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9.5 KiB
221 lines
9.5 KiB
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<title>Walkthrough: How to use the TQt SAX2 classes</title>
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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 TQt SAX2 classes</h1>
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<p>
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<p> For a general discussion of the XML topics in TQt please refer to
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the document <a href="xml.html">XML Module.</a>
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To learn more about SAX2 see the document describing
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<a href="xml.html#sax2">the TQt SAX2 implementation.</a>
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<p> Before reading on you should at least be familiar with
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the <a href="xml.html#sax2Intro">Introduction to SAX2.</a>
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<p> <a name="quickStart"></a>
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<h2>A tiny parser</h2>
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<p> In this section we will present a small example reader that outputs
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the names of all elements in an XML document on the command line.
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The element names are indented corresponding to their nesting level.
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<p> As mentioned in <a href="xml.html#sax2Intro">Introduction to SAX2</a>
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we have to implement the functions of the handler classes that we are
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interested in. In our case these are only three:
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<a href="tqxmlcontenthandler.html#startDocument">TQXmlContentHandler::startDocument</a>(),
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<a href="tqxmlcontenthandler.html#startElement">TQXmlContentHandler::startElement</a>() and
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<a href="tqxmlcontenthandler.html#endElement">TQXmlContentHandler::endElement</a>().
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<p> For this purpose we use a subclass of the <a href="tqxmldefaulthandler.html">TQXmlDefaultHandler</a> (remember
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that the special handler classes are all abstract and the default handler class
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provides an implementation that does not change the parsing behavior):
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<p> <pre>/****************************************************************************
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** $Id: qt/structureparser.h 3.3.8 edited Jan 11 14:37 $
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**
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** Copyright (C) 1992-2007 Trolltech ASA. All rights reserved.
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**
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** This file is part of an example program for TQt. This example
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** program may be used, distributed and modified without limitation.
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**
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*****************************************************************************/
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#ifndef STRUCTUREPARSER_H
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#define STRUCTUREPARSER_H
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#include <<a href="tqxml-h.html">tqxml.h</a>>
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class TQString;
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class StructureParser : public <a href="tqxmldefaulthandler.html">TQXmlDefaultHandler</a>
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{
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public:
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bool startDocument();
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bool startElement( const <a href="tqstring.html">TQString</a>&, const <a href="tqstring.html">TQString</a>&, const <a href="tqstring.html">TQString</a>& ,
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const <a href="tqxmlattributes.html">TQXmlAttributes</a>& );
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bool endElement( const <a href="tqstring.html">TQString</a>&, const <a href="tqstring.html">TQString</a>&, const <a href="tqstring.html">TQString</a>& );
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private:
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<a href="tqstring.html">TQString</a> indent;
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};
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#endif
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</pre>
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<p> Apart from the private helper variable <em>indent</em> that we will use to
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get indentation right, there is nothing special about our new
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<em>StructureParser</em> class.
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<p>
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<p> Even the implementation is straight-forward:
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<p> <pre> #include "structureparser.h"
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#include <stdio.h>
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#include <<a href="tqstring-h.html">tqstring.h</a>>
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</pre>
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<p> First we overload <a href="tqxmlcontenthandler.html#startDocument">TQXmlContentHandler::startDocument</a>() with a non-empty version.
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<p> <pre> <a name="x2137"></a>bool StructureParser::<a href="tqxmlcontenthandler.html#startDocument">startDocument</a>()
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{
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indent = "";
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return TRUE;
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}
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</pre>
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<p> At the beginning of the document we simply
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set <em>indent</em> to an empty string because we
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want to print out the root element without any indentation.
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Also we return TRUE so that the parser continues without
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reporting an error.
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<p> Because we want to be informed when the parser comes
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accross a start tag of an element and subsequently print it out, we
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have to overload <a href="tqxmlcontenthandler.html#startElement">TQXmlContentHandler::startElement</a>().
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<p> <pre> <a name="x2138"></a>bool StructureParser::<a href="tqxmlcontenthandler.html#startElement">startElement</a>( const <a href="tqstring.html">TQString</a>&, const <a href="tqstring.html">TQString</a>&,
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const <a href="tqstring.html">TQString</a>& qName,
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const <a href="tqxmlattributes.html">TQXmlAttributes</a>& )
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{
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printf( "%s%s\n", (const char*)indent, (const char*)qName );
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indent += " ";
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return TRUE;
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}
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</pre>
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<p> This is what the implementation does: The name of the element with
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preceding indentation is printed out followed by a linebreak.
