XSL is a template language, not a procedural language. That means a stylesheet specifies a sample of the output, not a sequence of programming steps to generate it. A stylesheet consists of a mixture of output samples with instructions of what to put in each sample. Each bit of output sample and instructions is called a template.
In general, you write a template for each element type in your document. That lets you concentrate on handling just one element at a time, and keeps a stylesheet modular. The power of XSL comes from processing the templates recursively. That is, each template handles the processing of its own element, and then calls other templates to process its children, and so on. Since an XML document is always a single root element at the top level that contains all of the nested descendent elements, the XSL templates also start at the top and work their way down through the hierarchy of elements.
Take the
DocBook <para> paragraph element as
an example. To convert this to HTML, you want to wrap the
paragraph content with the HTML
tags <p> and </p>.
But a DocBook <para> can contain
any number of in-line DocBook elements marking up the text.
Fortunately, you can let other templates take care of those
elements, so your XSL template
for <para> can be quite
simple:
<xsl:template match="para">
<p>
<xsl:apply-templates/>
</p>
</xsl:template>
The <xsl:template> element
starts a new template, and
its match attribute indicates where to
apply the template, in this case to
any <para> elements. The template
says to output a literal <p> string
and then execute
the <xsl:apply-templates/> instruction.
This tells the XSL processor to look among all the
templates in the stylesheet for any that should be applied
to the content of the paragraph. If each template in the
stylesheet includes
an <xsl:apply-templates/> instruction,
then all descendents will eventually be processed. When it
is through recursively applying templates to the paragraph
content, it outputs the </p> closing
tag.
Since you aren't writing a linear procedure to
process your document, the context of where and how to
apply each modular template is important.
The match attribute
of <xsl:template> provides that
context for most templates. There is an entire expression
language, XPath, for identifying what parts of your
document should be handled by each template. The simplest
context is just an element name, as in the example above.
But you can also specify elements as children of other
elements, elements with certain attribute values, the first
or last elements in a sequence, and so on. Here is how the
DocBook <formalpara> element is
handled:
<xsl:template match="formalpara">
<p>
<xsl:apply-templates/>
</p>
</xsl:template>
<xsl:template match="formalpara/title">
<b><xsl:apply-templates/></b>
<xsl:text> </xsl:text>
</xsl:template>
<xsl:template match="formalpara/para">
<xsl:apply-templates/>
</xsl:template>
There are three templates defined, one for
the <formalpara> element itself,
and one for each of its children elements. The match attribute
value formalpara/title in the second
template is an XPath expression indicating
a <title> element that is an
immediate child of
a <formalpara> element. This
distinguishes such titles from
other <title> elements used in
DocBook. XPath expressions are the key to controlling how
your templates are applied.
In general, the XSL processor has internal rules that
apply templates that are more specific before templates
that are less specific. That lets you control the details,
but also provides a fallback mechanism to a less specific
template when you don't supply the full context for every
combination of elements. This feature is illustrated by the
third template, for formalpara/para. By
including this template, the stylesheet processes a <para> within <formalpara> in
a special way, in this case by not outputting the HTML <p> tags already output by its parent. If this template had not been included, then the processor would have fallen back to the template
specified by match="para" described
above, which would have output a second set of <p> tags.
You can also control template context with
XSL modes, which are used extensively
in the DocBook stylesheets. Modes let you process the same
input more than once in different ways.
A mode attribute in
an <xsl:template> definition adds a
specific mode name to that template. When the same mode
name is used
in <xsl:apply-templates/>, it acts
as a filter to narrow the selection of templates to only
those selected by
the match expression and that
have that mode name. This lets you define two different
templates for the same element match that are applied under
different contexts. For example, there are two templates
defined for
DocBook <listitem> elements:
<xsl:template match="listitem"> <li><xsl:apply-templates/></li> </xsl:template> <xsl:template match="listitem" mode="xref"> <xsl:number format="1"/> </xsl:template>
The first template is for the normal list item
context where you want to output the
HTML <li> tags. The second template
is called with <xsl:apply-templates
select="$target" mode="xref"/> in the context
of processing <xref> elements. In
this case the select attribute locates
the ID of the specific list item and
the mode attribute selects the second
template, whose effect is to output its item number when it
is in an ordered list. Because there are many such special
needs when
processing <xref> elements, it is
convenient to define a mode name xref to
handle them all. Keep in mind that mode settings
do not automatically get passed down to
other templates
through <xsl:apply-templates/>.
Although XSL is template-driven, it also has some features of traditional programming languages. Here are some examples from the DocBook stylesheets.
Assign a value to a variable: <xsl:variable name="refelem" select="name($target)"/> If statement: <xsl:if test="$show.comments"> <i><xsl:call-template name="inline.charseq"/></i> </xsl:if> Case statement: <xsl:choose> <xsl:when test="@columns"> <xsl:value-of select="@columns"/> </xsl:when> <xsl:otherwise>1</xsl:otherwise> </xsl:choose> Call a template by name like a subroutine, passing parameter values and accepting a return value: <xsl:call-template name="xref.xreflabel"> <xsl:with-param name="target" select="$target"/> </xsl:call-template>
However, you can't always use these constructs as you do in other programming languages. Variables in particular have very different behavior.
