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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Walkthrough: How to use the Qt SAX2 classes</h1>
<p>
<p> For a general discussion of the XML topics in Qt please refer to
the document <a href="xml.html">XML Module.</a>
To learn more about SAX2 see the document describing
<a href="xml.html#sax2">the Qt SAX2 implementation.</a>
<p> Before reading on you should at least be familiar with
the <a href="xml.html#sax2Intro">Introduction to SAX2.</a>
<p> <a name="quickStart"></a>
<h2>A tiny parser</h2>
<p> 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.
<p> As mentioned in <a href="xml.html#sax2Intro">Introduction to SAX2</a>
we have to implement the functions of the handler classes that we are
interested in. In our case these are only three:
<a href="qxmlcontenthandler.html#startDocument">QXmlContentHandler::startDocument</a>(),
<a href="qxmlcontenthandler.html#startElement">QXmlContentHandler::startElement</a>() and
<a href="qxmlcontenthandler.html#endElement">QXmlContentHandler::endElement</a>().
<p> For this purpose we use a subclass of the <a href="qxmldefaulthandler.html">QXmlDefaultHandler</a> (remember
that the special handler classes are all abstract and the default handler class
provides an implementation that does not change the parsing behavior):
<p> <pre>/****************************************************************************
** $Id: qt/structureparser.h 3.3.8 edited Jan 11 14:37 $
**
** Copyright (C) 1992-2007 Trolltech ASA. All rights reserved.
**
** This file is part of an example program for Qt. This example
** program may be used, distributed and modified without limitation.
**
*****************************************************************************/
#ifndef STRUCTUREPARSER_H
#define STRUCTUREPARSER_H
#include <<a href="qxml-h.html">qxml.h</a>>
class QString;
class StructureParser : public <a href="qxmldefaulthandler.html">QXmlDefaultHandler</a>
{
public:
bool startDocument();
bool startElement( const <a href="qstring.html">QString</a>&, const <a href="qstring.html">QString</a>&, const <a href="qstring.html">QString</a>& ,
const <a href="qxmlattributes.html">QXmlAttributes</a>& );
bool endElement( const <a href="qstring.html">QString</a>&, const <a href="qstring.html">QString</a>&, const <a href="qstring.html">QString</a>& );
private:
<a href="qstring.html">QString</a> indent;
};
#endif
</pre>
<p> Apart from the private helper variable <em>indent</em> that we will use to
get indentation right, there is nothing special about our new
<em>StructureParser</em> class.
<p>
<p> Even the implementation is straight-forward:
<p> <pre> #include "structureparser.h"
#include <stdio.h>
#include <<a href="qstring-h.html">qstring.h</a>>
</pre>
<p> First we overload <a href="qxmlcontenthandler.html#startDocument">QXmlContentHandler::startDocument</a>() with a non-empty version.
<p> <pre> <a name="x2137"></a>bool StructureParser::<a href="qxmlcontenthandler.html#startDocument">startDocument</a>()
{
indent = "";
return TRUE;
}
</pre>
<p> At the beginning of the document we simply
set <em>indent</em> 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.
<p> 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 <a href="qxmlcontenthandler.html#startElement">QXmlContentHandler::startElement</a>().
<p> <pre> <a name="x2138"></a>bool StructureParser::<a href="qxmlcontenthandler.html#startElement">startElement</a>( const <a href="qstring.html">QString</a>&, const <a href="qstring.html">QString</a>&,
const <a href="qstring.html">QString</a>& qName,
const <a href="qxmlattributes.html">QXmlAttributes</a>& )
{
printf( "%s%s\n", (const char*)indent, (const char*)qName );
indent += " ";
return TRUE;
}
</pre>
<p> This is what the implementation does: The name of the element with
preceding indentation is printed out followed by a linebreak.
Strictly speaking <em>qName</em> contains the local element name
without an eventual prefix denoting the <a href="xml.html#namespaces">namespace.</a>
<p> If another element follows before the current element's end tag
it should be indented. Therefore we add four spaces to the
<em>indent</em> string.
<p> Finally we return TRUE in order to let the parser continue without
errors.
<p> The last functionality we need to add is the parser's behaviour when an
end tag occurs. This means overloading <a href="qxmlcontenthandler.html#endElement">QXmlContentHandler::endElement</a>().
<p> <pre> <a name="x2136"></a>bool StructureParser::<a href="qxmlcontenthandler.html#endElement">endElement</a>( const <a href="qstring.html">QString</a>&, const <a href="qstring.html">QString</a>&, const <a href="qstring.html">QString</a>& )
{
indent.remove( (uint)0, 4 );
return TRUE;
}
</pre>
<p> Obviously we then should shorten the <em>indent</em> string by the four
whitespaces added in startElement().
<p> With this we're done with our parser and can start writing the main()
program.
<p>
<p> <pre> #include "structureparser.h"
#include <<a href="qfile-h.html">qfile.h</a>>
#include <<a href="qxml-h.html">qxml.h</a>>
#include <<a href="qwindowdefs-h.html">qwindowdefs.h</a>>
int main( int argc, char **argv )
{
if ( argc < 2 ) {
fprintf( stderr, "Usage: %s <xmlfile> [<xmlfile> ...]\n", argv[0] );
return 1;
}
</pre>
<p> This check ensures that we have a sequence of files from the command
line to examine.
<p> <pre> StructureParser handler;
</pre>
<p> The next step is to create an instance of the <em>StructureParser</em>.
<p> <pre> <a href="qxmlsimplereader.html">QXmlSimpleReader</a> reader;
<a name="x2140"></a> reader.<a href="qxmlreader.html#setContentHandler">setContentHandler</a>( &handler );
</pre>
<p> After that we set up the reader. As our <em>StructureParser</em>
class deals with <a href="qxmlcontenthandler.html">QXmlContentHandler</a> functionality only
we simply register it as the content handler of our choice.
<p> <pre> for ( int i=1; i < argc; i++ ) {
</pre>
<p> Successively we deal with all files given as command line arguments.
<p> <pre> <a href="qfile.html">QFile</a> xmlFile( argv[i] );
<a href="qxmlinputsource.html">QXmlInputSource</a> source( &xmlFile );
</pre>
<p> Then we create a
<a href="qxmlinputsource.html">QXmlInputSource</a> for the XML file to be parsed.
<p> <pre> <a name="x2139"></a> reader.<a href="qxmlsimplereader.html#parse">parse</a>( source );
</pre>
<p> Now we take our input source and start parsing.
<p> <pre> }
return 0;
}
</pre>
<p> Running the program on the following XML file...
<p> <pre><animals>
<mammals>
<monkeys> <gorilla/> <orangutan/> </monkeys>
</mammals>
<birds> <pigeon/> <penguin/> </birds>
</animals>
</pre>
<p> ... produces the following output:
<pre>
animals
mammals
monkeys
gorilla
orang-utan
birds
pigeon
penguin
</pre>
<p> It will however refuse to produce the correct result if you e.g. insert
a whitespace between a < and the element name in your test-XML file.
To prevent such annoyances
you should always install an error handler with <a href="qxmlreader.html#setErrorHandler">QXmlReader::setErrorHandler</a>(). This allows you to report
parsing errors to the user.
<p> <p>See also <a href="step-by-step-examples.html">Step-by-step Examples</a>.
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