Weird behaviour when dragging QGraphicsItem

[MatheusIH]

Here's before I drag:

Here's after I drag it:

If I alt tab the blurry shadow line disappears, what I can't figure out is why it appears in the first place.

Here's the code I'm using for my objects:

//rectItem
customRect::customRect(const QRectF &rect) : QGraphicsRectItem(rect){
    setFlag(QGraphicsItem::ItemIsMovable);
    setFlag(QGraphicsItem::ItemSendsScenePositionChanges);
}

void customRect::addLine(myArrow *line) {
    arrows.append(line);
    this->line = line;
}

QVariant customRect::itemChange(GraphicsItemChange change, const QVariant &value)
{
    if (change == ItemPositionChange && scene()) {
        QPointF newPos = value.toPointF();
        moveLineToCenter(newPos);
    }
    return QGraphicsItem::itemChange(change, value);
}

void customRect::moveLineToCenter(QPointF newPos) {
    int xOffset = rect().x();
    int yOffset = rect().y();

    QPointF newCenterPos = QPointF(newPos.x() + xOffset, newPos.y() + yOffset);


    // Move the required point of the line to the center of the elipse
    QPointF p1 = newCenterPos;
    p1.setY( newCenterPos.y() - 200 );
    QPointF p2 = newCenterPos;

    foreach (myArrow *arrow, arrows) {
        p1.setX(p1.x() + 25);
        p2.setX(p2.x() + 25);
        arrow->updatePosition(p1,p2);
        update();
    }
}

//arrowItem
myArrow::myArrow(customRect *rect){
    myrect = rect;
    QPointF p = rect->boundingRect().topLeft();
    p.setY(p.y() - 200);
    setLine( QLineF( p , rect->boundingRect().topLeft() ));
    setFlag(QGraphicsLineItem::ItemIsSelectable);
}

void myArrow::updatePosition(QPointF p1, QPointF p2)
{
    QLineF line(p2,p1);
    setLine(line);
}

void myArrow::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    qreal arrowSize = 20;

    double angle = std::atan2(-line().dy(), line().dx());

    QPointF arrowP1 = line().p1() + QPointF(sin(angle + M_PI / 3) * arrowSize,
                                    cos(angle + M_PI / 3) * arrowSize);
    QPointF arrowP2 = line().p1() + QPointF(sin(angle + M_PI - M_PI / 3) * arrowSize,
                                    cos(angle + M_PI - M_PI / 3) * arrowSize);

    arrowHead.clear();
    arrowHead << line().p1() << arrowP1 << arrowP2;

    painter->drawLine(line());
    painter->drawPolygon(arrowHead);
}

I really have no clue, please help.
Thanks in advance!

(Editted myArrow paint event)

[SGaist]

Hi,

Isn't that your arrow being painted as you are moving your item ?

[MatheusIH]

Hi, thanks for the response!

I'm guessing yes, so it's a problem in my paint event being called too many times? I'm not seeing the problem or a way out of it, here.

[SGaist]

Why are you calling update from within paint ?

Then are you sure that you are painting the same thing each time ?

Take a look at the notes in the paint method documentation.

[MatheusIH]

@SGaist I'm trying to replicate this example with only 1 object (which I thought would be easier, since I'm still learning Qt) : http://doc.qt.io/qt-5/qtwidgets-graphicsview-diagramscene-arrow-cpp.html

The update inside paint event was misplaced but it was not causing the problem. If I take out this line:

painter->drawPolygon(arrowHead)

It works fine but I lose the head of my arrow. What really annoys me is that when I alt tab and tab back again to my program the blurry lines disappears, which makes me think its not a problem within my code (?).