XSL mockup for multipage rendering

Here is how an XSL would render a multipage document.

First, let’s consider the whole document defining the opus:

== One

Some text of level one.

== Two

Some text of level two.

=== Two-one

Some text of level two-one.

The XML form of the document above is:

<?xml version="1.0" encoding="UTF-8" ?>
<opus>
  <level>
    <title>One</title>
    <paragraph>Some text of level one.</paragraph>
  </level>
  <level>
    <title>Two</title>
    <paragraph>Some text of level two.</paragraph>
    <level>
      <title>Two-one</title>
      <paragraph>Some text of level two-one.</paragraph>
    </level>
  </level>
</opus>

Let’s take for granted that Novelang supports XSL metadata. Our multipage-enabled stylesheet would define an embedded stylesheet that transforms a whole opus into a simple map of page names and page paths. A path is whatever the stylesheet may reprocess, but an XPath expression is quite good. For the document above, here is how our map could look like, if we want to support 2 levels:

page1 -> /opus/level[1]
page2 -> /opus/level[2]
page3 -> /opus/level[2]/level[1]

Please note that, at this point, the decisision to support a given depth, or exclude some tagged levels, entirely belongs to the page-extracting stylesheet.

By merging the page map with the opus, we get the XML input for the rendering of one page. Novelang knows which page it is either because it is iterating over all known pages of the map (batch mode), or because the page name is a part of the request issued to the HTTP dæmon.

<op>us>
  <meta>
    <page>
      <name>page2</name>
      <path>/opus/level[2]</path>
    </page>
  </meta>

  <level>
    <title>One</title>
    <paragraph>Some text of level one.</paragraph>
  </level>
  <level>
    <title>Two</title>
    <paragraph>Some text of level two.</paragraph>
    <level>
      <title>Two-one</title>
      <paragraph>Some text of level two-one.</paragraph>
    </level>
  </level>
</opus>

(Note: the n: namespace prefix doesn’t appear here for brevity.)

The stylesheet gets this whole document as input for every page. All what changes is the name, path pair in the meta/page element. The stylesheet needs to know which page it is rendering, and the whole document tree as well, in order to create a navigation bar or any kind of header or footer corresponding to a specially-titled or tagged level of the document.

This involves some XSL trickery: evaluating an XPath expression at runtime. While it’s not part of XPath 1.0 specification, it is a part of semi-official EXSLT communitiy initiative. The dyn:evaluate http://www.exslt.org/dyn/functions/evaluate does that for us. It works well with Xalan-2.7.1 which is the XSLT engine bundled with Novelang (it works a slightly better than JDK’s one).

In the stylesheet below, we save useful expressions into variables.

The root template prints those variables, then a pseudo-navigation bar made of nested lists.

The nested loop for iterating over level elements is rather ugly but it makes sense as we don’t want infinite deph of titles in a navigation bar.

The title-with-locator template just adds bold on the title in the navigation bar that corresponds to current page.

All other templates mimic Novelang’s standard rendering.

<xsl:stylesheet
    version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:dyn="http://exslt.org/dynamic"
    extension-element-prefixes="dyn"
>
  <!-- Be sure to use Xalan-2.7.1 (not JDK's default). -->

  <!--
    Here, expect a meta section, embedding a stylesheet 
    that extracts the pages we'll find in the meta section 
    of input document.
  -->

  <xsl:output method="html" />

  <xsl:variable name="page-name" select="/opus/meta/page/name" />
  <xsl:variable name="page-path" select="/opus/meta/page/path" />
  <xsl:variable name="page-nodeset" 
      select="dyn:evaluate( $page-path )" />
  <xsl:variable name="page-id" 
      select="generate-id( $page-nodeset )" />

  <xsl:template match="meta/page" >
    $page-name=<xsl:value-of select="$page-name" />
    $page-path=<xsl:value-of select="$page-path" />
    $page-id=<xsl:value-of select="$page-id" />
  </xsl:template>

  <xsl:template match="/opus" >
    <html>
      <xsl:apply-templates select="meta" />

      <!-- Navigation bar -->
      <ul>
        <xsl:for-each select="level">
          <li>
            <xsl:call-template name="title-with-locator"/>
          </li>
          <xsl:if test="level">
            <ul>
              <xsl:for-each select="level">
                <li>
                  <xsl:call-template name="title-with-locator"/>
                </li>
              </xsl:for-each>
            </ul>
          </xsl:if>
        </xsl:for-each>
      </ul>

      <!-- Document body, same templates as usual -->
      <xsl:apply-templates select="$page-nodeset" />

    </html>

  </xsl:template>


  <xsl:template match="paragraph" >
    <p>
      <xsl:value-of select="." />
    </p>
  </xsl:template>

  <xsl:template match="title" />

  <xsl:template match="level" >
    <h2><xsl:value-of select="title" /></h2>
    <xsl:apply-templates/>
  </xsl:template>

  <xsl:template match="level/level" >
    <h3><xsl:value-of select="title" /></h3>
    <xsl:apply-templates/>
  </xsl:template>

  <xsl:template name="title-with-locator" >
    <xsl:text>
    </xsl:text>
    <xsl:choose>
      <xsl:when test="generate-id( . ) = $page-id" >
        <b><xsl:call-template name="title-alone" /></b>
      </xsl:when>
      <xsl:otherwise>
        <xsl:call-template name="title-alone" />
      </xsl:otherwise>
    </xsl:choose>
  </xsl:template>

  <xsl:template name="title-alone" >
    Title: <xsl:value-of select="title" />
  </xsl:template>

</xsl:stylesheet>

Finally, this is how the rendering looks like: