Question

A style in Qt implements the look and feel found in GUIs on different
platforms. For instance the Windows style used in Windows and the
Motif style that are common on many Unix platforms.

This is a short guide that describes the steps that are necessary to
get started creating and using custom styles with the style API in Qt
3.x. First, we go through the steps necessary to create a style: 1)
picking a base style to inherit from and 2) re-implementing the
necessary functions in the derived class. Then we show how to use the
new style from within your own applications, or as a plugin together
with existing Qt applications.

Creating a custom style

1. Pick a base style to inherit from.

The first step is to pick one of the base styles provided with Qt to
build your custom style on. Which of the available styles to start
from does of course depend on what look & feel you want. Basically you
should choose from the QWindowsStyle derived classes or the
QMotifStyle derived classes. These are the two base look & feel
classes in the Qt style engine.
Inheriting directly from QCommonStyle is also an option if you want to
start almost from scratch when implementing your style. In this simple
example we will inherit from QWindowsStyle.

2. Re-implement the necessary functions in your derived class.

Depending on which parts of the base style you want to change, you
have to re-implement the functions that are used to draw those parts
of the interface. If you take a look at the QStyle documentation,
you will find a list of the different primitives, controls and complex
controls. You will also find an illustration that shows where the
different primitives, controls and complex controls are used. In this
example we will first change the look of the standard arrows that are
used in the QWindowsStyle. The arrows are PrimitiveElements that are
drawn in the drawPrimitive() function, therefore we need to
re-implement that function. We get the following class declaration:

#include <qwindowsstyle.h>

class CustomStyle : public QWindowsStyle {
    Q_OBJECT
public:
    CustomStyle();
    ~CustomStyle();

    void drawPrimitive( PrimitiveElement pe,
                        QPainter *p,
                        const QRect & r,
                        const QColorGroup & cg,
                        SFlags flags = Style_Default,
                        const QStyleOption & = QStyleOption::Default ) const;

private:
    // Disabled copy constructor and operator=
    CustomStyle( const CustomStyle & );
    CustomStyle& operator=( const CustomStyle & );
};

Note that we disable the copy constructor and the '=' operator for our
style. QObject is the base class for all style classes in Qt, and a
QObject inherently cannot be copied; there are some aspects of it that
are not copyable.

From the QStyle docs we see that PE_ArrowUp, PE_ArrowDown,
PE_ArrowLeft and PE_ArrowRight are the primitives we need to do
something with. We get the following in our drawPrimitive() function:

CustomStyle::CustomStyle()
{
}

CustomStyle::~CustomStyle()
{
}

void CustomStyle::drawPrimitive( PrimitiveElement pe,
                                 QPainter * p,
                                 const QRect & r,
                                 const QColorGroup & cg,
                                 SFlags flags,
                                 const QStyleOption & opt ) const
{
    // we are only interested in the arrows
    if (pe >= PE_ArrowUp && pe <= PE_ArrowLeft) {
        QPointArray pa( 3 );
        // make the arrow cover half the area it is supposed to be 
        // painted on
        int x = r.x();
        int y = r.y();
        int w = r.width() / 2;
        int h = r.height() / 2;
        x += (r.width() - w) / 2;
        y += (r.height() - h) /2;

        switch( pe ) {
        case PE_ArrowDown:
            pa.setPoint( 0, x, y );
            pa.setPoint( 1, x + w, y );
            pa.setPoint( 2, x + w / 2, y + h );
            break;
        case PE_ArrowUp:
            pa.setPoint( 0, x, y + h );
            pa.setPoint( 1, x + w, y + h );
            pa.setPoint( 2, x + w / 2, y );
            break;
        case PE_ArrowLeft:
            pa.setPoint( 0, x + w, y );
            pa.setPoint( 1, x + w, y + h );
            pa.setPoint( 2, x, y + h / 2 );
            break;
        case PE_ArrowRight:
            pa.setPoint( 0, x, y );
            pa.setPoint( 1, x, y + h );
            pa.setPoint( 2, x + w, y + h / 2 );
            break;
        default: break;
            
        }

        // use different colors to indicate that the arrow is 
        // enabled/disabled
        if ( flags & Style_Enabled ) {
            p->setPen( cg.mid() );
            p->setBrush( cg.brush( QColorGroup::ButtonText ) );
        } else {
            p->setPen( cg.buttonText() );
            p->setBrush( cg.brush( QColorGroup::Mid ) );
        }
        p->drawPolygon( pa );
    } else {
        // let the base style handle the other primitives
        QWindowsStyle::drawPrimitive( pe, p, r, cg, flags, data );
    }
}

Using a custom style

There are several ways of using a custom style in a Qt
application. The easiest and most simple way is to include the following
lines of code in the application's main() function:

#include "customstyle.h"

int main( int argc, char ** argv )
{
    QApplication::setStyle( new CustomStyle() );
    // do the usual routine on creating your QApplication object etc.
}

Note that you also have to include the customstyle.h and
customstyle.cpp files in your project.

2. Creating and using a pluggable style

You may want to use your custom style in a Qt application that you
don't want to, or have the opportunity to recompile. The Qt Plugin
system makes it possible to create styles as plugins. Styles created
as plugins are loaded as shared objects at runtime by Qt itself.
Please refer to the Qt Plugin documentation
for more information on how to go about creating a style plugin.

Compile your plugin and put it into $QTDIR/plugins/styles. We now have
a pluggable style that Qt can load automatically. To use your new
style with existing applications, simply start the application with
the following argument:

./application -style custom

The application should appear with the look & feel from the custom
style you implemented.