Tobi Waves

taught by Evan Lenz

XSLT is a language for processing XML documents. XSLT itself is written in XML. The output generated by an XSLT 'program' can be anything, but normally it is used for generating (X)HTML documents which can then be displayed by a browser.

XSLT uses the XPATH language for addressing 'nodes' in an XML document.

XPATH Expressions

An XPath expression is made up from several steps separated by /.

step/step/step

A step consists of three elements: The axis to identify a set of nodes relative to the current-context node. The node test to filter out relevant nodes from the set of nodes selected by the axis and finally any number of optional predicates to further filter which nodes get selected.

axis::node-test[predicate][predicate]

XPath expressions can return 4 types of data:

node-set which is a number of zero or more nodes without duplicates

number a floating point number

string a unicode string

boolean true or false

XPath knows seven different types of nodes:

Root - the toplevel node of a document is called "/"

Element - 'tags'

PI - Processing instructions lt?xml ...?gt

Comment - Comment tags

Text - Character data, including white space!

Attribute - lttag attrib="xxx"gt

Namespace - lt?xml-stylesheet ...?gt

When selecting XPath nodes, you can use different 14(!) axis. By default, you use the child:: axis. Default means, that you don't even have to mention it. So an expression like section is actually child::section. The other common axis is attribute:: it also has an abbreviation, called @ so instead of writing attribute::section you can write @section

The other 12 axis are: descendant-or-self:: which can be abbreviated as // and parent:: aka .. and self:: aka . the remaining axis do not have abbreviations: ancestor:: following-sibling:: preceding-sibling:: following:: preceding:: namespace:: descendant:: ancestor-or-self::

XPath example

XPATH looks at an XML document as if it was a filesystem. Like when navigating a filesystem, there is a context node where from where XPATH looks at the document.

ltarticlegt
ltheadinggtHellolt/headindgt
ltparagtParagraph ltemphgt1lt/emphgtlt/paragt
ltparagtParagraph 2lt/paragt
lt/articlegt

If the first para node is the current context node the expressions would return the following:

* - emph
emph - emph
.. - para
../* - heading,para,para
../para[2] - the second paragraph
/article/* - heading,para,para

The XSLT Processing Model

The most important command in xslt is xsl:apply-templates. A lot of people do not use this function properly since they write only one big template, instead of writing many small ones for different purposes.

If you use multiple templates, xslt will invoke a conflict resolution protocol when several templates match for a particular element.

Selection happens by priority:

-0.5  match=""
-0.25 match="xyz:" 
 0    match="name"
  .5  match="nameA/nameB" 

You can override the priority of a template by setting the priority attribute explicitly.

Whenever XSLT processing is stared, the processor executes the template-rule for /. If you do not supply a template-rule the processor will use its built-in template rules (there is one for each of the seven node types).