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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 >
< p >
< p > For a general discussion of the XML topics in TQt 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 TQt 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 = "tqxmlcontenthandler.html#startDocument" > TQXmlContentHandler::startDocument< / a > (),
< a href = "tqxmlcontenthandler.html#startElement" > TQXmlContentHandler::startElement< / a > () and
< a href = "tqxmlcontenthandler.html#endElement" > TQXmlContentHandler::endElement< / a > ().
< p > For this purpose we use a subclass of the < a href = "tqxmldefaulthandler.html" > TQXmlDefaultHandler< / 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 TQt. This example
** program may be used, distributed and modified without limitation.
**
*****************************************************************************/
#ifndef STRUCTUREPARSER_H
#define STRUCTUREPARSER_H
#include < < a href = "tqxml-h.html" > tqxml.h< / a > >
class TQString;
class StructureParser : public < a href = "tqxmldefaulthandler.html" > TQXmlDefaultHandler< / a >
{
public:
bool startDocument();
bool startElement( const < a href = "tqstring.html" > TQString< / a > & , const < a href = "tqstring.html" > TQString< / a > & , const < a href = "tqstring.html" > TQString< / a > & ,
const < a href = "tqxmlattributes.html" > TQXmlAttributes< / a > & );
bool endElement( const < a href = "tqstring.html" > TQString< / a > & , const < a href = "tqstring.html" > TQString< / a > & , const < a href = "tqstring.html" > TQString< / a > & );
private:
< a href = "tqstring.html" > TQString< / 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 = "tqstring-h.html" > tqstring.h< / a > >
< / pre >
< p > First we overload < a href = "tqxmlcontenthandler.html#startDocument" > TQXmlContentHandler::startDocument< / a > () with a non-empty version.
< p > < pre > < a name = "x2137" > < / a > bool StructureParser::< a href = "tqxmlcontenthandler.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 = "tqxmlcontenthandler.html#startElement" > TQXmlContentHandler::startElement< / a > ().
< 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 > & ,
const < a href = "tqstring.html" > TQString< / a > & qName,
const < a href = "tqxmlattributes.html" > TQXmlAttributes< / 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 = "tqxmlcontenthandler.html#endElement" > TQXmlContentHandler::endElement< / a > ().
< 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 > & )
{
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 = "tqfile-h.html" > tqfile.h< / a > >
#include < < a href = "tqxml-h.html" > tqxml.h< / a > >
#include < < a href = "tqwindowdefs-h.html" > tqwindowdefs.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 = "tqxmlsimplereader.html" > TQXmlSimpleReader< / a > reader;
< a name = "x2140" > < / a > reader.< a href = "tqxmlreader.html#setContentHandler" > setContentHandler< / a > ( & handler );
< / pre >
< p > After that we set up the reader. As our < em > StructureParser< / em >
class deals with < a href = "tqxmlcontenthandler.html" > TQXmlContentHandler< / 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 = "tqfile.html" > TQFile< / a > xmlFile( argv[i] );
< a href = "tqxmlinputsource.html" > TQXmlInputSource< / a > source( & xmlFile );
< / pre >
< p > Then we create a
< a href = "tqxmlinputsource.html" > TQXmlInputSource< / a > for the XML file to be parsed.
< p > < pre > < a name = "x2139" > < / a > reader.< a href = "tqxmlsimplereader.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 = "tqxmlreader.html#setErrorHandler" > TQXmlReader::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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