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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: Using SAX2 features with the Qt XML classes</h1>
<p>
<p> This document assumes that you are familiar with <a href="xml.html#namespaces">namespaces</a> in XML and the concept of a <a href="xml.html#sax2">SAX2
parser</a>.
If features of SAX2 readers are new to you please read
<a href="xml.html#sax2Features">the feature section</a> of the SAX2 document.
<p> As a novice to the Qt XML classes it is advisable to have a look at the
<a href="xml-sax-walkthrough.html">tiny SAX2 parser walkthrough</a> before
reading on.
<p> This walkthrough covers two topics: First of all it shows how to
set SAX2 features and secondly how to integrate the Qt XML functionality
into a Qt GUI application.
<p> The resulting application allows you to compare the output of the reader
depending on how the two features
<em>http://xml.org/sax/features/namespace-prefixes</em>
and <em>http://xml.org/sax/features/namespaces</em> are set.
To do this it shows tree views of the read XML file
listing the qualified names of elements and attributes and the respective
namespace URIs.
<p> <h3>Setting features</h3>
<p>
<p> Let's begin with the main program of the application. First the boring
part: we include all the classes we need:
<p> <pre> #include "structureparser.h"
#include <<a href="qapplication-h.html">qapplication.h</a>>
#include <<a href="qfile-h.html">qfile.h</a>>
#include <<a href="qxml-h.html">qxml.h</a>>
#include <<a href="qlistview-h.html">qlistview.h</a>>
#include <<a href="qgrid-h.html">qgrid.h</a>>
#include <<a href="qmainwindow-h.html">qmainwindow.h</a>>
#include <<a href="qlabel-h.html">qlabel.h</a>>
</pre>
<p> <a href="#structureparser.h">structureparser.h</a> contains the API of
the XML parser that we implement in <a href="#structureparser.cpp">structureparser.cpp.</a>
<p> <pre> int main( int argc, char **argv )
{
<a href="qapplication.html">QApplication</a> app( argc, argv );
</pre>
<p> As usual we then create a Qt application object and hand command line arguments
over to it.
<p> <pre> <a href="qfile.html">QFile</a> xmlFile( argc == 2 ? argv[1] : "fnord.xml" );
</pre>
<p> If the user runs the program with one filename as
an argument we process this file, otherwise we use the <em>fnord.xml</em> file from
the example directory for demonstration purposes.
<p> <pre> <a href="qxmlinputsource.html">QXmlInputSource</a> source( &xmlFile );
</pre>
<p> We use <em>xmlFile</em> as the XML Input Source...
<p> <pre> <a href="qxmlsimplereader.html">QXmlSimpleReader</a> reader;
</pre>
<p> ... and instantiate a <em>reader</em> object. Later we will manipulate its features
and thus influence how the XML data are read.
<p> <pre> <a href="qgrid.html">QGrid</a> * container = new <a href="qgrid.html">QGrid</a>( 3 );
</pre>
<p> Now let's think about presenting the output: As described in the
<a href="xml.html#sax2Features">Qt SAX2 documentation</a>
there are three valid combinations of <em>http://xml.org/sax/features/namespace-prefixes</em>
and <em>http://xml.org/sax/features/namespaces</em>: TRUE/TRUE, TRUE/FALSE and
FALSE/TRUE. To show the relevant output side by side of each other
and mark them with three labels makes up for a grid layout consisting
of three columns (and thus two lines).
<p> <pre> <a href="qlistview.html">QListView</a> * nameSpace = new <a href="qlistview.html">QListView</a>( container, "table_namespace" );
</pre>
<p> The most natural way of presenting XML elements is in a tree.
Thus we use a listview. Its name <em>nameSpace</em> indicates that this
one will be used to present the combination of <em>http://xml.org/sax/features/namespaces</em> being TRUE and
<em>http://xml.org/sax/features/namespace-prefixes</em>
being FALSE -- the default configuration of a <a href="qxmlsimplereader.html">QXmlSimpleReader</a>.
<p> Being the first grid entry the <em>nameSpace</em> listview will
appear in the upper left corner of the virtual grid.
