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/*! \page xml.html

\title XML Module

This module is part of the \link editions.html Qt Enterprise Edition \endlink.

\tableofcontents

\target overview
\section1 Overview of the XML architecture in Qt

The XML module provides a well-formed XML parser using the SAX2 (Simple API for
XML) interface plus an implementation of the DOM Level 2 (Document Object
Model).

SAX is an event-based standard interface for XML parsers.
The Qt interface follows the design of the SAX2 Java implementation.
Its naming scheme was adapted to fit the Qt naming conventions.
Details on SAX2 can be found at
\link http://www.megginson.com/SAX/ http://www.megginson.com/SAX/ \endlink.

Support for SAX2 filters and the reader factory are under
development. Furthermore the Qt implementation does not include the
SAX1 compatibility classes present in the Java interface.

For an introduction to Qt's SAX2 classes see
"\link #sax2 The Qt SAX2 classes \endlink".
A code example is discussed in the "\link xml-sax-walkthrough.html tagreader
walkthrough \endlink".

DOM Level 2 is a W3C Recommendation for XML interfaces that maps the
constituents of an XML document to a tree structure. Details and the
specification of DOM Level 2 can be found at
\link http://www.w3.org/DOM/ http://www.w3.org/DOM/ \endlink.
More information about the DOM classes in Qt is provided in the
\link #dom Qt DOM classes \endlink.


Qt provides the following XML related classes:

\list
\i \l QDomAttr -- Represents one attribute of a QDomElement
\i \l QDomCDATASection -- Represents an XML CDATA section
\i \l QDomCharacterData -- Represents a generic string in the DOM
\i \l QDomComment -- Represents an XML comment
\i \l QDomDocument -- The representation of an XML document
\i \l QDomDocumentFragment -- Tree of QDomNodes which is usually not a complete QDomDocument
\i \l QDomDocumentType -- The representation of the DTD in the document tree
\i \l QDomElement -- Represents one element in the DOM tree
\i \l QDomEntity -- Represents an XML entity
\i \l QDomEntityReference -- Represents an XML entity reference
\i \l QDomImplementation -- Information about the features of the DOM implementation
\i \l QDomNamedNodeMap -- Collection of nodes that can be accessed by name
\i \l QDomNode -- The base class for all nodes of the DOM tree
\i \l QDomNodeList -- List of QDomNode objects
\i \l QDomNotation -- Represents an XML notation
\i \l QDomProcessingInstruction -- Represents an XML processing instruction
\i \l QDomText -- Represents textual data in the parsed XML document
\i \l QXmlAttributes -- XML attributes
\i \l QXmlContentHandler -- Interface to report logical content of XML data
\i \l QXmlDeclHandler -- Interface to report declaration content of XML data
\i \l QXmlDefaultHandler -- Default implementation of all XML handler classes
\i \l QXmlDTDHandler -- Interface to report DTD content of XML data
\i \l QXmlEntityResolver -- Interface to resolve extern entities contained in XML data
\i \l QXmlErrorHandler -- Interface to report errors in XML data
\i \l QXmlInputSource -- The input data for the QXmlReader subclasses
\i \l QXmlLexicalHandler -- Interface to report lexical content of XML data
\i \l QXmlLocator -- The XML handler classes with information about the actual parsing position
\i \l QXmlNamespaceSupport -- Helper class for XML readers which want to include namespace support
\i \l QXmlParseException -- Used to report errors with the QXmlErrorHandler interface
\i \l QXmlReader -- Interface for XML readers (i.e. for SAX2 parsers)
\i \l QXmlSimpleReader -- Implementation of a simple XML reader (a SAX2 parser)
\endlist



\target sax2
\section1 The Qt SAX2 classes

\target sax2Intro
\section2 Introduction to SAX2

The SAX2 interface is an event-driven mechanism to provide the user with
document information. "Event" in this context has nothing to do with the
term "event" you probably know from windowing systems; it means that the
parser reports certain document information while parsing the document.
These reported information is referred to as "event".

To make it less abstract consider the following example:
\code 
<quote>To make it less abstract consider the following example:</quote>
\endcode

Whilst reading (a SAX2 parser is usually referred to as "reader") 
the above document three events would be triggered:
\list 1
 \i A start tag occurs (\c{<quote>}).
 \i Character data (i.e. text) is found.
 \i An end tag is parsed (\c{</quote>}).
\endlist

Each time such an event occurs the parser reports it so that 
a suitable event handling routine can be invoked.

