File: xml-sax-walkthrough.doc

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/****************************************************************************
**
** Documentation on the sax interface of the xml module
**
** Copyright (C) 2005-2008 Trolltech ASA.  All rights reserved.
**
** This file is part of the Qt GUI Toolkit.
**
** This file may be used under the terms of the GNU General
** Public License versions 2.0 or 3.0 as published by the Free
** Software Foundation and appearing in the files LICENSE.GPL2
** and LICENSE.GPL3 included in the packaging of this file.
** Alternatively you may (at your option) use any later version
** of the GNU General Public License if such license has been
** publicly approved by Trolltech ASA (or its successors, if any)
** and the KDE Free Qt Foundation.
**
** Please review the following information to ensure GNU General
** Public Licensing requirements will be met:
** http://trolltech.com/products/qt/licenses/licensing/opensource/.
** If you are unsure which license is appropriate for your use, please
** review the following information:
** http://trolltech.com/products/qt/licenses/licensing/licensingoverview
** or contact the sales department at sales@trolltech.com.
**
** This file may be used under the terms of the Q Public License as
** defined by Trolltech ASA and appearing in the file LICENSE.QPL
** included in the packaging of this file.  Licensees holding valid Qt
** Commercial licenses may use this file in accordance with the Qt
** Commercial License Agreement provided with the Software.
**
** This file is provided "AS IS" with NO WARRANTY OF ANY KIND,
** INCLUDING THE WARRANTIES OF DESIGN, MERCHANTABILITY AND FITNESS FOR
** A PARTICULAR PURPOSE. Trolltech reserves all rights not granted
** herein.
**
**********************************************************************/

/*! \page xml-sax-walkthrough.html

\ingroup step-by-step-examples

\title Walkthrough: How to use the Qt SAX2 classes

For a general discussion of the XML topics in Qt please refer to
the document \link xml.html XML Module. \endlink
To learn more about SAX2  see the document describing
\link xml.html#sax2 the Qt SAX2 implementation. \endlink

Before reading on you should at least be familiar with 
the \link xml.html#sax2Intro Introduction to SAX2. \endlink


<a name="quickStart"></a>
<h2>A tiny parser</h2>

In this section we will present a small example reader that outputs
the names of all elements in an XML document on the command line. 
The element names are indented corresponding to their nesting level.

As mentioned in \link xml.html#sax2Intro Introduction to SAX2 \endlink
we have to implement the functions of the handler classes that we are
interested in. In our case these are only three:
\l QXmlContentHandler::startDocument(),
\l QXmlContentHandler::startElement() and
\l QXmlContentHandler::endElement().

For this purpose we use a subclass of the \l QXmlDefaultHandler (remember
that the special handler classes are all abstract and the default handler class
provides an implementation that does not change the parsing behavior):

\include xml/tagreader/structureparser.h

Apart from the private helper variable \e indent that we will use to
get indentation right, there is nothing special about our new 
\e StructureParser class.

\quotefile xml/tagreader/structureparser.cpp

Even the implementation is straight-forward: 

\skipto include
\printuntil qstring.h

First we overload \l QXmlContentHandler::startDocument() with a non-empty version.

\printline startDocument
\printuntil }

At the beginning of the document we simply 
set \e indent to an empty string because we
want to print out the root element without any indentation.
Also we return TRUE so that the parser continues without 
reporting an error.

Because we want to be informed when the parser comes
accross a start tag of an element and subsequently print it out, we
have to overload \l QXmlContentHandler::startElement(). 

\printline startElement
\printuntil } 

This is what the implementation does: The name of the element with
preceding indentation is printed out followed by a linebreak.
Strictly speaking \e qName contains the local element name 
without an eventual prefix denoting the \link xml.html#namespaces namespace. 
\endlink

If another element follows before the current element's end tag
it should be indented. Therefore we add four spaces to the
\e indent string.

Finally we return TRUE in order to let the parser continue without
errors.

The last functionality we need to add is the parser's behaviour when an 
end tag occurs. This means overloading \l QXmlContentHandler::endElement().

\printline endElement
\printuntil }  

Obviously we then should shorten the \e indent string by the four
whitespaces added in startElement().

With this we're done with our parser and can start writing the main() 
program.

\quotefile xml/tagreader/tagreader.cpp

\skipto include
\printto handler

This check ensures that we have a sequence of files from the command
line to examine.

\printline handler

The next step is to create an instance of the \e StructureParser. 

\printline reader
\printline setContentHandler

After that we set up the reader. As our \e StructureParser
class deals with \l QXmlContentHandler functionality only 
we simply register it as the content handler of our choice.

\printuntil for

Successively we deal with all files given as command line arguments.

\printline xmlFile
\printline QXmlInputSource

Then we create a
\l QXmlInputSource for the XML file to be parsed. 

\printline parse

Now we take our input source and start parsing.

\printline }
\printuntil }


Running the program on the following XML file...

\include xml/tagreader/animals.xml

... produces the following output:
\code
animals
    mammals
        monkeys
            gorilla
            orang-utan
    birds
        pigeon
        penguin 
\endcode

It will however refuse to produce the correct result if you e.g. insert
a whitespace between a &lt; and the element name in your test-XML file.
To prevent such annoyances
you should always install an error handler with \l
QXmlReader::setErrorHandler(). This allows you to report
parsing errors to the user.



*/