get the value of the "phone" field and stuff it in a variable called phone
var phone = document.getElementById("phone").value;
// Set some values on a form using an array called response
document.getElementById("order").value = response[0];
document.getElementById("address").value = response[1];
|
There's nothing particularly remarkable here and that's good! You should start to realize that there's nothing tremendously complicated about this. Once
you master XMLHttpRequest, much of the rest of your Ajax application will be simple JavaScript code like that shown in Listing 2, mixed in with a bit of clever HTML. Then, every once in a
while, there's a little DOM work...so let's look at that.
Last but not least, there's the DOM, the Document Object Model. For some of you, hearing about the DOM is going to be a little intimidating – it's not often used by HTML designers and is even somewhat unusual for JavaScript coders unless you're really into some high-end programming tasks. Where you will find the DOM in use a lot is in heavy-duty Java and C/C++ programs; in fact, that's probably where the DOM got a bit of its reputation for being difficult or hard to learn.
Fortunately, using the DOM in JavaScript technology is easy, and is mostly intuitive. At this point, I'd normally show you how to use the DOM or at least give you a few code examples, but even that would be misleading. You see, you can get pretty far into Ajax without having to mess with the DOM and that's the path I'm going to show you. I'll come back to the DOM in a future article, but for now, just know that it's out there. When you start to send XML back and forth between your JavaScript code and the server and really change the HTML form, you'll dig back into DOM. For now, it's easy to get some effective Ajax going without it, so put this on the back-burner for now.
With a basic overview under your belt, you're ready to look at a few specifics. Since XMLHttpRequest is central to Ajax applications – and
probably new to many of you – I'll start there. As you saw in Listing 1,
it should be pretty easy to create this object and use it, right? Wait a minute.
Remember those pesky browser wars from a few years back and how nothing worked the same across browsers? Well, believe it or not, those wars are still
going on albeit on a much smaller scale. And, surprise: XMLHttpRequest is one of the victims of this war. So you'll need to do a few different
things to get an XMLHttpRequest object going. I'll take your through it step by step.
Working with Microsoft browsers
Microsoft's browser, Internet Explorer, uses the MSXML parser for handling XML (you can find out more about MSXML in Resources). So when you write Ajax applications that need to work on Internet Explorer, you need to create the object in a particular way.
However, it's not that easy. MSXML actually has two different versions floating around depending on the version of JavaScript technology installed in
Internet Explorer, so you've got to write code that handles both cases. Look at Listing 3 for the code that you need to create an XMLHttpRequest on Microsoft browsers.
var xmlHttp = false;
try {
xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
} catch (e) {
try {
xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
} catch (e2) {
xmlHttp = false;
}
}
|
All of this won't make exact sense yet, but that's OK. You'll dig into JavaScript programming, error handling, conditional compilation, and more before this series is finished. For now, you want to get two core lines into your head:
xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
and
xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
.
In a nutshell, this code tries to create the object using one version of MSXML; if that fails, it then creates the object using the other version. Nice,
huh? If neither of these work, the xmlHttp variable is set to false, to tell your code know that something hasn't worked. If that's the case,
you've probably got a non-Microsoft browser and need to use different code to do the job.
Dealing with Mozilla and non-Microsoft browsers
If Internet Explorer isn't your browser of choice or you write code for non-Microsoft browsers, then you need different code. In fact, this is the really simple line of code you saw back in Listing 1:
var xmlHttp = new XMLHttpRequest object;
.
This much simpler line creates an XMLHttpRequest object in Mozilla, Firefox, Safari, Opera, and pretty much every other non-Microsoft browser
that supports Ajax in any form or fashion.
The key is to support all browsers. Who wants to write an application that works just on Internet Explorer or an application that works just on non-Microsoft browsers? Worse yet, do you want to write your application twice? Of course not! So your code combines support for both Internet Explorerand non-Microsoft browsers. Listing 4 shows the code to do just that.
