AJAX-Driven Web Sites: Under The Hood
Recently, a number of Web sites have begun to raise some eyebrows within the developer community. What's unique about these sites is that they behave more like a desktop application than a Web application. As you interact with them, they quickly display an endless amount of information to your browser without reloading the page.
At the Google Maps site for example (http://maps.google.com/), you can click on the map, zoom in, zoom out, and move around as much as you like. Your browser continues to be fed with data from the server, yet your browser doesn't have to refresh. They're not using applets, or anything like Flash, so how are they doing it?
Nuts & Bolts
An Ajax Example
When a user types a character and the SendValue(Val) is called, the HTTPRequest object is initialized first. At this point we determine if the current browser is IE, or some-thing else like Mozilla or Netscape, so we can create the objRequest (HTTPRequest) correctly. Next, to handle things on the server side, we load the "url" variable with the location of the aspx page that will do our processing.
Next, let's look at the three objRequest lines. First, the objRequest.onreadystatechange is called. The onreadystatechange property helps us set up a callback function. This callback will be called only when the readyState property changes; that is, when we get data back from the Web server. The callback function will handle it at that time.
The objRequest.open requires three parameters: a GET or POST, a string for the "url" we defined earlier, and a Boolean value that defines whether this call is asynchronous or not. Note that if this Boolean value is set to true, as it is here, a callback function is required.
The objRequest.send(null) line actually calls the aspx page defined in the "url" variable. However, before we go on to the aspx page, notice the callback function (Process()). Remember, this is the function that will be called (back) after the Web page code has processed our request; it's our reentry point. Here, we simply take the values returned from our aspx page, and put them into the second text box (txtEchoOutPut).
From the user's perspective, this is all done very quickly. When a user types in the first text box, their letters appear within the second text box immediately. The typed text is actually making a round trip to the server and back.
Ajax Is Not New
Here at Magenic, we're taking a look at how this methodology can benefit our clients. Ajax is not something that will replace every Web site, as we know it, but it has a place and it's a skill we want to have in our repertoire.
3. The name Ajax is not official. The folks at Adaptive Path are given credit for this catchy name. In ASP.Net 2.0, it is referred to as "script callbacks."
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Last Modified: March 9, 2008