Datasheet

You could then insert the XHTML document fragment into the HTML page dynamically to update the

page.

XSLT is another standard maintained by W3C. Both IE and Mozilla have XSLT processors; however, they

don’t always treat their XSLT documents in the same way. XSLT uses another language, XPath, to query

the XML document when applying its transformations. XPath queries are used to address elements

within the original XML document, such as the following:

<xsl:for-each select=”//Hotel”>

The //Hotel statement instructs the browser’s XSLT processor to look for elements called <Hotel> that

are descendants of the root element

<HotelList>. XPath queries can be used to locate specific items or

groups of items within an XML document, using a syntax that is superficially similar to the way

browsers can locate web pages, with the following as an example:

//HotelList/Hotel/Rooms

These techniques can be used to retrieve sections of data and display it on the page using the browser.

Again, they are able to offer instantaneous updates without the need for a trip back to the server. Both

XSLT and XPath are discussed in more detail in Chapter 8.

The XMLHttpRequest Object

If there’s one thing you haven’t come across before, it’s likely to be the XMLHttpRequestObject.

Microsoft introduced quite an obscure little ActiveX control in Internet Explorer version 5 (IE 5) called

the

XMLHttp object. Of course, ActiveX controls are tied to IE, so shortly afterward, Mozilla engineers

followed suit and created their own version for Mozilla 1 and Netscape 7, the corresponding

XMLHttpRequest object. The status of these objects was elevated considerably because of their inclusion

in the original Ajax article. They are now more commonly referred to singularly as the

XMLHttpRequest

object, and, in IE 7, there is a native XMLHttpRequest object in addition to the ActiveX Control. Versions

of this object have been included in Safari 1.2 and Opera as well.

But what does it do? Well, it allows a developer to submit and receive XML documents in the back-

ground. Previously, you could use hidden frames or IFrames to perform this task for you, but the

XMLHttpRequest is rather more sophisticated in the ways in which it allows you to send and pick up

data.

Unfortunately, because it isn’t yet standard, that means there are still two separate ways of creating it. In

versions of IE prior to version 7, it is created as an

ActiveXObject in JavaScript as follows:

var xHRObject = new ActiveXObject(“Microsoft.XMLHTTP”);

There are several versions of the MSXML library with respective ProgIDs to instan-

tiate

XMLHttp object. Specifically, you cannot instantiate any version of XMLHttp

object higher than ‘Msxml2.XMLHTTP.3.0’ with the given version independent

ProgID Microsoft.XMLHTTP. For the examples in this book, though, this will be

sufficient.

Chapter 1: Introducing Ajax

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