XSL provides two elements that let you assign a value
to a
name: <xsl:variable> and <xsl:param>.
These share the same name space and syntax for assigning
names and values. Both can be referred to using
the $name syntax. The main difference
between these two elements is that a param's value acts as
a default value that can be overridden when a template is
called using
a <xsl:with-param> element as in the
last example above.
Here are two examples from DocBook:
<xsl:param name="cols">1</xsl:param> <xsl:variable name="segnum" select="position()"/>
In both elements, the name of the parameter or
variable is specified with
the name attribute. So the name of
the param here
is cols and the name of
the variable is segnum.
The value of either can be supplied in two ways. The value
of the first example is the text node "1" and is supplied
as the content of the element. The value of the second
example is supplied as the result of the expression in
its select attribute, and the element
itself has no content.
The feature of XSL variables that is odd to new users is that once you assign a value to a variable, you cannot assign a new value within the same scope. Doing so will generate an error. So variables are not used as dynamic storage bins they way they are in other languages. They hold a fixed value within their scope of application, and then disappear when the scope is exited. This feature is a result of the design of XSL, which is template-driven and not procedural. This means there is no definite order of processing, so you can't rely on the values of changing variables. To use variables in XSL, you need to understand how their scope is defined.
Variables defined outside of all templates are considered global variables, and they are readable within all templates. The value of a global variable is fixed, and its global value can't be altered from within any template. However, a template can create a local variable of the same name and give it a different value. That local value remains in effect only within the scope of the local variable.
Variables defined within a template remain in effect
only within their permitted scope, which is defined as all
following siblings and their descendants. To understand
such a scope, you have to remember that XSL instructions
are true XML elements that are embedded in an XML family
hierarchy of XSL elements, often referred to as parents,
children, siblings, ancestors and descendants. Taking the
family analogy a step further, think of a variable
assignment as a piece of advice that you are allowed to
give to certain family members. You can give your advice
only to your younger siblings (those that follow you) and
their descendents. Your older siblings won't listen,
neither will your parents or any of your ancestors. To
stretch the analogy a bit, it is an error to try to give
different advice under the same name to the same group of
listeners (in other words, to redefine the variable). Keep
in mind that this family is not the elements of your
document, but just the XSL instructions in your stylesheet.
To help you keep track of such scopes in hand-written
stylesheets, it helps to indent nested XSL elements. Here
is an edited snippet from the DocBook stylesheet
file pi.xsl that illustrates different
scopes for two variables:
1 <xsl:template name="dbhtml-attribute"> 2 ... 3 <xsl:choose> 4 <xsl:when test="$count>count($pis)"> 5 <!-- not found --> 6 </xsl:when> 7 <xsl:otherwise> 8 <xsl:variable name="pi"> 9 <xsl:value-of select="$pis[$count]"/> 10 </xsl:variable> 11 <xsl:choose> 12 <xsl:when test="contains($pi,concat($attribute, '='))"> 13 <xsl:variable name="rest" select="substring-after($pi,concat($attribute,'='))"/> 14 <xsl:variable name="quote" select="substring($rest,1,1)"/> 15 <xsl:value-of select="substring-before(substring($rest,2),$quote)"/> 16 </xsl:when> 17 <xsl:otherwise> 18 ... 19 </xsl:otherwise> 20 </xsl:choose> 21 </xsl:otherwise> 22 </xsl:choose> 23 </xsl:template>
The scope of the variable pi begins
on line 8 where it is defined in this template, and ends on
line 20 when its last sibling ends.[1] The scope of the variable rest begins on line 13 and ends on line 15. Fortunately, line
15 outputs an expression using the value before it goes out of
scope.
What happens when
an <xsl:apply-templates/> element
is used within the scope of a local variable? Do the
templates that are applied to the document children get the
variable? The answer is no. The templates that are applied
are not actually within the scope of the variable. They
exist elsewhere in the stylesheet and are not following
siblings or their descendants.
To pass a value to another template, you pass a
parameter using
the <xsl:with-param> element. This
parameter passing is usually done with calls to a specific
named template
using <xsl:call-template>, although
it works
with <xsl:apply-templates> too.
That's because the called template must be expecting the
parameter by defining it using
a <xsl:param> element with the same
parameter name. Any passed parameters whose names are not
defined in the called template are ignored.
Here is an example of parameter passing
from docbook.xsl:
<xsl:call-template name="head.content"> <xsl:with-param name="node" select="$doc"/> </xsl:call-template>
Here a template
named head.content is being called and
passed a parameter named node whose
content is the value of the $doc variable
in the current context. The top of that template looks like
this:
<xsl:template name="head.content"> <xsl:param name="node" select="."/>
The template is expecting the parameter because it
has a <xsl:param> defined with the
same name. The value in this definition is the default
value. This would be the parameter value used in the
template if the template was called without passing that
parameter.