Whilst this is a fast and simple approach to read XML documents
manipulation is difficult because data are not stored, simply handled
and discarded serially. This is when the \link #dom DOM interface 
\endlink comes handy.

The Qt XML module provides an
abstract class, \l QXmlReader, that defines the interface for potential 
SAX2 readers.
At the moment Qt ships with one reader implementation, \l
QXmlSimpleReader.

The reader reports parsing events through special handler classes. In Qt
the following ones are available:
\list
 \i \l QXmlContentHandler
    reports events related to the content of a document (e.g. the start tag
    or characters).
 \i \l QXmlDTDHandler
    reports events related to the DTD (e.g. notation declarations).
 \i \l QXmlErrorHandler
    reports errors or warnings that occurred during parsing.
 \i \l QXmlEntityResolver
    reports external entities during parsing and allows the user to resolve
    external entities him- or herself instead of leaving it to the reader.
 \i \l QXmlDeclHandler
    reports further DTD related events (e.g. attribute declarations).
    Usually users are not interested in them, but under certain circumstances
    this class comes handy.
 \i \l QXmlLexicalHandler
    reports events related to the lexical structure of the document
    (the beginning of the DTD, comments etc.). Occasionally this
    might be useful.
\endlist

These classes are abstract classes describing the interface. The
\l QXmlDefaultHandler class provides a "do nothing" default implementation for
all of them. Therefore users need to overload only the
QXmlDefaultHandler functions they are interested in. 

To read input XML data a special class \l QXmlInputSource is used.

Apart from the already mentioned ones the following SAX2 support classes 
provide the user with useful functionality:
\list
 \i \l QXmlAttributes
    is used to pass attributes in a start element event.
 \i \l QXmlLocator
    is used to obtain the actual parsing position of an event.
 \i \l QXmlNamespaceSupport
    is used to easily implement \link xml.html#namespaces namespace \endlink
    support for a reader.
    Note that namespaces do not change the parsing 
    behavior. They are only reported through the handler.
\endlist


\target sax2Features
\section2 Features
 
The behaviour of an XML reader depends on whether it supports certain
optional features or not. 
As an example a reader can have the feature 
"report attributes used for \link xml.html#namespaces namespace \endlink declarations
and prefixes along with the local name of a tag".  
Like every other feature this has a unique name represented by a URI:
it is called \e http://xml.org/sax/features/namespace-prefixes.

The Qt SAX2 implementation allows you to find out whether the 
reader has this ability using \l QXmlReader::hasFeature().
If the return value is TRUE it is possible to
turn the relevant feature on and off.
To do this use \l QXmlReader::setFeature(). Whether a supported feature
is on or off (TRUE or FALSE) can be queried using  \l QXmlReader::feature().

Consider the example 
\code 
<document xmlns:book = 'http://trolltech.com/fnord/book/'
          xmlns      = 'http://trolltech.com/fnord/' >
\endcode
A reader not supporting the
\e http://xml.org/sax/features/namespace-prefixes feature would clearly
report the element name \e document but not its attributes \e xmlns:book and \e xmlns
with their values. A
reader with the feature \e http://xml.org/sax/features/namespace-prefixes
reports the namespace attributes if \l QXmlReader::feature() is TRUE and 
disregards them if the feature is FALSE.


Other features include \e http://xml.org/sax/features/namespace (namespace 
processing, implies \e http://xml.org/sax/features/namespace-prefixes) or
\e http://xml.org/sax/features/validation (the ability to report validation
errors). 

Whilst SAX2 leaves it to the user to define and implement whatever
features are required, support for \e http://xml.org/sax/features/namespace
(and thus \e http://xml.org/sax/features/namespace-prefixes) is
mandantory. Accordingly \l QXmlSimpleReader, the implementation
of \l QXmlReader that comes with the Qt XML module, supports both of them,
and therefore can do namespace processing.

Being a non-validating parser \l QXmlSimpleReader 
does not support \e http://xml.org/sax/features/validation
and other features.