<p> <pre> StructureParser * handler = new StructureParser( nameSpace );
</pre>
<p> Then we create a handler that deals with the XML data read by the reader.
As the provided handler class <a href="qxmldefaulthandler.html">QXmlDefaultHandler</a> simply does nothing
with the data from the reader,
we can't use it right away. Instead we have to subclass our
own <a href="#structureparser.cpp">StructureParser</a> from it.
<p> <pre> reader.<a href="qxmlreader.html#setContentHandler">setContentHandler</a>( handler );
</pre>
<p> The <em>handler</em> serves as content handler for the reader. Note that
for simplicity reasons we don't register e.g. an error handler. Thus
our program will not complain about for example missing closing tags
in the parsed XML document.
<p> <pre> reader.<a href="qxmlsimplereader.html#parse">parse</a>( source );
</pre>
<p> Finally we parse the document with the reader's default feature settings.
<p> <pre> <a href="qlistview.html">QListView</a> * namespacePrefix = new <a href="qlistview.html">QListView</a>( container,
"table_namespace_prefix" );
</pre>
<p> Now we prepare for the parsing of the same XML input source with
different reader settings. The output will be presented in
a second <a href="qlistview.html">QListView</a>, <em>namespacePrefix</em>. As it is the second
member of the <em>container</em> grid it will appear in the middle of
the upper grid row.
<p> <pre> handler->setListView( namespacePrefix );
</pre>
<p> Then we ask the <em>handler</em> to present the data in the <em>namespacePrefix</em>
listview.
<p> <pre> <a name="x2125"></a> reader.<a href="qxmlsimplereader.html#setFeature">setFeature</a>( "http://xml.org/sax/features/namespace-prefixes",
TRUE );
</pre>
<p> Now we modify the behaviour of the <em>reader</em> and change
<em>http://xml.org/sax/features/namespace-prefixes</em> from the default FALSE
to TRUE. The <em>http://xml.org/sax/features/namespaces</em> feature has
still its default setting TRUE.
<p> <pre> source.<a href="qxmlinputsource.html#reset">reset</a>();
</pre>
<p> We have to reset the input source to make the new parsing start from the
beginning of the document again.
<p> <pre> reader.<a href="qxmlsimplereader.html#parse">parse</a>( source );
</pre>
<p> Finally we parse the XML file a second time with the changed reader
settings (TRUE/TRUE).
<p> <pre> <a href="qlistview.html">QListView</a> * prefix = new <a href="qlistview.html">QListView</a>( container, "table_prefix");
handler->setListView( prefix );
reader.<a href="qxmlsimplereader.html#setFeature">setFeature</a>( "http://xml.org/sax/features/namespaces", FALSE );
source.<a href="qxmlinputsource.html#reset">reset</a>();
reader.<a href="qxmlsimplereader.html#parse">parse</a>( source );
</pre>
<p> Next we prepare and use the upper right listview to show the reader results
with the feature setting <em>http://xml.org/sax/features/namespaces</em>
FALSE and <em>http://xml.org/sax/features/namespace-prefixes</em> TRUE.
<p> <pre> // namespace label
(void) new <a href="qlabel.html">QLabel</a>(
"Default:\n"
"http://xml.org/sax/features/namespaces: TRUE\n"
"http://xml.org/sax/features/namespace-prefixes: FALSE\n",
container );
// namespace prefix label
(void) new <a href="qlabel.html">QLabel</a>(
"\n"
"http://xml.org/sax/features/namespaces: TRUE\n"
"http://xml.org/sax/features/namespace-prefixes: TRUE\n",
container );
// prefix label
(void) new <a href="qlabel.html">QLabel</a>(
"\n"
"http://xml.org/sax/features/namespaces: FALSE\n"
"http://xml.org/sax/features/namespace-prefixes: TRUE\n",
container );
</pre>
<p> The second row of the <em>container</em> grid is filled with three labels
denoting the reader settings that belong to the above listview.
<p> <pre> app.<a href="qapplication.html#setMainWidget">setMainWidget</a>( container );
container-><a href="qwidget.html#show">show</a>();
return app.<a href="qapplication.html#exec">exec</a>();
}
</pre>
<p> Same procedure as with every Qt GUI program: the grid serves as the
main widget of our application and is shown. After that we enter
the GUI's event loop.
