Weird behaviour when dragging QGraphicsItem

Question

I'm trying to create an arrow item in Qt, so I'm basing myself on this example: http://doc.qt.io/qt-5/qtwidgets-graphicsview-diagramscene-arrow-cpp.html

The only difference being that I want to connect an arrow to one item and not two. And it is all working except when I try to drag my item, it leaves a blurry trail behind as if it was painting the whole drag movement and if I alt tab it disapears.

Here's before I drag:

And here's after:

Only the arrow head is behaving like that, and I believe may be something wrong with its paint event:

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);
}

If I take out the arrow head by removing this line painter->drawPolygon(arrowHead) the line itself works fine.

Here is the whole source project: https://github.com/Matheusih/0000000123

Any hints, please?

Answer 1

The problem is that the boundingRect of MyArrow only takes the QLineF as a base and not the arrow, that generates that undesired effect.

My solution proposes to use a QGraphicsPathItem instead of a QGraphicsLineItem, besides I do not understand your logic implemented in moveLineToCenter, I see it as unnecessary, I have modified it to update its position:

myarrow.h

#ifndef MYARROW_H
#define MYARROW_H

class CustomRect;

#include <QGraphicsPathItem>

class MyArrow : public QGraphicsPathItem
{
public:
    MyArrow(CustomRect *rect);
    void updatePosition(QPointF p1, QPointF p2);
private:
    CustomRect *myrect;
};

#endif // MYARROW_H

myarrow.cpp

#include "customrect.h"
#include "myarrow.h"

#include <QPainter>

#include <cmath>

MyArrow::MyArrow(CustomRect *rect){
    myrect = rect;
    QPointF p = rect->boundingRect().center();
    updatePosition(p - QPointF(0, 50), p - QPointF(0, 250));
    setFlag(QGraphicsLineItem::ItemIsSelectable);
}

void MyArrow::updatePosition(QPointF p1, QPointF p2)
{

    const qreal arrowSize = 20;

    QPainterPath path;

    path.moveTo(p1);
    path.lineTo(p2);

    QPointF diff = p2-p1;
    double angle = std::atan2(-diff.y(), diff.x());

    QPointF arrowP1 = p1 + QPointF(sin(angle + M_PI / 3) * arrowSize,
                                    cos(angle + M_PI / 3) * arrowSize);
    QPointF arrowP2 = p1 + QPointF(sin(angle + M_PI - M_PI / 3) * arrowSize,
                                    cos(angle + M_PI - M_PI / 3) * arrowSize);
    QPolygonF arrowHead;
    arrowHead << p1 << arrowP1 << arrowP2 << p1;
    path.moveTo(p1);
    path.addPolygon(arrowHead);

    setPath(path);
}

customrect.h

#ifndef CUSTOMRECT_H
#define CUSTOMRECT_H

#include <QGraphicsPixmapItem>

class MyArrow;

class CustomRect : public QGraphicsRectItem
{
public:
    CustomRect (const QRectF& rect);
    void addLine(MyArrow *line);
    QVariant itemChange(GraphicsItemChange change, const QVariant &value);

    void moveLineToCenter(QPointF newPos);

private:
    QList<MyArrow *> arrows;
};
#endif // CUSTOMRECT_H

customrect.cpp

#include "customrect.h"
#include "myarrow.h"

CustomRect::CustomRect(const QRectF &rect) : QGraphicsRectItem(rect){
    setFlag(QGraphicsItem::ItemIsMovable);
    setFlag(QGraphicsItem::ItemSendsScenePositionChanges);
}

void CustomRect::addLine(MyArrow *line) {
    arrows << 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) {

    for(MyArrow *arrow: arrows) {
        arrow->setPos(newPos);
    }
}

_{The complete solution can be found in the following link}