Amazingly, the answer is essentially yes. Here's an example of what that means. Let's say that your client wants a set of pages to have tabs as one of the "minor" changes. Using AJAX and, for example, the Yahoo AJAX User Interface, it would actually take longer to copy existing content to the new tabbed pages than it would to implement those new tabs.
You can see some real-world examples of AJAX Web pages at two well-known Web sites: Google Maps and Ask. Play around with a map and try dragging it (left- and right-clicks will zoom in or out on the mapped area), or on Ask.com look at how responsive the query box is when asking a question. You can further notice some AJAX magic via the drop-down box that pops up on Google's main page: It should be obvious from the speed with which the items appear in the box that something out of the ordinary is happening. Note that items on the list change as you type.
Just about every AJAX script will always provide a routine called XMLHttpRequest, the event handler at the center of all AJAX-type code. Basically, this routine is set up to process all the events as a "callback" function, something at the heart of Windows programming and a routine that is translated well into other operating systems and browsers. This is why the majority of properly written AJAX code tends to be transportable across different operating systems and browsers.
Before diving headfirst into the code, you should carefully review the DOM (Document Object Model) thoroughly. Making efficient usage of some of the more intricate aspects of the DOM is paramount to becoming a more-then-competent AJAX programmer. A complete understanding of the DOM leads to truly sophisticated Web pages.
The absolute definition of the request object can be found at W3C's official site. This document requires a very careful and thoughtful read, however: there are a number of gotchas open to interpretation by the unwary, seemingly standard and required practice when dealing with W3C documents.
The AJAX programmer must implement the XMLHttpRequest object so that it is able to handle just about any browser that comes across The Web site, including Internet Explorer (early and recent versions), Firefox, Safari, Opera and others. The code sample at the end of this article will do just that when placed in the standard index or default .htm file:
Note that the Internet Explorer portion of the code is more complicated. One portion has to handle the earlier versions of the browser, while the other takes care of the newer ones. Further note how clean the code for the other browsers has become. This code basically creates a new request handler and assigns a variable, xmlHttp, to it for later processing.
A list of status codes and a simple event handler follow:
|0||The request is not initialized|
|1||The request has been set up|
|2||The request has been sent|
|3||The request is in process|
|4||The request is complete|
The .open method takes three arguments. The first will indicate whether the standard "GET" or "POST" form submission is used, defining whether the action to be taken will be exposed within the submitted form or through the URL. This determines how the "action" target will handle the eventual event. POST is usually used for sending information to the server, GET for retrieving information. Always put the method type in uppercase.
The second argument indicates the target of the submission (in this case "tryme.asp").
The third argument will indicate whether this is an asynchronous call, indicated by a "TRUE" value, or a synchronous call, indicated by a "FALSE" value. The underlying heart of AJAX is that it just about every call to the target will be asynchronous, not something most Web developers are used to. Generally speaking, a synchronous call will lock up the Web page until it is finished transmitting to the server, the server processes incoming data, then transmits an entire page back. Rather inefficient if you only need to update a single text field, for example.
Basically, a properly constructed AJAX page will allow that, once an field in the form is active, the request function will be called for every key entered into that field or upon selecting an item in a 'SELECT' field.
It is important to understand that he "Request Server" can either be on a remote machine or on the same machine as the browser. The secret of AJAX is that there is no need for a Submit button any longer: Once the page is loaded it can interact with the "Request Server" without having to reload the page. This gives extraordinary flexibility, interactive capabilities and lightning-fast responses.
Note that even though AJAX does not require entire pages be submitted or posted for processing allowing for individual form elements to submit, is still important to check the status return from a submission: A 200 code still means OK, a 404 code still indicates that the target was not found.
The following real-world examples show how useful AJAX can be.
A very interesting snippet from comes Janos Pal Toth and can be found at this page. Cutting to the chase, you can download this script, upload that to your server and access it. You can see an example at this site. Once you are running it, you'll see a magnifying rectangle in each image. Left-click and drag on the magnifier to increase/decrease its size. Right- click and drag to increase/decrease the magnification factor. As you'll see when you download the entire script it's trivial to replace the "test.jpg" image with one of your own — on your own server, of course!
Probably one of the easiest and most impressive AJAX enhancements both to show and implement is the ability to include a simple way to show the so-called "accordion effect" — a means for comfortably showing more information than the screen real estate would normally allow. This trick can be easily seen in the directory at this site.
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