Tag Libraries
for XSLT and XQuery
Erik Hennum, MarkLogic Inc.
Vyacheslav Zholudev, Jacobs University Bremen
August 2, 2011
Tag Libraries
Tag libraries in a nutshell
A Java technology for dynamic web pages:
- Develop a page component library
- data producers, widgets, ... - Define a simple API as a new XML vocabulary
- Assemble by editing page to embed vocabulary
- not by writing code
Lots of adoption, lots of tag libraries
The question on the table
- An approach useful only in Java?
or
- Fills a gap in the XML technology stack?
Tag libraries don't change what you can do with XML
but who can tap into XML technology
Canonical example: the tagdoc (1 of 2)
<html:table>
<tag:attendees>
<html:tr>
<html:td>${last-name}</html:td>
<html:td>${first-name}</html:td>
</html:tr>
</tag:attendees>
</html:table>
<tag:attendees>tag gets the list of attendees- repeats the
<html:tr>fragment for each attendee - replaces
${last-name}and${first-name}
Canonical example: the result document (2 of 2)
<html:table>
...
<html:tr>
<html:td>Fitzgerald</html:td>
<html:td>Elena</html:td>
</html:tr>
<html:tr>
<html:td>Green</html:td>
<html:td>Alfonso</html:td>
</html:tr>
<html:tr>
<html:td>Hutz</html:td>
<html:td>Egberto</html:td>
</html:tr>
...
</html:table>
Why would you care?
- Consultant
Budget used up, next client in the pipeline -
hand off to the client for indefinite refinement - Webapp library or framework developer
Branding and layout requirements -
hand off to the adopter to customize by editing - Site manager
Merge and compress resources used in the page -
parse the page to discover its dependencies
Expertise level vs number of practitioners
A closer look at a tag call
<c:set var="feed-uri"
value="http://news.ycombinator.com/rss"/>
<feed:summary feed="${feed-uri}" ...>
<p>
<a href="${feed-uri}">${title}</a> ${description}
</p>
</feed:summary>
pull -
<feed:summary> tag is a call
from the tagdoc to the taglib
push -
the tag content is a callback from taglib to tagdoc
with
${title} and
${description}
parameters
closure -
the tag content refers to
${feed-uri}
variable
defined before the tag call
taglib as DSL (Domain Specific Language)
General-purpose language with programming syntax
- do anything
ResultSet rs = stmt.executeQuery(query);
while (rs.next()) {
String lastName = rs.getString("LAST_NAME");
}
Domain-specific language with custom statements
- do one thing clearly (semantic transparency)
SELECT LAST_NAME, FIRST_NAME FROM ATTENDEES
Comparison with Simplified Stylesheets
- callbacks - push processing defined in the tagdoc
- imports - assembly of libraries by the consumer
- DSL - domain tag vocabulary vs
XSLT syntax
<xsl:apply-templates>vs<feed:summary> - page processing - vs extra transform input
Compatibility with XForms
- XForms defines form processing in a runtime document
- a tag library produces a runtime document
- the runtime document can use XForms - Encapsulate
- the model, view, or bindings for a specific form
- alternative handlers for a form's events
XSTag (XML Simple Tags)
Workflow
Adapting Java Tag Libraries for the XML toolkit
| Java | XSTag | |
|---|---|---|
| Expressions | Unified Expression Language | XPath |
| Tag bindings | Configuration files | QName of function or template |
| Callbacks | One unnamed tag body |
Many named handlers |
| Fragment reuse | Document inclusion | Basic tag libraries |
Fragment reuse with a basic tag library
<-- A basic taglib that defines page components -->
<xst:taglib version="0.2" ... xmlns:pg=".../pagecomp">
...
<xst:tag name="pg:footer" as="element()*">
<p>Copyright 2011, Some Legal Entity</p>
</xst:tag>
</xst:taglib>
<-- A tagdoc that uses the taglib to generate a page -->
<html xst:version="0.2" ... xmlns:pg=".../pagecomp">
<xst:import href="PageComponents.xstag"/>
...
<body>
...
<pg:footer/>
</body>
</html>
Example of feed processing -
demo and source
A basic taglib for feed iteration
<xst:taglib version="0.2" ... xmlns:feed=".../feed/">
... other tags get feed and properties ...
<xst:tag name="feed:list" as="node()*">
<xst:param name="feed" as="document-node()*"/>
<xst:param name="onItem" implicit="true" as="handler(
$title as xs:string?,
...
