Beginning Ajax With ASP.NET (2006).pdf

The XMLHttpRequest Object
As you can see, the data contained within the XML file is rendered in the page in exactly the same way it
is stored within the file.
As mentioned previously, this might be okay for simple scenarios that require only a single unit of data
or data that is in a proprietary format, but for more complex scenarios you’ll want to use the
responseXML property.
Using the responseXML Property
In a majority of scenarios, you will want to return multiple result items. This might be a list of names to
display in a drop-down list, a list of customers, or an object representation. The XML format is ideally
suited to this, and direct support for this format is provided by the responseXML property of the
XMLHttpRequest object. This property is a standard XML document object that can be examined and
parsed using W3C DOM node tree methods and properties.
Try It Out
For detailed reference material on the XML Document Object Model, visit http://msdn
.microsoft.com/library/en-us/xmlsdk30/htm/xmmscxmlreference.asp.
Returning Response Data As an XML Document Object
You can continue to modify the code example to extract values from the XML data that was retrieved
and displayed in the previous example. Rather than reproduce the entire previous code sample, we will
show only the modified section that replaces the added code from the previous example that used the
responseText property. Examine the following code:
if ( xmlHttpObj.readyState == READYSTATE_COMPLETE )
{
var doc = xmlHttpObj.responseXML;
var node = doc.selectSingleNode(“//Customers/Customer/Firstname/text()”);
document.getElementById(“divResults”).childNodes[0].nodeValue = node.nodeValue;
}
Instead of simply displaying the entire set of returned data, you are extracting the text value of the first
instance of the node. You do this by utilizing the selectSingleNode method, which
takes an X Path expression as an argument and returns an XML node object if one is found or NULL if the
X Path query is unsuccessful. With the returned node object, you assign the nodeValue property of the
first element of the childNodes property, which itself is a property of the divResults element to the
text value of the node.
For those unfamiliar with the X Path language, you can think of it as a dynamic query language for
XML documents that operates in a similar way to ANSI SQL for databases. Nodes, elements, and data
can be queried and returned using X Path expressions. X Path expressions can contain conditional
expressions and be quite complex. For a detailed reference on X Path expressions and the associated
syntax, visit the W3C site at www.w3.org/TR/xpath, or for something a little more readable, try
www.topxml.com/xsl/XPathRef.asp.
This example is still fairly simplistic, though. Now, you’re going to take your newfound knowledge and
apply this by creating a simple web page that allows you to make a selection from a list of customers
and display the fullname and email address by doing a server-side lookup into your XML data file
asynchronously.
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