The Extensible Stylesheet Language (XSL) W3C recommendation was created as a means to display XML data. The recommendation includes a transformation language (XSLT) and formatting object (or output format) language (XSL-FO), which together provide the XSL stylesheet developer with the tools necessary to present XML.

The XSL-FO language is like HTML on steroids, because it allows not only cascading stylesheet (CSS)-type functionality, but also pagination and page layout, which are not available using HTML alone (at least not to as great a degree as with XSL-FO). However, such robustness comes at a cost; the language is very (very) verbose, which makes writing XSL-FO XSL stylesheets very difficult; thus, XSL-FO stylesheets are supposed to be produced as the result of an XSL transformation. But someone has to write the XSL stylesheet that transforms the XML into XSL-FO, and that's where this article can help.

The purpose of this article is to show you, the XSL stylesheet developer, how to make XSL-FO XSL stylesheets that are flexible and reusable. In other words, I'm going to show you how to make modularized XSL-FO XSL stylesheets.

Many of the key concepts presented in this article are not XSL-FO specific and can be leveraged to write any kind of XSL stylesheet. However, I concentrated on XSL-FO XSL stylesheets because their verbosity highlights the need for modularization more than most stylesheets.

I'm assuming a working knowledge of both XSLT and XSL-FO. For an introduction, you can refer to many resources on the Internet, including my article in the January 2002 edition of XML-Journal.

Overview of the Solution
Although this modularization solution is very flexible, it is also very simple and uses the XSL <xsl:import> and <xsl:attribute-set> elements, along with the xsl:use-attribute-sets attribute. When used properly, these elements and this attribute not only allow you to externalize the way an XML document is transformed into an XSL-FO document (<xsl:import>), but also the look-and-feel of the eventual FO document (<xsl:attribute-set>, xsl:use-attribute-sets). Figure 1 depicts all the pieces of these concepts.

From a high level, the XSLT engine reads in an XML document and XSL stylesheet (nothing new there). The main stylesheet, the one that is read by the XSLT engine, then uses <xsl:import> to use two additional stylesheets - one for the layout of the XSL-FO document and another for the "look-and-feel." The layout document then uses the use-attribute-sets attribute to get the "look-and-feel" (i.e., element attribution information) data from the "look-and-feel" XSL stylesheet, which is implemented as a series of <xsl:attribute-set> elements.

With the layout and look-and-feel externalized and separated into different stylesheets, the stylesheet writer can easily exchange both/either the layout and/or look-and-feel of the XSL-FO document information. If the XSL-FO document has such flexibility, it follows that the layout and look-and-feel of the eventual output document, a PDF file for example, has equal flexibility.

A Presentation Example
The example I'm going to present is a good case for the advantages of using a modular design in XSL. In presentation graphics (e.g., Microsoft PowerPoint), there are templates that can change the layout of pages and the color/graphics scheme of your presentation. I am going to replicate, to some degree, that same kind of functionality using XSL. If you use the ideas in this example, you can impress your friends by creating PDF presentations that are cross-platform with data separated from the presentation (which implies that the same data could potentially be used in entirely different formats).

The XML file
The data for the sample is represented in a simple XML file, named presentation.xml, that depicts the idea of a presentation with pages containing concepts (such a format is probably a little too coupled to the presentation, but it'll work). The root element of the document is "Presentation," with descriptive child elements called "Title," "Author," "Date," and "CorporateLogo," and a series of "Page" elements that take the following form:

<Page style="bullet">
<Position>01</Position>
<Title>Title Text</Title>
<Concept>Some concept</Concept>
...more concepts
</Page>

The "style" attribute is referred to in the layout stylesheet to create the proper text layout in the presentation (e.g, PDF) output. The example is bullet, but you could extend the idea to be graphic, column, bullet-graphic, or any other style you require. You'll see shortly how the value of this attribute is used.

The position element allows you to put the "Page" elements in any order within the XML document and still have them laid out the way you want in the final output. Like the "style" attribute, the "Position" element is used by the layout stylesheet.

The main XSL stylesheet
Ultimately, the XSLT engine requires the name of a stylesheet to be processed. In this example, that file is called main.xsl and is very simple since its main purpose is to point to the correct layout and look-and-feel stylesheet. Listing 1 shows the contents of that stylesheet, minus the XML declaration and <xsl:stylesheet> element.

Within Listing 1, the <xsl:import> elements point to the layout and look-and-feel stylesheet, respectively. To change the template, you simply create a new layout and/or look-and-feel stylesheet and change the value of the import.

The other interesting point of the main.xsl stylesheet is simply that it creates the <fo:root> element and uses the <xsl:apply-templates> to transfer processing to other templates (which are defined in the plainPageLayout.xsl stylesheet).