\target sax2Namespaces
\section2 Namespace support via features

As we have seen in the \link #sax2Features previous section \endlink
we can configure the behavior of the reader when it comes to namespace
processing. This is done by setting and unsetting the 
\e http://xml.org/sax/features/namespaces and
\e http://xml.org/sax/features/namespace-prefixes features.
 
They influence the reporting behavior in the following way:
\list 1
 \i Namespace prefixes and local parts of elements and attributes can be
    reported.
 \i The qualified names of elements and attributes are reported.
 \i \l QXmlContentHandler::startPrefixMapping() and \l
    QXmlContentHandler::endPrefixMapping() are called by the reader.
 \i Attributes that declare namespaces (i.e. the attribute \e xmlns and
    attributes starting with \e xmlns: ) are reported.
\endlist

Consider the following element:

\code
<author xmlns:fnord = 'http://trolltech.com/fnord/'
             title="Ms" 
             fnord:title="Goddess" 
             name="Eris Kallisti"/>
\endcode

With \e http://xml.org/sax/features/namespace-prefixes set to TRUE 
the reader will report four attributes, with the \e namespace-prefixes
feature set to FALSE only three: The \e xmlns:fnord attribute defining
a namespace is then "unvisible" for the reader.

The \e http://xml.org/sax/features/namespaces feature on the other hand 
is responsible for reporting local names, namespace prefixes and -URIs.
With \e http://xml.org/sax/features/namespaces set to TRUE
the parser will report \e title as the local name of \e fnord:title
attribute, \e fnord being the namespace prefix and \e http://trolltech.com/fnord/
as the namespace URI.
When \e http://xml.org/sax/features/namespaces is FALSE none of them are
reported.

In the current implementation the Qt XML classes follow the definition
that the prefix \e xmlns itself isn't associated with any namespace at all
(see \link http://www.w3.org/TR/1999/REC-xml-names-19990114/#ns-using 
http://www.w3.org/TR/1999/REC-xml-names-19990114/#ns-using \endlink).
Therefore even with \e http://xml.org/sax/features/namespaces and
\e http://xml.org/sax/features/namespace-prefixes both set to TRUE
the reader won't return either a local name, a namespace prefix or
a namespace URI for \e  xmlns:fnord.

This might be changed in the future following the W3C suggestion
\link http://www.w3.org/2000/xmlns/ http://www.w3.org/2000/xmlns/ \endlink
to associate \e xmlns with the namespace \e http://www.w3.org/2000/xmlns. 


As the SAX2 standard suggests \l QXmlSimpleReader by default has
\e http://xml.org/sax/features/namespaces set to TRUE and
\e http://xml.org/sax/features/namespace-prefixes set to FALSE.
When changing this behavior using \l QXmlSimpleReader::setFeature() 
note that the combination of both features set to
FALSE is illegal.

For a practical demonstration of how the two features affect the 
output of the reader run the \link tagreader-with-features-example.html
tagreader with features example. \endlink 

\target sax2NamespacesSummary
\section3 Summary

\l QXmlSimpleReader implements the following behavior:

\table
\header \i (namespaces, namespace-prefixes)
	\i Namespace prefix and local part
	\i Qualified names
	\i Prefix mapping
	\i xmlns attributes
\row \i (TRUE, FALSE) \i Yes \i Yes* \i Yes \i No
\row \i (TRUE, TRUE) \i Yes \i Yes \i Yes \i Yes
\row \i (FALSE, TRUE) \i No* \i Yes \i No* \i Yes
\row \i (FALSE, FALSE) \i41 Illegal
\endtable

For the entries marked with a "*", SAX does not require a particuliar
behavior.

\target sax2Properties
\section2 Properties

Properties are a more general concept. They also have a unique name,
represented as an URI, but their value is \c void*. Thus nearly everything
can be used as a property value. This concept involves some danger,
though: there are no means to ensure type-safety; the user must take care
that he or she passes the correct type. Properties are useful if a reader supports
special handler classes.

\omit example! \endomit
 
The URIs used for features and properties often look like URLs, e.g. 
\c http://xml.org/sax/features/namespace. This does not mean that whatsoever
data is required at this address. It is simply a way to define unique names.
 
Everybody can define and use new SAX2 properties for his or her
readers. Property support is however not
required.
 
To set or query properties the following functions are provided:
\l QXmlReader::setProperty(), \l QXmlReader::property() and \l
QXmlReader::hasProperty().                      