/* Create a new XMLHttpRequest object to talk to the Web server */
var xmlHttp = false;
/*@cc_on @*/
/*@if (@_jscript_version >= 5)
try {
xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
} catch (e) {
try {
xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
} catch (e2) {
xmlHttp = false;
}
}
@end @*/
if (!xmlHttp && typeof XMLHttpRequest != 'undefined') {
xmlHttp = new XMLHttpRequest();
}
|
For now, ignore the commenting and weird tags like @cc_on; those are special JavaScript compiler commands that you'll explore in depth in my
next article, which will focus exclusively on XMLHttpRequest. The core of this code breaks down into three steps:
xmlHttp, to reference the XMLHttpRequest object that you will create.
Msxml2.XMLHTTP object.
Microsoft.XMLHTTP object.
xmlHttp still isn't set up, create the object in a non-Microsoft way.
At the end of this process, xmlHttp should reference a valid XMLHttpRequest object, no matter what browser your users run.
What about security? Today's browsers offer users the ability to crank their security levels up, to turn off JavaScript technology, and disable any number of options in their browser. In these cases, your code probably won't work under any circumstances. For these situations, you'll have to handle problems gracefully – that's at least one article in itself, one I will tackle later (it's going to be a long series, isn't it? Don't worry; you'll master all of this before you're through). For now, you're writing robust, but not perfect, code, which is great for getting a handle on Ajax. You'll come back to the finer details.
Request/Response in an Ajax world
So you now understand Ajax and have a basic idea about the XMLHttpRequest object and how to create it. If you've read closely, you even
realize that it's the JavaScript technology that talks to any Web application on the server rather than your HTML form being submitted to that application
directly.
What's the missing piece? How to actually use XMLHttpRequest. Since this is critical code that you'll use in some form in everyAjax
application you write, take a quick tour through what a basic request/response model with Ajax looks like.
You have your shiny new XMLHttpRequest object; now take it for a spin. First, you need a JavaScript method that your Web page can call (like
when a user types in text or selects an option from a menu). Then, you'll follow the same basic outline in almost all of your Ajax applications:
Listing 5 is a sample of an Ajax method that does these very things, in this order:
function callServer() {
// Get the city and state from the web form
var city = document.getElementById("city").value;
var state = document.getElementById("state").value;
// Only go on if there are values for both fields
if ((city == null) || (city == "")) return;
if ((state == null) || (state == "")) return;
// Build the URL to connect to
var url = "/scripts/getZipCode.php?city=" + escape(city) + "&state=" + escape(state);
// Open a connection to the server
xmlHttp.open("GET", url, true);
// Setup a function for the server to run when it's done
xmlHttp.onreadystatechange = updatePage;
// Send the request
xmlHttp.send(null);
}
|
A lot of this is self-explanatory. The first bit of the code uses basic JavaScript code to grab the values of a few form fields. Then the code sets up a PHP script as the destination to connect to. Notice how the URL of the script is specified and then the city and state (from the form) are appended to this using simple GET parameters.
Next, a connection is opened; here's the first place you see XMLHttpRequest in action again. The method of connection is indicated (GET), as
well as the URL to connect to. The final parameter, when set to true, requests an asynchronous connection (thus making this Ajax). If you usedfalse, the code would wait around on the server when the request was made and not continue until a response was received. By setting this to true, your users can still use the form (and even call other JavaScript methods) while the server is processing this request in the
background.
The onreadystatechange property of xmlHttp (remember, that's your instance of the XMLHttpRequest object) allows you
to tell the server what to do when it does finish running (which could be in five minutes or five hours). Since the code isn't going to wait
around for the server, you'll need to let the server know what to do so you can respond to it. In this case, a specific method – called updatePage() – will be triggered when the server is finished processing your request.
Finally, send() is called with a value of null. Since you've added the data to send to the server (the city and state) in the
request URL, you don't need to send anything in the request. So this fires off your request and the server can do what you asked it to do.
If you don't get anything else out of this, notice how straightforward and simple this is! Other than getting the asynchronous nature of Ajax into your head, this is relatively simple stuff. You'll appreciate how it frees you up to concentrate on cool applications and interfaces rather than complicated HTTP request/response code.
