| By Jeffry Houser |
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| February 9, 2007 06:30 PM EST |
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43,214 |
The first line initializes the request variable, giving it a Boolean value of false. You can check against this later to make sure that your request object was properly initialized. The next code creates an instance of the XMLHttpRequest object using 'new XMLHttpRequest.' This works fine for most non-Microsoft browsers, but won't work in IE. If it doesn't initialize, then an error is thrown, which is where the first catch statement comes into play. It attempts to re-initialize using the method available in the current version of Internet Explorer: new ActiveXObject("Msxml2.XMLHTTP").
If that fails, it tries the version from older versions of Internet Explorer new ActiveXObject("Microsoft.XMLHTTP"). That's kind of complicated right? I borrowed this initialization code from the article www-128.ibm.com/developerworks/Web/library/
wa-ajaxintro2/?ca=dgr-lnxw07AJAX-Request (which you should all read because it goes into a lot more details about error checking that are outside the scope of this article).
So, now the code has created the object, but what can you do with it? As you can probably see, it doesn't do much yet. You want to assign it a URL, load the data, and then process the data in some way. This function will do that:
function getRSS() {
var url = "/htdocs/jeffhousercom/wwwroot/rss.cfm";
request.open("GET", url, true);
request.onreadystatechange = updatePage;
request.send(null);
}
This is function to grab an RSS feed. The first line defines the URL. This is standard JavaScript for defining a variable. You should bear in mind that you can only request URLs from the current Web site. In this case, I'm grabbing a RSS feed from a local copy of my blog. The next line calls a method on the request object entitled open. Open prepares the request to be sent, but doesn't actual send it. There are five arguments to the open function (although we only use three in this example):
- Request-type: This specifies the request type, either get or post, for the URL that you want to send.
- url: The URL specifies the URL that you're getting (or posting) to.
- Asynchronous: The async value is a Boolean value that specifies whether this request is asynchronous or not. For AJAX use, you'll want to specify true. If set to false, the application will hold until this request is finished, which defeats the purpose of an RIA in the first place.
- username: The username specifies the username required for accessing the URL, if applicable.
- password: The username specifies the username required for accessing the URL, if applicable.
Okay, you've set up the request to be ready to send. The next line specifies the onreadystatechange property. This property specifies what function to call once the request is complete. If you don't specify a method to be called then nothing will happen once the request is complete. The final line calls the send method. The send method is the one that will execute the request. Its parameter is the data that you need to send along with the request. In our example, we aren't sending parameters, so we pass a null value.
Once the request complete, the updatePage function will execute, so lets look at that next:
function updatePage() {
if (request.readyState == 4){
Form1.XMLValues.value = request.responseText;
}
}
This function first checks the readyState of the XMLHttpRequest object. Exploring that is beyond the scope of this article, but there are a lot more details in some of the references provided at the end. For now, all you need to know is that if the readyState is 4, then the request is complete. This takes the resulting value from the server response and assigns it to a form textbox. The form is below:
<form name="Form1">
<textarea name="XMLValues" rows="20" cols="50"></textarea>
</form>
The text box will contain the XML from the RSS feed. There's another property associated with the XMLHttpRequest object called responseXML. This will contain the results as an XML Document object, which JavaScript can access natively. Unfortunately specific details are beyond the scope of this article, but I found this good resource to get you started on that: www.peachpit.com/articles/article.asp?p=29307&seqNum=4&rl=1.
Where To Go from Here
If you want to learn more about AJAX, there are some great resources you can read. The article by Jesse James Garrett that started it all is at www.adaptivepath.com/publications/essays/archives/000385.php. Garrett is credited with coining the term AJAX. You can follow an AJAX blog at http://www.ajax-powered.net, which talks about all things AJAX. Finally, there are a few people who are working on ways to integrate ColdFusion and AJAX together. One is the CFAJAX project www.indiankey.com/cfajax. Another is the AJAXCFC project at www.robgonda.com/blog/projects/ajaxcfc.
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