Beginning Ajax With ASP.NET (2006).pdf

The XMLHttpRequest Object
while the user is still manipulating the information within the browser. This represents the heart of Ajax
and is the core advantage that it represents within traditional browser applications. A user can continue
to work within the browser application uninterrupted, while in the background a request is sent and a
response that contains the result of some server-side processing is received.
Synchronous Requests
Take a look at a following simple code example of a synchronous operation and then take a look at the
explanation of exactly what is occurring that follows. Note: The code for the inclusion of the script
include file mentioned previously has been omitted for brevity.
Try It Out
A Synchronous Operation
function MakeXMLHTTPCall()
{
var xmlHttpObj;
xmlHttpObj = CreateXmlHttpRequestObject();
if (xmlHttpObj)
{
xmlHttpObj.open(“GET”,”http://” + location.host +
“/XmlHttpExample1/DataFile.xml”, false);
xmlHttpObj.send(null);
alert(“Request/Response Complete.”);
}
}
How It Works
The preceding code sample is very simple, however it does show the basic usage of the XMLHTTP
object. If you examine the code in detail, you’ll see the following:
1. First you create a new XMLHTTP object and assign it to a variable.
2. After checking if the object is not null, that is, that the object creation in Step 1 was successful,
you execute the open method passing in three parameters:
xmlHttpObj.open(“GET”,”http://” + location.host + “/XmlHttpExample1/DataFile.xml”,
false);
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The first parameter, “GET”, is the type of request to make (this can be any of the standard
HTTP verbs “GET”, “POST”, “PUT”, or “HEAD”)
The second parameter is the server address, or endpoint, to make the request to. In this
case, it’s an XML file located at http://localhost/XmlHttpExample1DataFile.xml.
The third parameter, false, indicates whether a synchronous or asynchronous request
should take place. In this case, false indicates that a synchronous request should
occur.
3. The send method is executed on the XMLHTTP object instance to perform the actual request.
xmlHttpObj.send(null);
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