<p> <h3><a name="structureparser.h">The handler API</a></h3>
<p> Let's have a brief look at the API of our handler class
<em>StructureParser</em>:
<p>
<pre> #include <<a href="qxml-h.html">qxml.h</a>>
#include <<a href="qptrstack-h.html">qptrstack.h</a>>
class QListView;
class QListViewItem;
class QString;
</pre>
<p> <pre> class StructureParser: public <a href="qxmldefaulthandler.html">QXmlDefaultHandler</a>
{
</pre>
<p> We derive it from the <a href="qxmldefaulthandler.html">QXmlDefaultHandler</a> class that
implements a handler that simply does nothing.
<p> <pre> public:
StructureParser( <a href="qlistview.html">QListView</a> * );
</pre>
<p> This makes it easy for us to implement only the functionality
we in fact need. In our case this is the constructor that
takes a <a href="qlistview.html">QListView</a> as an argument,
<p> <pre> 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>& );
</pre>
<p> the function to execute at the occurrence of element start tags
(inherited from <a href="qxmlcontenthandler.html">QXmlContentHandler</a>), and
<p> <pre> bool endElement( const <a href="qstring.html">QString</a>&, const <a href="qstring.html">QString</a>&, const <a href="qstring.html">QString</a>& );
</pre>
<p> the code to run when an end tag occurs.
<p> All we have to implement so far is content handling.
<p> <pre> void setListView( <a href="qlistview.html">QListView</a> * );
</pre>
<p> In addition we have a function that selects a listview
for the output.
<p> <pre> private:
<a href="qptrstack.html">QPtrStack</a><QListViewItem> stack;
</pre>
<p> Keep in mind that we write a SAX2 parser that doesn't
have an object model to keep all elements and attributes
in memory. To display the elements and attributes in a tree like
structure we must however keep track of all elements
that haven't been closed yet.
<p> To do this we use a LIFO stack
of QListItems. An element will be added to the stack when
its start tag appears and removed
as soon as its end tag is parsed.
<p> <pre> <a href="qlistview.html">QListView</a> * table;
};
</pre>
<p> Apart from this we define a member variable that contains
the currently used listview.
<p> <h3><a name="structureparser.cpp">The handler itself</a></h3>
<p> Now that we defined the API we have to implement the
relevant functions.
<p>
<pre> #include "structureparser.h"
#include <<a href="qstring-h.html">qstring.h</a>>
#include <<a href="qlistview-h.html">qlistview.h</a>>
</pre>
<p> <pre> StructureParser::StructureParser( <a href="qlistview.html">QListView</a> * t )
: <a href="qxmldefaulthandler.html">QXmlDefaultHandler</a>()
{
</pre>
<p> First we have the constructor that takes a listview pointer as
its argument.
<p> <pre> setListView( t );
}
</pre>
<p> All we have to do here is to prepare the argument <a href="qlistview.html">QListView</a>
before usage. This we do with the <a href="#setListView()">setListView()</a> function.
<p> <a name="setListView()"></a>
<pre> void StructureParser::setListView( <a href="qlistview.html">QListView</a> * t )
{
table = t;
</pre>
<p> First we store the argument away.
<p> <pre> table->setSorting( -1 );
</pre>
<p> We want the elements to be listed as they appear in the
document -- and not for example sorted alphabetically. That's
why we switch off sorting at all.
<p> <pre> table->addColumn( "Qualified name" );
table->addColumn( "Namespace" );
}
</pre>
<p> The listview now consists of two columns: one for the
element's or attribute's qualified names and one for
their namespace URIs. Columns are added from left to right
and with the title as an argument.
<p> Now let's deal with XML content handling.
<p> <pre> bool StructureParser::<a href="qxmlcontenthandler.html#startElement">startElement</a>( const <a href="qstring.html">QString</a>& namespaceURI,
const <a href="qstring.html">QString</a>& ,
const <a href="qstring.html">QString</a>& qName,
const <a href="qxmlattributes.html">QXmlAttributes</a>& attributes)
{
</pre>
<p> When we come across the start tag of an element the handler does
the real work. Although <em>startElement</em> is called with four
arguments we keep track of only three: the namespace URI
of the element, its qualified name and its attributes.
