Built-in XSLT and CSS stylesheets

XSLT

XSLT is a functional, Turing-complete XML transformation language.

LinkedDataHub's XSLT 2.0 stylesheets work by transforming RDF/XML response body from the underlying HTTP API. Additional metadata from RDF vocabularies is used to improve user experience.

Plain RDF/XML

RDF/XML is an important RDF syntax which functions as a bridge to the XML technology stack. The stylesheets use Jena's "plain" RDF/XML output which groups statements by subject and does not nest resource descriptions. This allows for predictable XPath patterns:

  • /rdf:RDF — represents the RDF graph
  • /rdf:RDF/rdf:Description or /rdf:RDF/*[*][@rdf:about] | /rdf:RDF/*[*][@rdf:nodeID]— resource description which contains properties
  • /rdf:RDF/rdf:Description/@rdf:about — subject resource URI
  • /rdf:RDF/rdf:Description/@rdf:nodeID — subject blank node ID
  • /rdf:RDF/rdf:Description/* predicate (e.g. rdf:type) which URI is concat(namespace-uri(), local-name())
  • /rdf:RDF/rdf:Description/*/@rdf:resource — object resource
  • /rdf:RDF/rdf:Description/*/@rdf:nodeID — object blank node ID
  • /rdf:RDF/rdf:Description/*/text() — literal value

Stylesheets

XSLT stylesheet components used by LinkedDataHub:

Includes
<xsl:include> is used to inclkude one stylesheet into another. The import mechanism is specified in 3.10.2 Stylesheet Inclusion the XSLT 2.0 specification. The templates from the included stylesheets have the same priority as those of the importing stylesheet.
Imports
<xsl:import> is used to import one stylesheet into another. The import mechanism is specified in 3.10.3 Stylesheet Import the XSLT 2.0 specification. The templates from the imported stylesheets have lower priority than those of the importing stylesheet.
Parameters
XSD-typed global parameters passed to the stylesheet
Keys
Lookup keys
Templates
Template rules for XML node processing

One XSLT stylesheet can be specified per application. In order to reuse LinkedDataHub's built-in templates, it should import the system stylesheet layout.xsl and only override the necessary templates. That is however not a requirement, the stylesheet could also use its own independent transformation logic.

If there is no stylesheet specified for the application, the system stylesheet is used. It defines the overall layout and imports resource-level and container-specific stylesheets, as well as per-vocabulary stylesheets.

Note that LinkedDataHub itself imports stylesheets from Web-Client, which uses the same template modes but produces a much simpler layout.

There is also a special client-side stylesheet which is not used to render a full layout, but only manipulate DOM elements in the browser in response to user or system events. Currently it is processed using Saxon-CE which provides client-side extensions for XSLT 2.0, but a move to XSLT 3.0 and Saxon-JS is planned. It imports and reuses some of the same sub-stylesheets as the server-side system stylesheet does, but avoids loading per-vocabulary stylesheets in order to improve page load time. Templates of the client-side stylesheet can also be overridden.

Parameters

Both global (i.e. stylesheet-level) and template parameters are declared using <xsl:param>. For example:

<xsl:param name="ac:uri" as="xs:anyURI"/>

This parameter can be accessed using $ac:uri. For example:

<xsl:if test="$ac:uri">
    <xsl:value-of select="$ac:uri"/>
</xsl:if>

LinkedDataHub sets these global parameters by default (the list is not exhaustive):

ac:uri
The absolute URI of the current document (without query string)
ac:mode
The URI of the current layout mode
ac:forClass
The URI of the class which instance is being created. See also aplt:ForClass.
ac:sitemap
RDF/XML document of the application's ontology, with a transitive closure of imports
lapp:Application
RDF/XML document with the metadata of the current LDT application
lacl:Agent
RDF/XML document with the metadata of the currently authenticated agent (if any)

Templates

XSLT template components:

Match
XPath-based match pattern which either does or does not match an XML node
Mode
Allows to group templates and distinguish them from other groups which have the same match patterns (e.g. different layout modes)
Parameters
XSD-typed parameters passed to the template invocation
Body
Contains the XML output nodes as well as XSLT processing instructions

XSLT processing starts at the root of the RDF/XML document and produces HTML elements by applying templates on all of the RDF/XML nodes while moving down the XML tree. In other words, it starts at the graph level, moves down to resource description elements, then to property elements, and ends with identifier attributes and literal text nodes.

The content of the RDF/XML document normally includes the description of the requested document, and optionally a "thing"/concept paired with it. However, it can be completely customized by overriding the LDT template that matches that document and specifying a custom query. For example, for a document with a SKOS concept it might make sense to include its broader and/or narrower concepts in the RDF description as well. XML documents (which by definition includes RDF/XML) can also be loaded separately from the main RDF/XML document, but at a cost of an HTTP request per document. There is no rule on how to split data between the main RDF/XML document and the additional ones — it is a balancing act in terms of query maintainability and stylesheet performance.

Templates are applied (invoked) using <xsl:apply-templates>. Mode can be specified, e.g. <xsl:apply-templates mode="bs2:Header">. To stay in the current mode without explicitly specifying it, use <xsl:apply-templates mode="#current">. <xsl:with-param> is used to supply parameters.

