Drawing an overlay on top of an application's window - qt

Question

I want to be able to paint on top of my application's window so that I can annotate all the widgets with some extra diagnostic information, similar to the CSS developer tools in Firefox (eg add widget classes, styles, highlight borders etc).
I can walk the widget tree and extract the relevant information, but the question is how can I overlay all the application windows with this information?
One way would be to override my QMainWindow's paint event, but this has to be done for all top level windows. Is there an alternative method where you can paint on the QDesktopWidget for instance? Or any hooks into each QWidget's paint method? Anything that involves subclassing QWidget itself won't work with the standard widgets.
This follows on from my previous question:
Are there any useful tools for diagnosing Qt layout and spacing problems?
cheers
Mandrill
EDIT:
Thanks to Dmitry I've now got a really simple method that is easily extensible:
class DiagnosticStyle : public QWindowsVistaStyle
{
Q_OBJECT
public:
typedef QWindowsVistaStyle BaseStyle;
void drawControl(ControlElement element, const QStyleOption* option, QPainter* painter, const QWidget* widget) const;
};
void DiagnosticStyle::drawControl(ControlElement element, const QStyleOption* option, QPainter* painter, const QWidget* widget) const
{
BaseStyle::drawControl(element, option, painter, widget);
if (widget && painter) {
// draw a border around the widget
painter->setPen(QColor("red"));
painter->drawRect(widget->rect());
// show the classname of the widget
QBrush translucentBrush(QColor(255,246,240, 100));
painter->fillRect(widget->rect(), translucentBrush);
painter->setPen(QColor("darkblue"));
painter->drawText(widget->rect(), Qt::AlignLeft | Qt::AlignVCenter, widget->metaObject()->className());
}
}
qApp->setStyle(new DiagnosticStyle());

Answer 1

You can create own style class based on QMotifStyle or other ... and paint on any widget/control related to him information.
void MyStyle::drawPrimitive(PrimitiveElement element, const QStyleOption *option,QPainter *painter, const QWidget *widget) const
{
QStyle::State flags = option->state;
QRect rect = option->rect;
QPalette pal = option->palette;
QBrush brush;
switch (element)
{
case PE_FrameTabWidget:
{
painter->save();
// for example: draw anything on TabWidget
painter->drawPixmap(rect,centerPm,centerPm.rect());
painter->restore();
}
break;
default:
QMotifStyle::drawPrimitive(element, option, painter, widget);
break;
}
}

Answer 2

Somewhere in Qt5 the styles (GTK, Windows, etc) were made internal. Now you need to use QCommonStyle.
If anyone's wondering how to do this with Qt5+. Here's a self-contained version of #the_mandrill's code above.
class DiagnosticStyle : public QCommonStyle
{
Q_OBJECT
public:
typedef QStyle BaseStyle;
void drawControl(ControlElement element, const QStyleOption* option, QPainter* painter, const QWidget* widget) const
{
QCommonStyle::drawControl(element, option, painter, widget);
if (widget && painter) {
// draw a border around the widget
painter->setPen(QColor("red"));
painter->drawRect(widget->rect());
// show the classname of the widget
QBrush translucentBrush(QColor(255,246,240, 100));
painter->fillRect(widget->rect(), translucentBrush);
painter->setPen(QColor("darkblue"));
painter->drawText(widget->rect(), Qt::AlignLeft | Qt::AlignVCenter, widget->metaObject()->className());
}
};
};
Then, in your main window constructor call
qApp->setStyle(new DiagnosticStyle());