If an element has no namespace assigned or if the feature
settings of the reader don't provide the handler with
namespace URIs at all <em>namespaceURI</em> contains an empty
string.
<p> Note that we don't assign a variable to the second argument --
we're simply not interested in the local name of the element.
<p> <pre> <a href="qlistviewitem.html">QListViewItem</a> * element;
</pre>
<p> Whenever an element occurs we want to show it in the listview.
Therefore we define a <a href="qlistviewitem.html">QListViewItem</a> variable.
<p> <pre> if ( ! stack.isEmpty() ){
<a href="qlistviewitem.html">QListViewItem</a> *lastChild = stack.top()->firstChild();
</pre>
<p> As long as the element <em>stack</em> isn't empty the current element
is a child of the topmost (last unclosed) element on the stack. Thus we
create a new <a href="qlistviewitem.html">QListViewItem</a> as a child of QPtrStack::stack.top() with
the new element's qualified name in the first column and the according
namespace URI (or nothing) in the second one.
<p> The <a href="qlistviewitem.html">QListViewItem</a> is usally inserted as the first child. This means that we
would get the elements in reverse order. So we first search for the last
child of the QPtrStack::stack.top() element and insert it after this
element.
<p> In a valid XML document this applies to all elements except
the document root.
<p> <pre> if ( lastChild ) {
while ( lastChild-><a href="qlistviewitem.html#nextSibling">nextSibling</a>() )
lastChild = lastChild-><a href="qlistviewitem.html#nextSibling">nextSibling</a>();
}
element = new <a href="qlistviewitem.html">QListViewItem</a>( stack.top(), lastChild, qName, namespaceURI );
} else {
element = new <a href="qlistviewitem.html">QListViewItem</a>( table, qName, namespaceURI );
}
</pre>
<p> The root element we have to handle separately because it is
the first element to go onto the <a href="qlistviewitem.html">QListViewItem</a> stack.
Its listview item is therefore a direct child of the
<em>table</em> listview itself.
<p> <pre> stack.push( element );
</pre>
<p> Now we put the element's listview item on top of the stack.
<p> <pre> element-><a href="qlistviewitem.html#setOpen">setOpen</a>( TRUE );
</pre>
<p> By default a <a href="qlistview.html">QListView</a> presents all of its nodes closed.
The user may then click on the <em>+</em> icon to see the child
entries.
<p> We however want to see the entire element tree
at once when we run the program.
Therefore we open each listview item manually.
<p> <pre> if ( attributes.<a href="qxmlattributes.html#length">length</a>() > 0 ) {
</pre>
<p> What do we do if an element has attributes?
<p> <pre> <a name="x2105"></a> for ( int i = 0 ; i < attributes.<a href="qxmlattributes.html#length">length</a>(); i++ ) {
<a name="x2107"></a><a name="x2106"></a> new <a href="qlistviewitem.html">QListViewItem</a>( element, attributes.<a href="qxmlattributes.html#qName">qName</a>(i), attributes.<a href="qxmlattributes.html#uri">uri</a>(i) );
}
}
</pre>
<p> For each of them we create a new listview item to present the attribute's
qualified name and the relevant namespace URI (or nothing).
Obviously <em>attribute</em> is a child of
the current <em>element</em>.
<p> <pre> return TRUE;
}
</pre>
<p> To prevent the reader from throwing an error we have to
return TRUE when we successfully dealt with an
element's start tag.
<p> <pre> 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>& )
{
stack.pop();
</pre>
<p> Whenever we come across an element's closing tag we
have to remove its listview item from the stack as
it can't have children any longer.
<p> <pre> return TRUE;
}
</pre>
<p> And so we're done.
<p> <p>See also <a href="step-by-step-examples.html">Step-by-step Examples</a>.
<!-- eof -->
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<td>Copyright © 2007
<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
<td align=right><div align=right>Qt 3.3.8</div>
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