You generate HTML from your DocBook XML files by
applying the HTML version of the stylesheets. This is done
by using the HTML driver
file docbook/html/docbook.xsl as your
stylesheet. That is the master stylesheet file that
uses <xsl:include> to pull in the
component files it needs to assemble a complete stylesheet
for producing HTML.
The way the DocBook stylesheet generates HTML is to
apply templates that output a mix of text content and HTML
elements. Starting at the top level in the main
file docbook.xsl:
<xsl:template match="/">
<xsl:variable name="doc" select="*[1]"/>
<html>
<head>
<xsl:call-template name="head.content">
<xsl:with-param name="node" select="$doc"/>
</xsl:call-template>
</head>
<body>
<xsl:apply-templates/>
</body>
</html>
</xsl:template>
This template matches the root element of your input
document, and starts the process of recursively applying
templates. It first defines a variable
named doc and then outputs two literal
HTML elements <html> and <head>.
Then it calls a named
template head.content to process the
content of the HTML <head>, closes
the <head> and starts
the <body>. There it
uses <xsl:apply-templates/> to
recursively process the entire input document. Then it just
closes out the HTML file.
Simple HTML elements can generated as literal
elements as shown here. But if the HTML being output
depends on the context, you need something more powerful to
select the element name and possibly add attributes and
their values. Here is a fragment
from sections.xsl that shows how a
heading tag is generated using
the <xsl:element> and <xsl:attribute> elements:
1 <xsl:element name="h{$level}">
2 <xsl:attribute name="class">title</xsl:attribute>
3 <xsl:if test="$level<3">
4 <xsl:attribute name="style">clear: all</xsl:attribute>
5 </xsl:if>
6 <a>
7 <xsl:attribute name="name">
8 <xsl:call-template name="object.id"/>
9 </xsl:attribute>
10 <b><xsl:copy-of select="$title"/></b>
11 </a>
12 </xsl:element>
This whole example is generating a single HTML
heading element. Line 1 begins the HTML element definition
by identifying the name of the element. In this case, the
name is an expression that includes the
variable $level passed as a parameter to
this template. Thus a single template can
generate <h1>, <h2>,
etc. depending on the context in which it is called. Line 2
defines a class="title" attribute that is
added to this element. Lines 3 to 5 add
a style="clear all" attribute, but only
if the heading level is less than 3. Line 6 opens
an <a> anchor element. Although this
looks like a literal output string, it is actually modified
by lines 7 to 9 that insert
the name attribute into
the <a> element. This illustrates
that XSL is managing output elements as active element
nodes, not just text strings. Line 10 outputs the text of
the heading title, also passed as a parameter to the
template, enclosed in HTML boldface tags. Line 11 closes
the anchor tag with the
literal </a> syntax, while line 12
closes the heading tag by closing the element definition.
Since the actual element name is a variable, it couldn't
use the literal syntax.
As you follow the sequence of nested templates
processing elements, you might be wondering how the
ordinary text of your input document gets to the output. In
the file docbook.xsl you will find
this template that handles any text not processed by any
other template:
<xsl:template match="text()"> <xsl:value-of select="."/> </xsl:template>
This template's body consists of the "value" of the text node, which is just its text. In general, all XSL processors have some built-in templates to handle any content for which your stylesheet doesn't supply a matching template. This template serves the same function but appears explicitly in the stylesheet.
You generate formatting objects from your DocBook XML
files by applying the fo version of the stylesheets. This
is done by using the fo driver
file docbook/fo/docbook.xsl as your
stylesheet. That is the master stylesheet file that
uses <xsl:include> to pull in the
component files it needs to assemble a complete stylesheet
for producing formatting objects. Generating a formatting
objects file is only half the process of producing typeset
output. You also need a formatting object processor such as
the Apache XML Project's FOP as described in an earlier
section.
The DocBook fo stylesheet works in a similar manner
to the HTML stylesheet. Instead of outputting HTML tags, it
outputs text marked up
with <fo: tags.
For example, to indicate that some text should be kept
in-line and typeset with a monospace font, it might look
like this:something>
<fo:inline-sequence font-family="monospace">/usr/man</fo:inline-sequence>
The templates
in docbook/fo/inline.xsl that produce
this output for a
DocBook <filename> element look
like this:
<xsl:template match="filename">
<xsl:call-template name="inline.monoseq"/>
</xsl:template>
<xsl:template name="inline.monoseq">
<xsl:param name="content">
<xsl:apply-templates/>
</xsl:param>
<fo:inline-sequence font-family="monospace">
<xsl:copy-of select="$content"/>
</fo:inline-sequence>
</xsl:template>
There are dozens of fo tags and attributes specified in the XSL standard. It is beyond the scope of this document to cover how all of them are used in the DocBook stylesheets. Fortunately, this is only an intermediate format that you probably won't have to deal with very much directly unless you are writing your own stylesheets.
[1] Technically, the scope extends to the end tag of the parent of the <xsl:variable> element. That is effectively the last sibling.