Your First AxKit Page (Practical mod

E.2. Your First AxKit Page

Now we're going to see how AxKit works, by transforming an XML file containing data about Camelids (note the dubious Perl reference) into HTML.

First you will need a sample XML file. Open the text editor of your choice and type the code shown in Example E-1.

Example E-1. firstxml.xml

<?xml version="1.0"?>
<dromedaries>
  <species name="Camel">
    <humps>1 or 2</humps>
    <disposition>Cranky</disposition>
  </species>
  <species name="Llama">
    <humps>1</humps>
    <disposition>Aloof</disposition>
  </species>
  <species name="Alpaca">
    <humps>(see Llama)</humps>
    <disposition>Friendly</disposition>
  </species>
</dromedaries>

Save this file in your web server document root (e.g., /home/httpd/httpd_perl/htdocs/) as firstxml.xml.

Now we need a stylesheet to transform the XML to HTML. For this first example we are going to use XPathScript, an XML transformation language specific to AxKit. Later we will give a brief introduction to XSLT.

Create a new file and type the code shown in Example E-2.

Example E-2. firstxml.xps

<%
$t->{'humps'}{pre} = "<td>";
$t->{'humps'}{post} = "</td>";
$t->{'disposition'}{pre} = "<td>";
$t->{'disposition'}{post} = "</td>";
$t->{'species'}{pre} = "<tr><td>{\@name}</td>";
$t->{'species'}{post} = "</tr>";
%>
<html>
<head>
<title>Know Your Dromedaries</title>
</head>
<body>
  <table border="1">
    <tr><th>Species</th>
        <th>No. of Humps</th>
        <th>Disposition</th></tr>
    <%= apply_templates('/dromedaries/species') %>
  </table>
</body>
</html>

Save this file as firstxml.xps.

Now to get the original file, firstxml.xml, to be transformed on the server by text.xps, we need to somehow associate that file with the stylesheet. Under AxKit there are a number of ways to do that, with varying flexibility. The simplest way is to edit your firstxml.xml file and, immediately after the <?xml version="1.0"?> declaration, add the following:

<?xml-stylesheet href="firstxml.xps"
                 type="application/x-xpathscript"?>

Now assuming the files are both in the same directory under your httpd document root, you should be able to make a request for text.xml and see server-side transformed XML in your browser. Now try changing the source XML file, and watch AxKit detect the change next time you load the file in the browser.

E.2.1. If Something Goes Wrong

If you don't see HTML in your browser but instead get the source XML, you will need to check your error log. (In Internet Explorer you will see a tree-based representation of the XML, and in Mozilla, Netscape, or Opera you will see all the text of the document joined together.)

AxKit sends out varying amounts of debug information depending on the value of AxDebugLevel (which we set to the maximum value of 10). If you can't decipher the contents of the error log, contact the AxKit user's mailing list at [email protected] with details of your problem.

E.2.2. How it Works?

The stylesheet above specifies how the various tags work. The ASP <% %>syntax delimits Perl code from HTML. You can execute any code within the stylesheet.

In this example, we use the special XPathScript $t hash reference, which specifies the names of tags and how they should be output to the browser. There are several options for the second level of the hash, and here we see two of those options: pre and post. pre and postspecify (respectfully) what appears before the tag and what appears after it. These values in $t take effect only when we call the apply_templates( ) function, which iterates over the nodes in the XML, executing the matching values in $t.

E.2.3. XPath

One of the key specifications being used in XML technologies is XPath. This is a little language used within other languages for selecting nodes within an XML document (just as regular expressions is a language of its own within Perl). The initial appearance of an XPath is similar to that of a Unix directory path. In Example E-2 we can see the XPath /dromedaries/species, which starts at the root of the document, finds the dromedaries root element, then finds the species children of the dromedaries element. Note that unlike Unix directory paths, XPaths can match multiple nodes; so in the case above, we select all of the species elements in the document.

Documenting all of XPath here would take up many pages. The grammar for XPath allows many constructs of a full programming language, such as functions, string literals, and Boolean expressions. What's important to know is that the syntax we are using to find nodes in our XML documents is not just something invented for AxKit!