Dark transparent layer over a QMainWindow in Qt

Question

I need to implement a \"Loading...\" window in my application but I do prefer to cover the whole QMainWindow with a dark transparent layer with a text above. Does anybody kn

Answer 1

I would suggest execute a modal, frameless dialog on top and add a graphics effect on the background widget. This is IMHO a very flexible and short solution without touching the event system directly.

The performance might be bad - one could improve that by calling drawSource(), but I haven't a reliable solution here yet.

class DarkenEffect : public QGraphicsEffect
{
public:
    void draw( QPainter* painter ) override
    {
        QPixmap pixmap;
        QPoint offset;
        if( sourceIsPixmap() ) // No point in drawing in device coordinates (pixmap will be scaled anyways)
            pixmap = sourcePixmap( Qt::LogicalCoordinates, &offset );
        else // Draw pixmap in device coordinates to avoid pixmap scaling;
        {
            pixmap = sourcePixmap( Qt::DeviceCoordinates, &offset ); 
            painter->setWorldTransform( QTransform() );
        }
        painter->setBrush( QColor( 0, 0, 0, 255 ) ); // black bg
        painter->drawRect( pixmap.rect() );
        painter->setOpacity( 0.5 );
        painter->drawPixmap( offset, pixmap );
    }
};

// prepare overlay widget 
overlayWidget->setWindowFlags( Qt::FramelessWindowHint | Qt::Dialog | Qt::WindowStaysOnTopHint );

// usage
parentWidget->setGraphicsEffect( new DarkenEffect );
overlayWidget->exec();
parentWidget->setGraphicsEffect( nullptr );