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Strictly speaking <em>qName</em> contains the local element name
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without an eventual prefix denoting the <a href="xml.html#namespaces">namespace.</a>
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<p> If another element follows before the current element's end tag
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it should be indented. Therefore we add four spaces to the
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<em>indent</em> string.
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<p> Finally we return TRUE in order to let the parser continue without
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errors.
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<p> The last functionality we need to add is the parser's behaviour when an
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end tag occurs. This means overloading <a href="tqxmlcontenthandler.html#endElement">TQXmlContentHandler::endElement</a>().
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<p> <pre> <a name="x2136"></a>bool StructureParser::<a href="tqxmlcontenthandler.html#endElement">endElement</a>( const <a href="tqstring.html">TQString</a>&, const <a href="tqstring.html">TQString</a>&, const <a href="tqstring.html">TQString</a>& )
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{
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indent.remove( (uint)0, 4 );
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return TRUE;
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}
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</pre>
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<p> Obviously we then should shorten the <em>indent</em> string by the four
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whitespaces added in startElement().
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<p> With this we're done with our parser and can start writing the main()
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program.
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<p>
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<p> <pre> #include "structureparser.h"
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#include <<a href="tqfile-h.html">tqfile.h</a>>
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#include <<a href="tqxml-h.html">tqxml.h</a>>
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#include <<a href="tqwindowdefs-h.html">tqwindowdefs.h</a>>
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int main( int argc, char **argv )
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{
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if ( argc < 2 ) {
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fprintf( stderr, "Usage: %s <xmlfile> [<xmlfile> ...]\n", argv[0] );
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return 1;
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}
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</pre>
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<p> This check ensures that we have a sequence of files from the command
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line to examine.
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<p> <pre> StructureParser handler;
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</pre>
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<p> The next step is to create an instance of the <em>StructureParser</em>.
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<p> <pre> <a href="tqxmlsimplereader.html">TQXmlSimpleReader</a> reader;
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<a name="x2140"></a> reader.<a href="tqxmlreader.html#setContentHandler">setContentHandler</a>( &handler );
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</pre>
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<p> After that we set up the reader. As our <em>StructureParser</em>
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class deals with <a href="tqxmlcontenthandler.html">TQXmlContentHandler</a> functionality only
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we simply register it as the content handler of our choice.
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<p> <pre> for ( int i=1; i < argc; i++ ) {
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</pre>
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<p> Successively we deal with all files given as command line arguments.
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<p> <pre> <a href="tqfile.html">TQFile</a> xmlFile( argv[i] );
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<a href="tqxmlinputsource.html">TQXmlInputSource</a> source( &xmlFile );
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</pre>
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<p> Then we create a
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<a href="tqxmlinputsource.html">TQXmlInputSource</a> for the XML file to be parsed.
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<p> <pre> <a name="x2139"></a> reader.<a href="tqxmlsimplereader.html#parse">parse</a>( source );
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</pre>
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<p> Now we take our input source and start parsing.
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<p> <pre> }
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return 0;
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}
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</pre>
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<p> Running the program on the following XML file...
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<p> <pre><animals>
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<mammals>
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<monkeys> <gorilla/> <orangutan/> </monkeys>
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</mammals>
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<birds> <pigeon/> <penguin/> </birds>
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</animals>
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</pre>
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<p> ... produces the following output:
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<pre>
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animals
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mammals
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monkeys
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gorilla
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orang-utan
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birds
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pigeon
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penguin
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</pre>
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<p> It will however refuse to produce the correct result if you e.g. insert
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a whitespace between a < and the element name in your test-XML file.
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To prevent such annoyances
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you should always install an error handler with <a href="tqxmlreader.html#setErrorHandler">TQXmlReader::setErrorHandler</a>(). This allows you to report
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parsing errors to the user.
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<p> <p>See also <a href="step-by-step-examples.html">Step-by-step Examples</a>.
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<!-- eof -->
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<p><address><hr><div align=center>
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<table width=100% cellspacing=0 border=0><tr>
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<td>Copyright © 2007
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<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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<td align=right><div align=right>TQt 3.3.8</div>
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</table></div></address></body>
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