100% CPU usage when overriding QGraphicsLineItem::

Question

I have a class inheriting from QGraphicsLineItem and as soon as I override the paint method, it looks like Qt starts painting it at every "main loop", instead of drawing according to some events (like moving the item, etc).

Does anyone know more about the good practices when inheriting from a QGraphicsItem ? I look at other projets' code and it looks like it does not come from my paint method. I was thinking that maybe I'm doing something wrong in the paint method that changes the items states to "to be painted again", and so Qt paints it again. I joined the method code in case of. The method paints an arrow.

void Message::paint(QPainter* painter, const QStyleOptionGraphicsItem* option, QWidget* widget)
{
  QLineF line = this->line();
  Instance* from = dynamic_cast<Instance*> (this->from_get());
  Instance* to = dynamic_cast<Instance*> (this->to_get());

  QPointF from_pt(from->x() + from_pos_.x(), from->y() + from_pos_.y());
  line.setP1(from_pt);
  this->setLine(line);

  QPointF to_pt(to->x() + to_pos_.x(), to->y() + to_pos_.y());
  line.setP2(to_pt);
  this->setLine(line);

  textItem_->setPos(this->boundingRect().center().x() - textItem_->boundingRect().width() / 2,
                    this->boundingRect().center().y() - textItem_->boundingRect().height() / 2);
  rectItem_->setRect(textItem_->x(), textItem_->y(), textItem_->boundingRect().width(), textItem_->boundingRect().height());

  if (this->line().dy() >= 0)
  {
    int arrowSize = 14;
    double angle = ::acos(this->line().dx() / this->line().length());
    QPointF arrowP1;
    QPointF arrowP2;
    QPolygonF p;

    angle = (Pi * 2) - angle;
    arrowP1 = this->line().p2() - QPointF(sin(angle + Pi / 3) * arrowSize, cos(angle + Pi / 3) * arrowSize);
    arrowP2 = this->line().p2() - QPointF(sin(angle + Pi - Pi / 3) * arrowSize, cos(angle + Pi - Pi / 3) * arrowSize);
    p << this->line().p2() << arrowP1 << arrowP2;
    extremity_->setPolygon(p);
    extremity_->update(extremity_->boundingRect());
  }

  extremity_->paint(painter, option, widget);

  QGraphicsLineItem::paint(painter, option, widget);
}

Thank you for your help !

Answer 1

You probably shouldn't call methods like setPos, setRect, setPolygon or update inside your paint() method. These methods are likely to schedule a new paint event which will lead to infinite recursion.