XSLT Keys

XSLT Keys As your XSLT transformations get more complicated, they'll also start to take a little longer to run. This can become a problem when (for instance) you're generating content on-the-fly for a website. Time is often wasted because the same template performs the same expensive operation many times. xsl:key can help make your templates more efficient.

The nature of the problem

Imagine that we want to transform our Hamlet file (again). This time, we want to add a copy of the speaker's roleDesc immediately following the character's name at the beginning of every speech. This template would do it: ( ) Open parenthesis. Get the xml:id of the role from the who attribute of the parent sp element and stash it in a variable. Retrieve the matching castItem element in the cast list, and output its roleDesc content. Close parenthesis.

Inefficiencies in the previous template

Look again at this line: Every time this is executed, the XSLT processor has to look through all the castItem elements to find the matching one. This happens for every speaker element matched by the template.

xsl:key to the rescue To avoid repeating the same lookup operation every time a template is invoked, we can use xsl:key. xsl:key builds an index to a specific node. It builds the index once, and keeps it available for quick lookups during the rest of the transformation. The index works like an associative array.

Building an xsl:key index

An xsl:key index is built using a single command. The command appears at the top level of your stylesheet, usually near the beginning of the file. It looks like this:

This builds a key index named roles. The match attribute specifies the nodes which will be indexed. In this case, it's indexing all castItem elements whose type attribute is role. The last attribute, use, specifies what we use to look up a particular item in the index. Here, we use the xml:id attribute on the role child of the target castItem node.

Using an xsl:key index

Here's our index:

Here's how we can use it to accomplish the same job as before: ( )

xsl:key Task (and review of identity transforms and xsl:copy)

Using the eventDemo.xml file:

Create an identity transform. Add an xsl:key like this: name demoPlaces match place/placeName/orgName use ancestor::place/@xml:id Use the key to add the location of the meeting to the end of the label element, so that this: TEI MM 2004 becomes this: TEI MM 2004 (Johns Hopkins University)

xsl:key Task: creating the key

The key constructor will look like this:

xsl:key Task: invoking the key in a template

This is one way to use the key to accomplish the task: ( )

What is the call to "replace()" doing? Are there other ways of doing that?

Among the other ways of handling the hash problem in the where attribute would be translate(), substring-after(), and prepending the hash to the use attribute with concat() when creating the key, so the where attribute can be used directly.