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<title>XSL processing model</title><link rel="stylesheet" href="reference.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.69.0"><link rel="start" href="index.html" title="DocBook XSL Stylesheet Documentation"><link rel="up" href="publishing.html" title="Chapter 1. DocBook XSL"><link rel="prev" href="ch01s02.html" title="A brief introduction to XSL"><link rel="next" href="extensions.html" title="Chapter 2. Extensions"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">XSL processing model</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch01s02.html">Prev</a> </td><th width="60%" align="center">Chapter 1. DocBook XSL</th><td width="20%" align="right"> <a accesskey="n" href="extensions.html">Next</a></td></tr></table><hr></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="d0e321"></a>XSL processing model</h2></div></div></div><p>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 <span class="emphasis"><em>template</em></span>.</p><p>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.</p><p>Take the
DocBook <code class="literal"><para></code> paragraph element as
an example. To convert this to HTML, you want to wrap the
paragraph content with the HTML
tags <code class="literal"><p></code> and <code class="literal"></p></code>.
But a DocBook <code class="literal"><para></code> 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 <code class="literal"><para></code> can be quite
simple:</p><pre class="programlisting"><xsl:template match="para">
<p>
<xsl:apply-templates/>
</p>
</xsl:template>
</pre><p>The <code class="literal"><xsl:template></code> element
starts a new template, and
its <code class="literal">match</code> attribute indicates where to
apply the template, in this case to
any <code class="literal"><para></code> elements. The template
says to output a literal <code class="literal"><p></code> string
and then execute
the <code class="literal"><xsl:apply-templates/></code> 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 <code class="literal"><xsl:apply-templates/></code> instruction,
then all descendents will eventually be processed. When it
is through recursively applying templates to the paragraph
content, it outputs the <code class="literal"></p></code> closing
tag.</p><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="d0e373"></a>Context is important</h3></div></div></div><p>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 <code class="literal">match</code> attribute
of <code class="literal"><xsl:template></code> 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 <code class="literal"><formalpara></code> element is
handled:</p><pre class="programlisting"><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>
</pre><p>There are three templates defined, one for
the <code class="literal"><formalpara></code> element itself,
and one for each of its children elements. The <code class="literal">match</code> attribute
value <code class="literal">formalpara/title</code> in the second
template is an XPath expression indicating
a <code class="literal"><title></code> element that is an
immediate child of
a <code class="literal"><formalpara></code> element. This
distinguishes such titles from
other <code class="literal"><title></code> elements used in
DocBook. XPath expressions are the key to controlling how
your templates are applied.</p><p>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 <code class="literal">formalpara/para</code>. By
including this template, the stylesheet processes a <code class="literal"><para></code> within <code class="literal"><formalpara></code> in
a special way, in this case by not outputting the HTML <code class="literal"><p></code> 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 <code class="literal">match="para"</code> described
above, which would have output a second set of <code class="literal"><p></code> tags.</p><p>You can also control template context with
XSL <span class="emphasis"><em>modes</em></span>, which are used extensively
in the DocBook stylesheets. Modes let you process the same
input more than once in different ways.
