E.4 More XPathScript Details

XPathScript aims to provide the power and flexibility of XSLT as an XML transformation language, without the restriction of XSLT's XML-based syntax. Unlike XSLT, which has special modes for outputting in text, XML, and HTML, XPathScript outputs only plain text. This makes it a lot easier than XSLT for people coming from a Perl background to learn. However, XPathScript is not a W3C specification, despite being based on XPath, which is a W3C recommendation.

XPathScript follows the basic ASP syntax for introducing code and outputting code to the browser: use <% %> to introduce Perl code, and <%= %> to output a value.

E.4.1 The XPathScript API

Along with the code delimiters, XPathScript provides stylesheet developers with a full API for accessing and transforming the source XML file. This API can be used in conjunction with the delimiters listed above to provide a stylesheet language that is as powerful as XSLT, yet supports all the features of a full programming language such as Perl. (Other implementations, such as Python or Java, also are possible.)

E.4.1.1 Extracting values

A simple example to get us started is to use the API to bring in the title from a DocBook article. A DocBook article title looks like this:

<article>
 <artheader>
  <title>XPathScript - A Viable Alternative to XSLT?</title>
  ...

The XPath expression to retrieve the text in the <title> element is:

/article/artheader/title/text( )

Putting all this together to make this text into the HTML title, we get the following XPathScript stylesheet:

<html>
<head>
 <title><%= findvalue("/article/artheader/title") %></title>
</head>
<body>
  This was a DocBook Article. 
  We're only extracting the title for now!
<p>
The title was: <%= findvalue("/article/artheader/title") %>
</body>
</html>

Again, we see the XPath syntax being used to find the nodes in the document, along with the function findvalue( ). Similarly, a list of nodes can be extracted (and thus looped over) using the findnodes( ) function:

...
<%
for my $sect1 (findnodes("/article/sect1")) {
  print $sect1->findvalue("title"), "<br>\n";
  for my $sect2 ($sect1->findnodes("sect2")) {
    print " + ", $sect2->findvalue("title"), "<br>\n";
    for my $sect3 ($sect2->findnodes("sect3")) {
      print " + + ", $sect3->findvalue("title"), "<br>\n";
    }
  }
}
%>
...

Here we see how we can apply the find* functions to individual nodes as methods, which makes the node the context node to search from. That is, $node->findnodes("title") finds <title> child nodes of $node.

E.4.1.2 Declarative templates

We saw declarative templates earlier in this appendix, in Section E.2. The $t hash is the key to declarative templates. The apply_templates( ) function iterates over the nodes of your XML file, applying the templates defined in the $t hash reference as it meets matching tags. This is the most important feature of XpathScript, because it allows you to define the appearance of individual tags without having to do your own iteration logic. We call this declarative templating.

The keys of $t are the names of the elements, including namespace prefixes where appropriate. When apply_templates( ) is called, XPathScript tries to find a member of $t that matches the element name.

The following subkeys define the transformation:

pre: Output to occur before the tag
post: Output to occur after the tag
prechildren: Output to occur before the children of this tag are written
postchildren: Output to occur after the children of this tag are written
prechild: Output to occur before every child element of this tag
postchild: Output to occur after every child element of this tag
showtag: Set to a false value (generally zero) to disable rendering of the tag itself
testcode: Code to execute upon visiting this tag

More details about XPathScript can be found on the AxKit web site, at http://axkit.org/.

[ Team LiB ]