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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><!-- /home/reggie/tmp/qt-3.0-reggie-5401/qt-x11-commercial-3.0.5/doc/xml-sax-walkthrough.doc:36 --><html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Walkthrough: How to use the Qt SAX2 classes</title><style type="text/css"><!--h3.fn,span.fn { margin-left: 1cm; text-indent: -1cm; }a:link { color: #004faf; text-decoration: none }a:visited { color: #672967; text-decoration: none }body { background: #ffffff; color: black; }--></style></head><body><table border="0" cellpadding="0" cellspacing="0" width="100%"><tr bgcolor="#E5E5E5"><td valign=center> <a href="index.html"><font color="#004faf">Home</font></a> | <a href="classes.html"><font color="#004faf">All Classes</font></a> | <a href="mainclasses.html"><font color="#004faf">Main Classes</font></a> | <a href="annotated.html"><font color="#004faf">Annotated</font></a> | <a href="groups.html"><font color="#004faf">Grouped Classes</font></a> | <a href="functions.html"><font color="#004faf">Functions</font></a></td><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 tothe 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 outputsthe 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 areinterested 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> (rememberthat the special handler classes are all abstract and the default handler classprovides an implementation that does not change the parsing behavior):<p> <pre>/****************************************************************************** $Id: qt/structureparser.h 3.0.5 edited Oct 12 2001 $**** Copyright (C) 1992-2000 Trolltech AS. 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 toget 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 wewant 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 comesaccross a start tag of an element and subsequently print it out, wehave 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 withpreceding 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 tagit 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 withouterrors.<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( 0, 4 ); return TRUE; }</pre><p> Obviously we then should shorten the <em>indent</em> string by the fourwhitespaces 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>></pre><p> <pre> #include <<a href="qwindowdefs-h.html">qwindowdefs.h</a>> int main( int argc, char **argv ) { if ( argc < 2 ) { fprintf( stderr, "Usage: %s <xmlfile>\n", argv[0] ); return 1; } for ( int i=1; i < argc; i++ ) {</pre><p> Successively we deal with all files given as command line arguments.<p> <pre> StructureParser handler;</pre><p> The next step is to create an instance of the <em>StructureParser</em>. <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 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> <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/> <orang-utan/> </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. inserta whitespace between a < and the element name in your test-XML file.To prevent such annoyancesyou should always install an error handler with <a href="qxmlreader.html#setErrorHandler">QXmlReader::setErrorHandler</a>(). This allows you to reportparsing errors to the user.<p> <p>See also <a href="step-by-step-examples.html">Step-by-step Examples</a>.<!-- eof --><p><address><hr><div align=center><table width=100% cellspacing=0 border=0><tr><td>Copyright © 2002 <a href="http://www.trolltech.com">Trolltech</a><td><a href="http://www.trolltech.com/trademarks.html">Trademarks</a><td align=right><div align=right>Qt version 3.0.5</div></table></div></address></body></html>?? 快捷鍵說明
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