LinkedDataHub provides the following default template modes, which are used to render the layout modes:

  • Graph-level modes that apply to rdf:RDF
    • bs2:Block which renders full resource description with all properties
    • bs2:BlockList renders a list of resources
    • xhtml:Table renders a table with resources as rows and properties as columns
    • bs2:Grid renders a gallery of thumbnails
    • bs2:Form which renders an RDF/POST form for for creation of new resources (when $ac:forClass parameter is set) or editing of existing resource
  • Resource-level templates modes that apply to rdf:Description
    • bs2:Block renders full resource description (by default header and property list)
    • bs2:Header renders resource header (by default with type information)
    • bs2:PropertyList renders definition list with property names and values (by default grouped by resource types)

bs2: prefix is used for Bootstrap 2.3.2-specific templates, xhtml: prefix is used for generic HTML templates. When adding new user-defined modes, it is recommended to choose a new namespace for them as well as a user-defined prefix.

An example of a template that matches rdf:Description:

<xsl:template match="*[*][@rdf:about] | *[*][@rdf:nodeID]" mode="bs2:Block">
    <xsl:param name="id" as="xs:string?"/>
    <xsl:param name="class" as="xs:string?"/>

    <div>
        <xsl:if test="$id">
            <xsl:attribute name="id"><xsl:value-of select="$id"/></xsl:attribute>
        </xsl:if>
        <xsl:if test="$class">
            <xsl:attribute name="class"><xsl:value-of select="$class"/></xsl:attribute>
        </xsl:if>

        <xsl:apply-templates select="." mode="bs2:Header"/>

        <xsl:apply-templates select="." mode="bs2:PropertyList"/>
    </div>
</xsl:template>

apl:Content mode embeds XHTML from the sioc:content value of the current document.

There are a few special template modes such as ac:label and ac:description and related functions ac:label() and ac:description() which are used not to render layout but to extract metadata from resource descriptions. They can be used to retrieve a resource label and description no matter which RDF vocabularies are used in the data. They do so by invoking templates of respective mode from vocabulary-specific stylesheets.

Overriding templates

Templates are overridden by redefining them in the importing stylesheet and providing the same or more specific match pattern and the same mode. The XSLT specification specifies exactly how template priorities are determined in 6.4 Conflict Resolution for Template Rules.

The overriding template can then get the output of the overridden template by invoking either <xsl:apply-imports> or <xsl:next-match>. Read more about 6.7 Overriding Template Rules.

Always override the most specific template, i.e. if you want to change how a property is rendered, do not override the template for resource description, only the one for the property.

Keys

Keys are a lookup mechanism. They are defined on the stylesheet level using <xsl:key> and invoked using the key() function. For example:

<xsl:key name="resources" match="*[*][@rdf:about] | *[*][@rdf:nodeID]" use="@rdf:about | @rdf:nodeID"/>

<xsl:template match="*">
    <xsl:for-each select="key('resources', $ac:uri)">
        <xsl:value-of select="ac:label(.)"/>
    </xsl:for-each>
</xsl:template>

They key definition matches rdf:Description elements and uses their identifiers (URI or blank node ID). The template then looks up the RDF description of the current resource, i.e. the resource with URI that equals $ac:uri which is the absolute URI of the current request, and outputs its label.

Loading data

The stylesheet is processing one main RDF/XML document at a time, which is supplied by the LinkedDataHub's HTML writer. However it is possible to load additional XML documents over HTTP using the document() XSLT function. To avoid XSLT errors on any possible error responses, it is advisable to do a conditional check using the doc-available() function before doing the actual document() call.

For example, instead of hardcoding the title of this document as Stylesheets, we can use the following code to load it and output it on the fly:

<xsl:value-of select="key('resources', 'https://linkeddatahub.com/linkeddatahub/docs/reference/stylesheet/', document('https://linkeddatahub.com/linkeddatahub/docs/reference/stylesheet/'))"/>

In case this document changes its title, all such references would automatically render the updated title. On the other hand, it incurs the overhead of making an HTTP request.

LinkedDataHub's default stylesheets are using this feature extensively. In fact, one HTML page is rendered from a dozen of RDF/XML documents.

Built-in model ontologies and sitemap ontologies, as well as some other system and well-known ontologies, have a local copy in each LinkedDataHub instance. As a result, retrieving their descriptions by dereferencing their URIs using document() does not incur an HTTP request and is much faster. The URI-to-file mapping is defined as Jena's location mapping and can be found in location-mapping.n3 and prefix-mapping.n3.

CSS

Bootstrap 2.3.2 theme is used with some customizations.

The CSS stylesheets is specified in the xhtml:Style XSLT template mode.

JavaScript

The JavaScript files are specified in the xhtml:Script XSLT template mode.

LinkedDataHub only uses JavaScript for the functionality that cannot be achieved using client-side XSLT.

Date Created
17 March 2020 20:44
Maker
Martynas Jusevičius
Owner
Martynas Jusevičius