Look-and-feel
The lookandfeel.xsl stylesheet contains a number of <xsl:attribute-set> definitions, which can be referenced using the xsl:use-attribute-sets property with an FO element. The contents of each <xsl:attribute-set> are the equivalent of attributes on XSL elements.

Take, for instance, a relatively simple <fo:simple-page-master> element, which defines a template for a page.

<fo:simple-page-master
master-name="Title"
margin-top="1in"
margin-bottom="1in"
margin-left="1in"
margin-right="1in"
page-height="8.5in"
page-width="11in"
>

<fo:region-body/>
<fo:region-after
extent="1in"/>
</fo:simple-page-master>

You can externalize all the attributes of the <fo:simple-page-master> element into the lookandfeel.xsl file by defining an <xsl:attribute-set> element as follows:

<xsl:attribute-set name="titlePage
Layout">
<xsl:attribute name="margin-
top">3in</xsl:attribute>
<xsl:attribute name="margin-
bottom">1in</xsl:attribute>
<xsl:attribute name="margin-
left">1in</xsl:attribute>
<xsl:attribute name="margin-
right">1in</xsl:attribute>
<xsl:attribute name="page-
height">8.5in</xsl:attribute>
<xsl:attribute name="page-
width">11in</xsl:attribute>
</xsl:attribute-set>

Then, you can refer to the attribute set within the <fo:simple-page-master> by using the "xsl:use-attribute-sets" attribute, such as:

<fo:simple-page-master
master-name="Title"
xsl:use-attribute-sets="titlePageLayout">

Not only is the syntax easier to read, but the attributes values themselves are now externalized from the layout in what's essentially the FO equivalent of an external CSS.

Page layout
In XSL-FO, layout is a matter of setting up page definitions and their contents. In my example, I use the page layout XSL document, named plainPageLayout.xsl, to perform the template matching on the XML document contents. I did this because the layout is responsible for understanding the output document, something that requires knowledge of the XML input, since the data from the latter becomes part of the former.

The plainPageLayout.xsl document itself looks like most XSL-FO stylesheets. However, instead of very long attribute listings (for look-and-feel type attributes), there are several xsl:use-attribute-sets attributes.

In terms of XSLT, plainPageLayout. xsl contains three templates: one each to process the "Presentation," "Page," and "Concept" elements, respectively. That presentation template creates the basic layout of the resulting XSL-FO document and then creates the title page, using data from the XML document. Throughout this stylesheet, xsl:use-attribute-sets attributes are used within <fo> elements to retrieve attributes from lookandfeel.xsl.

The Page template actually processes Page elements by style. That is, the

<xsl:template match="Page[@style='bullet']">

element will catch the "bullet" style pages only. Within the template, the non-title pages are set up with a title and two blank lines, after which the "Concept" elements are selected within a <xsl:apply templates select="Concept"/> statement. Finally, the "Concept" template processes each "Concept," creating an XSL-FO list with bullets.

As a side note, the stylesheet can be made even more modularized if you were to externalize your "Page" and/or "Concept" templates. Doing so would let you use the "bullet" style across different layout XSL documents. I did not do so for the sake of simplicity.

Listing 2 shows the entire contents of plainPageLayout.xsl. Within Listing 2, notice the comment that reads "<!-- Shows how to override attributes. -->." What I'm showing at that point in the stylesheet is that even if an XSL-FO element uses an attribute set, you can still override and/or amend the attribute set within the element itself, much in the same way you can override/add to an HTML element if a CSS is defined - further evidence of the CSS-like nature of attribute sets.

The result
I believe that the proof of any technology is in the pudding. When you run presentation.xml and main.xsl through an FO processor (I used FOP 0.20.3), you will end up with a three-page PDF (if you choose the PDF output option within FOP). Figures 2 and 3 show the first two pages. Page 3 is just like page 2 but contains different concepts (refer to presentation.xml).

Figure 2 shows not only the main "Presentation" element (and the relevant non-page children of "Presentation"), but it also shows some of the XSL-FO design I created. For instance, the logo and yellow background color. The color and font characteristics are externalized within lookandfeel.xsl, of course.

Figure 3 uses the same basic layout characteristics of the title page, but also contains the list of concepts. Every "Page" that uses the "bullet" style will be rendered just as Figure 3 (refer to the third page of presentation.pdf if you download the source code, available at www.sys-con.com/xml/sourcec.cfm).

Conclusion
XSL-FO XSL stylesheets are long and difficult to read and write. Furthermore, it's not easy to change layout and look-and-feel without additional modularization. Using the XSL <xsl:import> and <xsl:attribute-sets> elements along with the xsl:use-attribute-sets attribute can go a very long way toward making such modularization a reality.

Resources

• Figure 1
• Figure 2
• Figure 3
• Source Code