Qt 5: QGraphicsItem update() mechanism

[A Former User]

(Qt 5.4 on Win system)

Hello Qt'ies!

I have a QGraphicsScene, which can be zoomed and panned. In the picture attached there's a gray line which is a zoomed-in view of arc (ArcGraphItem is a QGraphicsItem-derived class).
The two yellow lines beside are helper lines painted to demonstrate the clicking/picking area of the arc (implementation in shape() method).

void 
ArcGraphItem::paint(
		QPainter						* painter,
		const QStyleOptionGraphicsItem	* option,
		QWidget							* widget) {
		
	
	qDebug() << "repaint";

	ArcGraphItem::scaleFactor = painter->transform().m11();

	painter->save();
	if (this->hovered) {
		painter->setPen(AbstractGraphItem::hoverPen);
	} else if (this->selected) {
		painter->setPen(AbstractGraphItem::selectPen);
	} else {
		painter->setPen(AbstractGraphItem::unselectPen);
	}

	/* draw arc by start/end point incl. re-connection to end point because of glitchy 1/16 integer angles */
	QPainterPath path(this->startPoint);
	path.arcTo(this->rectangle, this->startAngle, this->spanAngle);
	path.lineTo(this->endPoint);
	painter->drawPath(path);
	

  #if SHOW_PICK_AREA == 1
    /* yellow outline of the picking area */
	painter->setPen(AbstractGraphItem::pickAreaPen);
	painter->drawRect(this->boundingBox);
	painter->drawPath(this->shape());
  #endif

	painter->restore();
}

When clicking the arc, I call a selection method, which calls update() method of the AbstractGraphItem:

void
ArcGraphItem::setSelected(
		bool							selected) {

	if (this->selected != selected) {
		this->selected = selected;

		qDebug() << (selected ? "" : "not") << "select";
		this->update();
	}
}

The debug output clearly show that setSelected() is called and toggles the selection status, which is interpreted in paint() method. Unfortunately, the update() does not trigger a call to paint(), if only this single AbstractGraphItem is visible at the moment. If I a add another QGraphicsItem to the scene ("small point"), then the paint() method of the clicked arc is triggered.

According to a thread on StackOverflow [http://stackoverflow.com/questions/22900106/qt-update-doesnt-work#] I also have to call the update method of the scene. Do I have to?

Thank you for reading,
Riff Raff

Btw: shape() and boundingRect() methods are impl'd for ArcGraphItem.