A <code class="literal">mode</code> attribute in
an <code class="literal"><xsl:template></code> definition adds a
specific mode name to that template. When the same mode
name is used
in <code class="literal"><xsl:apply-templates/></code>, it acts
as a filter to narrow the selection of templates to only
those selected by
the <code class="literal">match</code> expression <span class="emphasis"><em>and</em></span> 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 <code class="literal"><listitem></code> elements:</p><pre class="programlisting"><xsl:template match="listitem">
<li><xsl:apply-templates/></li>
</xsl:template>
<xsl:template match="listitem" mode="xref">
<xsl:number format="1"/>
</xsl:template>
</pre><p>The first template is for the normal list item
context where you want to output the
HTML <code class="literal"><li></code> tags. The second template
is called with <code class="literal"><xsl:apply-templates
select="$target" mode="xref"/></code> in the context
of processing <code class="literal"><xref></code> elements. In
this case the <code class="literal">select</code> attribute locates
the ID of the specific list item and
the <code class="literal">mode</code> attribute selects the second
template, whose effect is to output its item number when it
is in an ordered list. Because there are many such special
needs when
processing <code class="literal"><xref></code> elements, it is
convenient to define a mode name <code class="literal">xref</code> to
handle them all. Keep in mind that mode settings
do <span class="emphasis"><em>not</em></span> automatically get passed down to
other templates
through <code class="literal"><xsl:apply-templates/></code>.</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="d0e483"></a>Programming features</h3></div></div></div><p>Although XSL is template-driven, it also has some
features of traditional programming languages. Here are
some examples from the DocBook stylesheets. </p><pre class="programlisting"><em class="lineannotation"><span class="lineannotation">Assign a value to a variable:</span></em>
<xsl:variable name="refelem" select="name($target)"/>
<em class="lineannotation"><span class="lineannotation">If statement:</span></em>
<xsl:if test="$show.comments">
<i><xsl:call-template name="inline.charseq"/></i>
</xsl:if>
<em class="lineannotation"><span class="lineannotation">Case statement:</span></em>
<xsl:choose>
<xsl:when test="@columns">
<xsl:value-of select="@columns"/>
</xsl:when>
<xsl:otherwise>1</xsl:otherwise>
</xsl:choose>
<em class="lineannotation"><span class="lineannotation">Call a template by name like a subroutine, passing parameter values and accepting a return value:</span></em>
<xsl:call-template name="xref.xreflabel">
<xsl:with-param name="target" select="$target"/>
</xsl:call-template>
</pre><p>However, you can't always use these constructs as you
do in other programming languages. Variables in particular
have very different behavior.</p><div class="sect3" lang="en"><div class="titlepage"><div><div><h4 class="title"><a name="d0e503"></a>Using variables and parameters</h4></div></div></div><p>XSL provides two elements that let you assign a value
to a
name: <code class="literal"><xsl:variable></code> and <code class="literal"><xsl:param></code>.
These share the same name space and syntax for assigning
names and values. Both can be referred to using
the <code class="literal">$name</code> 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 <code class="literal"><xsl:with-param></code> element as in the
last example above.</p><p>Here are two examples from DocBook:</p><pre class="programlisting"><xsl:param name="cols">1</xsl:param>
<xsl:variable name="segnum" select="position()"/>
</pre><p>In both elements, the name of the parameter or
variable is specified with
the <code class="literal">name</code> attribute. So the name of
the <code class="literal">param</code> here
is <code class="literal">cols</code> and the name of
the <code class="literal">variable</code> is <code class="literal">segnum</code>.
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 <code class="literal">select</code> attribute, and the element
itself has no content.</p><p>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.</p><p>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.</p><p>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 <code class="filename">pi.xsl</code> that illustrates different
scopes for two variables:</p><pre class="programlisting">
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>
</pre><p>The scope of the variable <code class="literal">pi</code> begins
on line 8 where it is defined in this template, and ends on
line 20 when its last sibling ends.<sup>[<a name="d0e560" href="#ftn.d0e560">1</a>]</sup> The scope of the variable <code class="literal">rest</code> begins on line 13 and ends on line 15. Fortunately, line
15 outputs an expression using the value before it goes out of
scope.</p><p>What happens when
an <code class="literal"><xsl:apply-templates/></code> 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. </p><p>To pass a value to another template, you pass a
parameter using
the <code class="literal"><xsl:with-param></code> element. This
parameter passing is usually done with calls to a specific
named template
using <code class="literal"><xsl:call-template></code>, although
it works
with <code class="literal"><xsl:apply-templates></code> too.
