Beginning XML - Part II (XML Style Language) by Amrit Hallan
In order to work with XML, it becomes necessary to know a little about the XML
Style Language.
XSL provides for two forms of output flow objects. The first set is the set of
displayable objects defined for HTML, which allows XML data to be mapped into
HTML-aware browsers. The second set is based on the DSSSL-O specifications
(Document Style Semantics and Specification Language - Online), and allows XML
data to be mapped to DSSSL-based text formatters, such as JADE. Both sets of
flow objects are described using XML markup.
XSL defines a set of rules which define a set of actions that are to be
associated with various patterns of target elements. The selection of target
elements can be qualified in a number of ways. For example, XSL allows
different rules to be applied to the same element type dependent on what its
ancestors, siblings or contents are. In addition, processing rules can be
specified for application when particular attribute values have been associated
with an element, or when the element has specific contents. This means that
specific rules can be applied to elements with unique identifiers or identified
content types (classes).
XSL allows for the definition of sharable sets of style rules. A style rule
applies a set of processing characteristics to a target element without
creating a new flow object. Where the same style is to be applied to a number
of elements, a uniquely named style can be defined for future reference. This
provides XSL with the facilities for creating cascading sets of style sheet
specifications similar in effect to those defined in the more limited Cascading
Style Sheet specification used to process HTML documents.
XSL style sheets can use the ECMAScript programming language to evaluate the
contents of elements or attributes prior to or during the creation of flow
objects. ECMAScript is a variant of JavaScript and Jscript that has been
formally defined by the European Computer Manufacturers Association. It allows
tools containing a Java Virtual Machine to process data contained within an XML
document. The language has been designed to support only a limited set of
processing side-effects to ensure that evaluation cannot inhibit the
progressive rendering of large documents.
Now coming back to XML, it was originally developed to allow structured
documents of the type typically encoded in SGML to be delivered over the
Internet as an integrated part of the World Wide Web of documents. Typically
these documents require the specification of element types over and above those
permitted in HTML (e.g. specific elements for parts number and other forms of
article identification, prices and other forms of calculable measurements, and
special classes of displayable text such as health warnings and controlled task
lists). XML allows users to define their own sets of document elements and
describe how each of these elements should be displayed on a screen in
conformance with the supplier's house style.
One area where XML is anticipated to be particularly important is in the area
of electronic commerce.
Traditional mechanisms for electronic data interchange (EDI) are based on the
interchange of messages between the computer systems of two or more businesses.
Each message has to be decoded before its contents can be processed or
presented to users. Web-based commerce has, by contrast, been based on the
concept of completing an HTML form and then posting the results back to the
server for processing, without any details of the transaction being retained by
the party completing the form.
XML-coded files are, by their nature, ideal for storing in databases. Because
XML files are both object-orientated and hierarchical in nature they can be
adapted to virtually any type of database, though care sometimes needs to be
taken to ensure that enough structural data is retained in the database to
reconstruct the original file.
Data stored using non-XML notations will need appropriate application software
to process it, but the XML-coded file will correctly identify where each piece
of such data belongs in the completed document and where it has been stored
prior to use.
By storing data in the clearly defined format provided by XML you can ensure
that your data will be transferable to a wide range of hardware and software
environments. New techniques in programming and processing data will not affect
the logical structure of your document's message. If more detail needs to be
added to the file all you need to do is to update the model and then add new
markup tags where required in the document instance. If a completely new style
is required then the existing document model can be linked to the new one to
provide automatic updating of document structures.
About the Author
Amrit Hallan is a freelance web designer. For all web site development and web
promotion needs, you can get in touch with him at
http://www.bytesworth.com. For more such articles, visit
http://www.bytesworth.com/articles and
http://www.bytesworth.com/learn You can subscribe to his newsletter
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