\target sax2Reading
\section2 Further reading

For a practical example on how to use the Qt SAX2 classes see the 
\link xml-sax-walkthrough.html tagreader walkthrough. \endlink

More information about XML (e.g. \link xml.html#namespaces namespaces \endlink)
can be found in the \link xml.html introduction to the Qt XML module. \endlink



\target dom
\section1 The Qt DOM classes

\target domIntro
\section2 Introduction to DOM

DOM provides an interface to access and change the content and structure of
an XML file. It makes a hierarchical view of the document (tree) 
available with the root element of the XML file serving as its root. 
Thus -- in contrast to the SAX2 interface -- an object model of the document 
is resident in memory after parsing which makes manipulation easy. 

In the Qt implementation of the DOM all
nodes in the document tree are subclasses of \l QDomNode.
The document itself is represented as a \l QDomDocument object.

Here are the available node classes and their potential children classes:

\list
\i \l QDomDocument: Possible children are
	\list
	\i \l QDomElement (at most one)
	\i \l QDomProcessingInstruction
	\i \l QDomComment
	\i \l QDomDocumentType
	\endlist
\i \l QDomDocumentFragment: Possible children are
	\list
	\i \l QDomElement
	\i \l QDomProcessingInstruction
	\i \l QDomComment
	\i \l QDomText
	\i \l QDomCDATASection
	\i \l QDomEntityReference
	\endlist
\i \l QDomDocumentType: No children
\i \l QDomEntityReference: Possible children are
	\list
	\i \l QDomElement
	\i \l QDomProcessingInstruction
	\i \l QDomComment
	\i \l QDomText
	\i \l QDomCDATASection
	\i \l QDomEntityReference
	\endlist
\i \l QDomElement: Possible children are
	\list
	\i \l QDomElement
	\i \l QDomText
	\i \l QDomComment
	\i \l QDomProcessingInstruction
	\i \l QDomCDATASection
	\i \l QDomEntityReference
	\endlist
\i \l QDomAttr: Possible children are
	\list
	\i \l QDomText
	\i \l QDomEntityReference
	\endlist
\i \l QDomProcessingInstruction: No children
\i \l QDomComment: No children
\i \l QDomText: No children
\i \l QDomCDATASection: No children
\i \l QDomEntity: Possible children are
	\list
	\i \l QDomElement
	\i \l QDomProcessingInstruction
	\i \l QDomComment
	\i \l QDomText
	\i \l QDomCDATASection
	\i \l QDomEntityReference
	\endlist
\i \l QDomNotation: No children
\endlist

With \l QDomNodeList and \l QDomNamedNodeMap two collection classes 
are provided: \l QDomNodeList is a list of nodes
whereas \l QDomNamedNodeMap is used to handle unordered sets of nodes
(often used for attributes).

The \l QDomImplementation class allows the user to query features of the
DOM implementation.

\section2 Further reading

To get started please refer to the \l QDomDocument documentation that
describes basic usage.

More information about Qt and XML  can be found in the \link xml.html
Introduction to the Qt XML module. \endlink   



\target namespaces
\section1 An introduction to namespaces

Parts of the Qt XML module documentation assume that you are
familiar with XML namespaces. Here we present a brief introduction;
skip to \link #namespacesConventions Qt XML
documentation conventions \endlink if you know this material.

Namespaces are a concept introduced into XML to allow a more modular design.
With their help data processing software can easily
resolve naming conflicts in XML documents.

Consider the following example:

\code
<document>
<book>
  <title>Practical XML</title>
  <author title="Ms" name="Eris Kallisti"/>
  <chapter>
    <title>A Namespace Called fnord</title>
  </chapter>
</book>
</document>
\endcode

Here we find three different uses of the name \e title. If you wish
to process this document you will encounter problems
because each of the \e titles should be displayed in a different manner --
even though they have the same name.

The solution would be to have some means of identifying the
first occurrence of \e title as the title of a book, i.e.
to use the \e title element of a
book namespace to distinguish it from for example the chapter title, e.g.:
\code
<book:title>Practical XML</book:title>
\endcode

\e book in this case is
a \e prefix denoting the namespace.

Before we can apply a
namespace to element or attribute names we must declare it.