The code in Listing 5 is about as easy as it gets. The data is simple text and can be included as part of the request URL. GET sends the request rather than the more complicated POST. There's no XML or content headers to add, no data to send in the body of the request – this is Ajax Utopia, in other words.
Have no fear; things will become more complicated as this series progresses. You'll learn how to send POST requests, how to set request headers and content types, how to encode XML in your message, how to add security to your request – the list is pretty long! Don't worry about the hard stuff for now; get your head around the basics, and you'll soon build up a whole arsenal of Ajax tools.
Now you need to actually deal with the server's response. You really only need to know two things at this point:
xmlHttp.readyState property is equal to 4.
xmlHttp.responseText property.
The first of these – ready states – is going to take up the bulk of the next article; you'll learn more about the stages of an HTTP request than you ever
wanted to know. For now, if you simply check for a certain value (4), things will work (and you'll have something to look forward to in the next article).
The second item – using the xmlHttp.responseText property to get the server's response – is easy. Listing 6 shows an example of a method that the server can call based on
the values sent in Listing 5.
function updatePage() {
if (xmlHttp.readyState == 4) {
var response = xmlHttp.responseText;
document.getElementById("zipCode").value = response;
}
}
|
Again, this code isn't so difficult or complicated. It waits for the server to call it with the right ready state and then uses the value that the server
returns (in this case, the ZIP code for the user-entered city and state) to set the value of another form field. The result is that the zipCode field suddenly appears with the ZIP code – but the user never had to click a button!. That's the desktop application feel I
talked about earlier. Responsiveness, a dynamic feel, and more, all with a little Ajax code.
Observant readers might notice that the zipCode field is a normal text field. Once the server returns the ZIP code and the updatePage() method sets the value of that field with the city/state ZIP code, users can override the value. That's intentional for two
reasons: To keep things in the example simple and to show you that sometimes you want users to be able to override what a server says. Keep both
in mind; they're important in good user-interface design.
So what's left? Actually, not much. You have a JavaScript method that grabs information that the user put into a form, sends it to the server, provides another JavaScript method to listen for and handle a response, and even sets the value of a field when that response comes back. All that's really left is to call that first JavaScript method and start the whole process. You could obviously add a button to your HTML form, but that's pretty 2001, don't you think? Take advantage of JavaScript technology like in Listing 7 .
<form>
<p>City: <input type="text" name="city" id="city" size="25"
onChange="callServer();" /></p>
<p>State: <input type="text" name="state" id="state" size="25"
onChange="callServer();" /></p>
<p>Zip Code: <input type="text" name="zipCode" id="zipCode" size="5" /></p>
</form>
|
If this feels like yet one more piece of fairly routine code, then you're right – it is! When a user puts in a new value for either the city or state
field, the callServer() method fires off and the Ajax fun begins. Starting to feel like you've got a handle on things? Good; that's the idea!
At this point, you're probably not ready to go out and write your first Ajax application – at least, not unless you're willing to do some real digging in
the Resources section. However, you can start to get the basic idea of
how these applications work and a basic understanding of the XMLHttpRequest object. In the articles to come, you'll learn to master this
object, how to handle JavaScript-to-server communication, how to work with HTML forms, and even get a handle on the DOM.
For now, though, spend some time thinking about just how powerful Ajax applications can be. Imagine a Web form that responds to you not just when you click a button, but when you type into a field, when you select an option from a combo box...even when you drag your mouse around the screen. Think about exactly what asynchronous means; think about JavaScript code running and not waiting on the server to respond to its requests. What sorts of problems can you run into? What areas do you watch out for? And how will the design of your forms change to account for this new approach in programming?
If you spend some real time with these issues, you'll be better served than just having some code you can cut-and-paste and throw into an application that you really don't understand. In the next article, you'll put these ideas into practice and I'll give you the details on the code you need to really make applications like this work. So, until then, enjoy the possibilities of Ajax.
Learn
XMLHttpRequest object, by checking out this excellent article on the XMLHttpRequest object.
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