#include <QtCore/QtCore>
#include <QtGui/QtGui>
#include <QtWidgets/QtWidgets>


class ChildWidget : public QWidget
{

public:

	ChildWidget( QWidget* parent=0 ) : QWidget( parent ) {}

	~ChildWidget() {}

protected:

	void paintEvent( QPaintEvent* event )
	{
		QPainter painter( this );
		for( const QRect& rect : event->region().rects() )
		{
			qDebug() << rect;
			painter.fillRect( rect, Qt::red );
		}
	}

};


int main( int argc, char* argv[] )
{
	QApplication app( argc, argv );
	QWidget mainWidget;
	ChildWidget* cw = new ChildWidget( &mainWidget );
	cw->setAttribute( Qt::WA_StaticContents );
	cw->setAttribute( Qt::WA_OpaquePaintEvent );
	cw->resize( 200, 200 );

	QPushButton button;
	button.setText( "Click Me" );
	QObject::connect( &button, &QPushButton::clicked,
					  [ &mainWidget ]() { mainWidget.resize( 250, 250 ); } );

	mainWidget.resize( 150, 150 );
	button.resize( 150, 40 );

	mainWidget.show();
	button.show();
	button.raise();

	return app.exec();
}