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Apply-templates revisited ...: how to design and use XSL templates.

Article Excerpt
The beginning of every XSLT book or tutorial introduces "template matching" and how to use Apply-Templates. This article explains the benefits of using XSLT templates as well as some examples that show how a "real-world" application can use Apply-Templates along with other XSLT techniques.

Previously I've skirted around using Apply-Templates, opting instead for XSLT programming logic. The result was always a lot of XSLT code with , , and statements. Most of the "if" and "choose" statements checked the values or existence of nodes and elements in an XML file before parsing and displaying the output as HTML or XML. These applications required a lot of logic, and most continue to perform well, so maybe not using Apply-Templates was feasible, but in the future I intend to be more careful in designing applications that use stylesheets and how those stylesheets will be developed.

I recently ran into a situation where Apply-Templates should be used -- otherwise the XSL stylesheet would expand to over a hundred lines of unmanageable code. After piecing together a few examples from books and online, Apply-Templates is making everything much easier. I realize that thinking of templates as objects makes a lot of sense (to parse this node, use this object -- I mean template). Thinking this way produces a better design, which is explained more near the end of this article.

Two Types of Templates

Contrary to the introductory examples mentioned above, Apply-Templates is only part of the solution when building an application, and many XML/XSLT specifications are required in designing a larger application. The two basic concepts of XSLT are calling templates by name and applying templates.

* Calling templates: A fairly straightforward technique in which a template is named and then called to parse the selected data. When calling a template, parameters can be passed and these values can be used later to filter by or as another piece of information for the output.

* Applying templates: Relies on "matching" elements found in the data, then parsing each node -- actually performing two operations.

Applications that use both types are much easier to design than trying to make everything conform to one technique or the other.

Analyzing an XML Document

Understanding how an application needs to be developed and the structure of the XML document are the most important aspects of designing an XML-based program. Begin by looking for parts of the data that are unique. Alter analyzing a document, you can break the content or data into sections. Less

frequent data, such...

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