Namespaces are URIs like \e http://trolltech.com/fnord/book/.
This does not mean that data must be available at this
address; the URI is simply used to provide a unique name.

We declare namespaces in the same way as
attributes; strictly speaking they \e are attributes.
To make for example \e http://trolltech.com/fnord/ the document's
default XML namespace \e xmlns we write

\code
xmlns="http://trolltech.com/fnord/"
\endcode

To distinguish the \e http://trolltech.com/fnord/book/ namespace
from the default, we have to supply it with a prefix:

\code
xmlns:book="http://trolltech.com/fnord/book/"
\endcode

A namespace that is declared like this can be applied
to element and attribute names by prepending the appropriate
prefix and a ":" delimiter. We have already seen this with
the \e book:title element.

Element names without a prefix belong to the default namespace.
This rule does not apply to attributes: an attribute
without a prefix does not belong to any of the declared
XML namespaces at all.
Attributes always belong to the "traditional" namespace
of the element in which they appear. A "traditional" namespace
is not an XML namespace, it simply means that all attribute names
belonging to one element must be different. Later we will see how
to assign an XML namespace to an attribute.

Due to the fact that attributes without prefixes are not in any
XML namespace there is
no collision between the attribute \e title (that belongs to the
\e author element) and for example the \e title element within a \e chapter.

Let's clarify matters with an example:
\code
<document xmlns:book = 'http://trolltech.com/fnord/book/'
          xmlns      = 'http://trolltech.com/fnord/' >
<book>
  <book:title>Practical XML</book:title>
  <book:author xmlns:fnord = 'http://trolltech.com/fnord/'
               title="Ms"
               fnord:title="Goddess"
               name="Eris Kallisti"/>
  <chapter>
    <title>A Namespace Called fnord</title>
  </chapter>
</book>
</document>
\endcode

Within the \e document element we have two namespaces declared.
The default namespace \e http://trolltech.com/fnord/
applies to the \e book element, the \e chapter element,
the appropriate \e title element and of course to \e document itself.

The \e book:author and \e book:title elements
belong to the namespace with the
URI \e http://trolltech.com/fnord/book/.

The two \e book:author attributes \e title and \e name have no XML namespace
assigned.
They are only members of the "traditional" namespace of the element
\e book:author, meaning that for example two \e title attributes
in \e book:author are forbidden.

In the above example we circumvent the last rule by adding a \e title
attribute from the \e http://trolltech.com/fnord/ namespace to \e book:author:
the \e fnord:title comes from the namespace with the prefix \e fnord
that is declared in the \e book:author element.

Clearly the \e fnord namespace has the same namespace URI as the
default namespace. So why didn't we simply use the
default namespace we'd already declared? The answer is quite complex:
\list
\i attributes without a prefix don't belong to any XML namespace at all,
even not to the default namespace;
\i additionally omitting the prefix would lead to a \e title-title clash;
\i writing it as \e xmlns:title would declare a new namespace with
the prefix \e title instead of applying the default \e xmlns namespace.
\endlist

With the Qt XML classes elements and attributes can be accessed in two ways: either
by refering to their qualified names consisting of the namespace prefix
and the "real" name (or \e local name) or
by the combination of local name and namespace URI.

More information on XML namespaces can be found at
\l http://www.w3.org/TR/REC-xml-names/.


\target namespacesConventions
\section2 Conventions used in Qt XML documentation

The following terms are used to distinguish the parts of names within the context of
namespaces:
\list
\i  The \e {qualified name}
    is the name as it appears in the document. (In the above example \e
    book:title is a qualified name.)
\i  A \e {namespace prefix} in a qualified name
    is the part to the left of the ":".  (\e book is the namespace prefix in
    \e book:title.)
\i  The \e {local part} of a name (also refered to as the \e {local name}) appears
    to the right of the ":".
    (Thus \e title is the local part of \e book:title.)
\i  The \e {namespace URI} ("Uniform Resource Identifier") is a unique
    identifier for a namespace. It looks like a URL
    (e.g. \e http://trolltech.com/fnord/ ) but does not require
    data to be accessible by the given protocol at the named address.
\endlist

Elements without a ":" (like \e chapter in the example) do not have a
namespace prefix. In this case the local part and the qualified name
are identical (i.e. \e chapter).
*/