Warning QPainter inside paintEvent

Question

I tried to draw some more information in QChartView so I re-implement paintEvent

  virtual void paintEvent(QPaintEvent *event) {
    QChartView::paintEvent(event);
    OmenChart *mchr = dynamic_cast<OmenChart *>(this->chart());
    if(mchr == nullptr)
        return;
    QPainter painter(this);

    const int labelOffset = 2 + 2;
    painter.setFont(this->font());
    painter.setPen(QPen(Qt::black));
    QFontMetrics fm(painter.font());


    const OmenScatterSeries *omnSr = mchr->serie();
    QVector<QPointF> points = omnSr->pointsVector();
    QStringList labels = omnSr->pointLabels();

    for (int i(0); i < labels.count(); i++) {
        QString pointLabel = labels[i];

        // Position text in relation to the point
        int pointLabelWidth = fm.width(pointLabel);
        QPointF position(points.at(i));
        position.setX(position.x() - pointLabelWidth / 2);
        position.setY(position.y() - labelOffset);
        painter.drawText(position, pointLabel);
    }
}

and I am taking these errors

QWidget::paintEngine: Should no longer be called
QPainter::begin: Paint device returned engine == 0, type: 1
QPainter::setFont: Painter not active
QPainter::setPen: Painter not active
QPainter::font: Painter not active
QWidget::paintEngine: Should no longer be called

Any ideas on this ? I had also used painter.begin and end but I took the same errors

Answer 1

Do not draw on your subclass of QChartView, but instead on its viewport.

QChartView is derived from QGraphicsView, which in turn is derived from QAbstractScrollArea and according to this answer (as well as the cited there documentation) you should use the viewport as a paint device for your QPainter, not the widget itself.

So, instead of

QPainter painter(this);

write

QPainter painter(viewport());

The same could be seen in the source of QGraphicsView, i.e.:

// Set up the painter
QPainter painter(viewport());