) as node()*"/>
<xst:for-each name="item" at="number" select="...">
...
<xst:call handler="$onItem">
<xst:with-param name="title" select="..."/>
...
</xst:call>
</xst:for-each>
</xst:tag>
</xst:taglib>
A plain HTML tagdoc (1 of 4)
<html xst:version="0.2" ... xmlns:feed=".../feed/">
<xst:import href="feed-lib/feeds.xstag"/>
<xst:variable name="feed">
<feed:get>http://news.ycombinator.com/rss</feed:get>
</xst:variable>
...
<body>
<h1><feed:title feed="{$feed}"/></h1>
<ul>
<feed:list feed="{$feed}" xst:handles="$uri, $title">
<li><a href="{$uri}">{$title}</a></li>
</feed:list>
</ul>
</body>
</html>
A tagdoc with feeds in a widget taglib (2 of 4)
<html xst:version="0.2" ... xmlns:widget=".../widget/">
<xst:import href="feed-lib/feeds.xstag"/>
<xst:import href="widget-lib/widgets.xstag"/>
...
<body>
...
<widget:accordion-list>
<feed:list feed="{$feed}" xst:handles="$uri, $title, ...">
<widget:accordion-summary>
{$title}
</widget:accordion-summary>
<widget:accordion-detail>
<p>Source: <a href="{$uri}">{$uri}</a> ... </p>
</widget:accordion-detail>
</feed:list>
</widget:accordion-list>
An AJAX client tagdoc (3a of 4)
<html xst:version="0.2" ... xmlns:rfsh=".../refresh/">
<xst:import href="refresh-lib/refresh.xstag"/>
...
<rfsh:refresh>
<rfsh:trigger>feed-view</rfsh:trigger>
<rfsh:provider>feed-ajax-server.html</rfsh:provider>
<rfsh:params>feed-uri=feed-select</rfsh:params>
<rfsh:targets>feed-title, feed-list</rfsh:targets>
</rfsh:refresh>
...
<form action="#">...
<select id="feed-select">...</select>
<button id="feed-view" type="button">View</button>
</form>
<h1 id="feed-title"/>
<ul id="feed-list"/>
...
An AJAX server tagdoc (3b of 4)
<div xst:version="0.2" ... xmlns:feed=".../feed/">
<xst:import href="feed-lib/feeds.xstag"/>
<xst:param name="feed-uri">...</xst:param>
<xst:variable name="feed">
<feed:get>{$feed-uri}</feed:get>
</xst:variable>
<h1 id="feed-title"><feed:title feed="{$feed}"/></h1>
<ul id="feed-list">
<feed:list feed="{$feed}" xst:handles="$uri, $title">
<li><a href="{$uri}">{$title}</a></li>
</feed:list>
</ul>
</div>
An XSL-FO tagdoc (4 of 4)
<root xst:version="0.2" ... xmlns:feed=".../feed/">
<xst:import href="feed-lib/feeds.xstag"/>
...
<block ...>Table of Contents</block>
<feed:list feed="{$feed}" xst:handles="$number, ...">
<block ...>
<basic-link ...>{$title}</basic-link>
...
<page-number-citation ref-id="f{$number}"/>
</block>
</feed:list>
...
<feed:list feed="{$feed}" xst:handles="$number, ...">
<block id="f{$number}" ...>{$title}</block>
...
</feed:list>
...
Processor-specific tag libraries
- When a taglib needs full-blown XQuery or XSLT
- Declared by the
xst:taglib/@forattribute - Same
xst:tagandxst:paramdeclaration
as for a basic taglib - Body of tag implemented in XQuery or XSLT
- exceptxst:callto handler - A tagdoc can import multiple taglibs implementing
the same tags for different processors
Concluding points
Limitations
Glue for component configuration and assembly -
not for:
- Low-level programming statements
- Transform from one complex vocabulary to another
Open questions
- Overloaded HTML? special attributes for tag call
- JavaScript processor for tagdoc? XPath in JQuery
- Mobile (similar to AJAX)?
- tags in the XQuery / XSLT processor?
- inline functions do the heavy lifting for handlers
- a document-oriented interface / DSL
Summary: a collaboration strategy
- taglibs implemented by XQuery / XSLT developers
- a domain language for feeds, widgets, AJAX, ... - tagdocs edited by HTML authors
- components assembled by the consumer
- handlers for push from taglib to tagdoc
Preprocessor implementation for experiments at
http://code.google.com/p/xstag