That's because the called template must be expecting the
parameter by defining it using
a <code class="literal"><xsl:param></code> element with the same
parameter name. Any passed parameters whose names are not
defined in the called template are ignored.</p><p>Here is an example of parameter passing
from <code class="filename">docbook.xsl</code>:</p><pre class="programlisting"><xsl:call-template name="head.content">
<xsl:with-param name="node" select="$doc"/>
</xsl:call-template>
</pre><p>Here a template
named <code class="literal">head.content</code> is being called and
passed a parameter named <code class="literal">node</code> whose
content is the value of the <code class="literal">$doc</code> variable
in the current context. The top of that template looks like
this:</p><pre class="programlisting"><xsl:template name="head.content">
<xsl:param name="node" select="."/>
</pre><p>The template is expecting the parameter because it
has a <code class="literal"><xsl:param></code> 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.</p></div></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="d0e614"></a>Generating HTML output.</h3></div></div></div><p>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 <code class="filename">docbook/html/docbook.xsl</code> as your
stylesheet. That is the master stylesheet file that
uses <code class="literal"><xsl:include></code> to pull in the
component files it needs to assemble a complete stylesheet
for producing HTML. </p><p>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 <code class="filename">docbook.xsl</code>:</p><pre class="programlisting"><xsl:template match="/">
<xsl:variable name="doc" select="*[1]"/>
<html>
<head>
<xsl:call-template name="head.content">
<xsl:with-param name="node" select="$doc"/>
</xsl:call-template>
</head>
<body>
<xsl:apply-templates/>
</body>
</html>
</xsl:template>
</pre><p>This template matches the root element of your input
document, and starts the process of recursively applying
templates. It first defines a variable
named <code class="literal">doc</code> and then outputs two literal
HTML elements <code class="literal"><html></code> and <code class="literal"><head></code>.
Then it calls a named
template <code class="literal">head.content</code> to process the
content of the HTML <code class="literal"><head></code>, closes
the <code class="literal"><head></code> and starts
the <code class="literal"><body></code>. There it
uses <code class="literal"><xsl:apply-templates/></code> to
recursively process the entire input document. Then it just
closes out the HTML file.</p><p>Simple HTML elements can generated as literal
elements as shown here. But if the HTML being output
depends on the context, you need something more powerful to
select the element name and possibly add attributes and
their values. Here is a fragment
from <code class="filename">sections.xsl</code> that shows how a
heading tag is generated using
the <code class="literal"><xsl:element></code> and <code class="literal"><xsl:attribute></code> elements:</p><pre class="programlisting">
1 <xsl:element name="h{$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>
</pre><p>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 <code class="literal">$level</code> passed as a parameter to
this template. Thus a single template can
generate <code class="literal"><h1></code>, <code class="literal"><h2></code>,
etc. depending on the context in which it is called. Line 2
defines a <code class="literal">class="title"</code> attribute that is
added to this element. Lines 3 to 5 add
a <code class="literal">style="clear all"</code> attribute, but only
if the heading level is less than 3. Line 6 opens
an <code class="literal"><a></code> anchor element. Although this
looks like a literal output string, it is actually modified
by lines 7 to 9 that insert
the <code class="literal">name</code> attribute into
the <code class="literal"><a></code> 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 <code class="literal"></a></code> 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.</p><p>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 <code class="filename">docbook.xsl</code> you will find
this template that handles any text not processed by any
other template:</p><pre class="programlisting"><xsl:template match="text()">
<xsl:value-of select="."/>
</xsl:template>
</pre><p>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.</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="d0e709"></a>Generating formatting objects.</h3></div></div></div><p>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 <code class="filename">docbook/fo/docbook.xsl</code> as your
stylesheet. That is the master stylesheet file that
uses <code class="literal"><xsl:include></code> 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.</p><p>The DocBook fo stylesheet works in a similar manner
to the HTML stylesheet. Instead of outputting HTML tags, it
outputs text marked up
with <code class="literal"><fo:<em class="replaceable"><code>something</code></em>></code> tags.
For example, to indicate that some text should be kept
in-line and typeset with a monospace font, it might look
like this:</p><pre class="programlisting"><fo:inline-sequence font-family="monospace">/usr/man</fo:inline-sequence></pre><p>The templates
in <code class="filename">docbook/fo/inline.xsl</code> that produce
this output for a
DocBook <code class="literal"><filename></code> element look
like this:</p><pre class="programlisting"><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>
</pre><p>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.</p></div><div xmlns:cf="http://docbook.sourceforge.net/xmlns/chunkfast/1.0" class="footnotes"><br><hr width="100" align="left"><div class="footnote"><p><sup>[<a name="ftn.d0e560" href="#d0e560">1</a>] </sup>Technically, the scope extends to the end tag of the parent of the <code class="literal"><xsl:variable></code> element. That is effectively the last sibling.</p></div></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch01s02.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="publishing.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="extensions.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">A brief introduction to XSL </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 2. Extensions</td></tr></table></div></body></html>