Russian Qt Forum

C++ (Qt)
#include <QWidget>
#include <QBackingStore>
#include <QPainter>
#include <QGridLayout>
 
class GridManager : public QWidget
{
	bool in_paint_ = false;
public:
	GridManager( QWidget* parent = nullptr ) : QWidget( parent )
	{
		setLayout( new QGridLayout() );
	}
	void addWidget( QWidget* w, int row, int column )
	{
		qobject_cast< QGridLayout* >( layout() )->addWidget( w, row, column );
	}
 
protected:
	bool event( QEvent* e )
	{
		if( e->type() == QEvent::ChildAdded )
			static_cast< QChildEvent* >( e )->child()->installEventFilter( this );
	}
	bool eventFilter( QObject* o, QEvent* e )
	{
		if( e->type() == QEvent::Paint )
		{
			if( !in_paint_ )
			{
				drawChild( qobject_cast< QWidget* >( o ) );
				return true;
			}
		}
		return QWidget::eventFilter( o, e );
	}
	void drawChild( QWidget* w )
	{
		w->setAttribute( Qt::WA_WState_InPaintEvent, false );
		auto bs = window()->backingStore();
		auto r = w->rect().translated( - w->mapFrom( w->window(), QPoint( 0, 0 ) ) );
		bs->beginPaint( QRegion( r ) );
		QPainter p( bs->paintDevice() );
		p.translate( r.topLeft() );
		in_paint_ = true;
		w->render( &p );
		in_paint_ = false;
		drawOverChild( &p, w );
		bs->endPaint();
	}
	virtual void drawOverChild( QPainter* p, QWidget* w )
	{
		p->setBrush( QColor( 0xff, 0xff, 0xff, 200 ) );
		p->drawRect( w->rect() );
	}
};
 

C++ (Qt)
class Dummy : public QWidget {
public:
 using QWidget::paintEvent;
};
 
bool eventFilter( QObject* o, QEvent* e )
{
	if (e->type() == QEvent::Paint) {
		Dummy * w = static_cast<Dummy*> (o);
		w->paintEvent(static_cast<QPaintEvent *> (e));
		QPainter painter(w);
		drawOverChild(&painter, w);
		return true;
	}
	return QWidget::eventFilter( o, e );
}