XPointer Basics - A Fragmented View (
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So that takes care of XPath - but where
does XPointer fit in?
According to the XPointer specification, XPointer serves as the basis for "fragment
identifiers" in XLinks. You probably don't know this, but you use fragment identifiers
pretty frequently in your daily Web development activities, to link to specific
segments of an HTML document - they're the bits following the hash (#) symbol
in a standard anchor tag. For example, in the hyperlink
<a href="http://www.paint.server/colors.html#tangerine">See what tangerine
looks
like</a>
the segment
#tangerine
would be a fragment identifier.
Just as fragment identifiers, when attached to an HTML hyperlink, direct the
browser to a specific section of an HTML document, XPointers, when attached to
an XLink, identify a specific location (or set of locations) within an XML document.
This location could be a single character, a text string, a block of contiguous
content, or a collection of elements; XPointer allows link authors to specify
the location using XPath's built-in functions and conditional tests.
Let's take a quick example. Consider the following XML document, which I'll call
"movie.xml":
<?xml version="1.0"?>
<movie id="67" genre="sci-fi">
<title>X-Men</title>
<cast>Hugh
Jackman, Patrick Stewart and Ian McKellen</cast>
<director>Bryan Singer</director>
<year>2000</year>
<?play_trailer?>
</movie>
Here's a simple XLink, which uses an XPointer to link to the data for "X-Men"
using the movie's "id" attribute:
<item xmlns:xlink="http://www.w3.org/1999/xlink" xlink:type="simple"
xlink:href="movie.xml#xpointer(id('67'))">X-Men</item>
As you can see, XPointer, with the able assistance of XPath's location paths,
is used to create the fragment identifier that identifies a particular section
of the XML data. Without XPointer, link authors would merely be able to reference
XML documents; they would have no way of drilling down to specific points within
those documents.
As the example above demonstrates, the syntax for an XPointer is usually
xpointer(expression)
For example,
xpointer(id('67')/title)
In order to offer link authors the flexibility to use "fallback links" - say,
if the document structure or content changes - the XPointer specification allows
for more than one XPointer to be specified simultaneously. For example, [code]
xpointer(id('67')/cast) xpointer(id('67')/director)
The XPointers are evaluated in left-to-right order, and the first successful
evaluation is used.
The XPointer specification also defines two additional "shortcut" forms. The
first of these is the so-called "bare name" form, which has been included primarily
to maintain compatibility with the existing HTML fragment identifier. An XPointer
of this form merely specifies a name, and refers to any element within the document
which has an "id" attribute matching that name. Which means that I could write
the XPointer above
xpointer(id('67'))
as
67
and the two would still be equivalent.
An XPointer may also consist of a "child sequence" - essentially, a series of
steps beginning with the document element and drilling down from there to the
required resource. Note that the steps are defined in terms of integers, not element
names - this XPointer references the first child element of the document element
(the movie title.)
xpointer(/1/1)
Bare names and child sequences may be combined together if desired.
As with XPath, you can use predicates and conditional tests to filter out unwanted
nodes, and select from a variety of built-in function to further classify the
result nodeset. I'm not going to spend any more time on this, though - instead,
I'd like to concentrate on XPointer's extensions to XPath, which are primarily
in the form of additional constructs to define "points" and "ranges".
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