XML +Framework Overview

The XML Framework is a collection of components for event-based XML parsing +and provides content-processing architecture.

Purpose

The XML Framework provides configurable +features for parsing XML and WBXML (WAP +Binary XML), with options for validating against a specification and auto-correcting +for spelling errors in the validated text, using a single interface. It is +based on the SAX (Simple +API for XML) specification.

Required background

You must have a basic understanding +on XML before using XML Framework.

Key concepts

The following are the key concepts +of XML Framework:

Attribute

A name-value pair separated +by an equals sign, for example author="Jane Austen"

Attribute +type

One of certain data types defined for attributes, for instance CDATA.

Client

An +application which uses the XML framework for parsing or generating a document.

Document +Type Definition (DTD)

A document which defines a particular use +of XML entities (the names, attributes and values permitted).

Extension

WBXML +extends XML syntax with extension tokens which are used differently by different +applications. For example, extension token is used to refer to a string table +created specifically for each message and transmitted in the introduction +of the message.

Parser

It is an interface to the XML framework +which allows a client to access the parser plug-ins, which are specific for +a mark-up language. For example, XML Expat Parser and WBXML Parser.

String +dictionary collection

A class that holds a collection of string +dictionaries.

String dictionary plug-ins

The XML Framework +allows strings to be stored in DTD document, XML namespace or WBXML code page +in an ECOM plug-in that could be accessed as required by the parser and the +client. These plug-ins are referred as string dictionary plug-ins.

String +pool

A string pool is a mechanism for storing strings in a particular +way using which the strings can be compared quickly.

String table

A +WBXML document is encoded and decoded using a table of frequently encountered +strings which the body of the document references by index to compress the +data.

Uniform Resource Identifier (URI)

The web address +associated with a prefix. For instance, http://www.w3.org/XML/1998/namespace.

WBXML

WAP +Binary XML (WBXML) is a binary representation of XML. It was developed by +the Open Mobile Alliance as a standard to allow XML documents to be transmitted +in a compact manner over mobile networks and was proposed as an addition to +the World Wide Web Consortium's Wireless Application Protocol family of standards.

Architecture

The following diagram illustrates +the XML framework, consisting of client and a parser:

+ Block diagram of XML framework + +

The XML framework consists of classes which model the main constituents +of the architecture - the framework as a whole, the parser plug-ins and extensions +to XML, the content processor chain and the content handler mechanism.

The +XML and WBXML parsers convert the contents of a document to UTF-8 format. +This is to ensure that extended characters are not lost from the document +by the String Pool. Expat is the engine behind the XML parser plug-in.

The +XML Framework allows strings to be stored for a particular DTD, XML namespace +or WBXML codepage in an ECOM plug-in that can be accessed when requireded +by the Parser and the Client. These plug-ins are referred as String Dictionary +Plug-ins and they are managed through a string dictionary collection object. +See String Dictionary

Libxml2 +provides XML processing, parsing and validation APIs. See libxml2.

Plug-in +1 and Plug-in 2 are examples of optional processors, which may be chained +together with the parser output to allow further processing of the data, before +the client receives it. Such plug-ins can be a DTD validator or a document +auto-corrector. The chain is not limited to just two plug-ins.

Parser +framework

The XML framework contains Parser framework which is +represented by the CParser class. A client with an XML +document to be parsed creates a CParser object and calls +its parse functions. CParser obtains the data about plug-ins +and the document to be parsed fromCMatchData and RDocumentParameters classes +respectively.

The parser framework conforms to the event-based SAX +specification. It outputs an event when it starts or finishes reading one +of the following:

a document
a start tag
an end tag
a prefix mapping
a processing instruction
character data
ignorable white space

For more information on XML-related concepts, refer to W3C or similar +sources.

Parser plug-ins

The CParser is +the interface to the XML framework allowing the client to access the parser +plug-ins, each one of which is specific to a mark-up language (e.g. XML, WBXML). +Individual parser plug-in implements the MParser interface. +It is associated through the TParserInitParams class, with +a character set converter (to convert other formats to Unicode), a string +dictionary and an element stack.

The Symbian platform framework is +delivered with three parser plug-ins, two for XML and one for WBXML.

The first XML parser +consists of CXmlParser class, which is wrapped around the CExpat class, +an implementation of the stream-based Expat parser.
The second XML parser +consists of CXMLEngineSAXPlugin class, which encapsulates +the SAX parser of the libxml2 component. It is not available +if the Symbian platform build excludes this component.
The WBXML parser is +implemented as the CWmxmlParser class.

Extensions to XML

The XML framework provides extensions +to XML. At present WBXML is implemented. WBXML requires use of string dictionaries +and extension tokens to store the element strings specific to the WBXML, which +are represented by the RStringDictionaryCollection, MWbxmlExtensionHandler and TExtensionTokens classes.

Content processors

Content processors are +plug-ins which perform further operations on the output of a parser plug-in. +They implement the MContentProcessor interface and are +associated through the TContentProcessorInitParams class, +with a string dictionary and element stack. They are organised into chains +by the MContentSource class which directs the output of +each plug-in to the next plug-in in the chain.

Content handlers

A +client application which is designed to react to the output of the XML framework +event must implement the MContentHandler interface. The +functions to be implemented correspond to the SAX specification discussed +in the Parser Framework section.

APIs

The XML Framework exports the following APIs:

+ + + +API +Description + + + + +

CExpat

Encapsulates the Expat XML parser.

+ + +

CXmlParser

Implementation of the stream-based Expat parser.

+ + +

CXMLEngineSAXPlugin

Encapsulates the SAX parser of the libxml2 component.

+ + +

CParser

Represents the entire parser framework.

+ + +

CMatchData

Consists of the data of the plug-ins.

+ + +

CWmxmlParser

WBXML parser implementation.

+ + +

RDocumentParameters

Consists of the data about the document to be parsed.

+ + +

RElementStack

Data structure used to store XML elements and check the tag ordering.

+ + +

RStringDictionaryCollection

Holds a collection of dictionaries requested by the user.

+ + + +

Typical uses

The following tasks can be performed +using XML Framework:

Parsing an XML document.
Choosing a parser plug-in.
Using content processor.
Writing a parser plug-in.
Customising a parser +plug-in.
Creating a resource +file for a parser plug-in.