QGraphicsPolygonItem not updating QPolygonF co-ordinates when dragged - c++

Question

I have a custom version of QGraphicsPolygonItem which is defined here:
#ifndef CUSTOMGPOLYGON_H
#define CUSTOMGPOLYGON_H
#include <QObject>
#include <QGraphicsPolygonItem>
#include <string>
#include <QGraphicsSceneMouseEvent>
#include <QMenu>
#include <QGraphicsTextItem>
class CustomGPolygon : public QObject, public QGraphicsPolygonItem
{
Q_OBJECT
public:
CustomGPolygon(QPolygonF poly, QObject *parent);
~CustomGPolygon();
using QGraphicsPolygonItem::boundingRect;
using QGraphicsPolygonItem::paint;
void mousePressEvent(QGraphicsSceneMouseEvent *event);
void mouseMoveEvent(QGraphicsSceneMouseEvent *event);
QGraphicsTextItem *className;
private slots:
void deletePolygon();
void copyPolygon();
signals:
void duplicatePoly(QPolygonF);
private:
QMenu menu;
};
#endif // CUSTOMGPOLYGON_H
This is the .cpp for my CustomGPolygon:
#include "customgpolygon.h"
#include <iostream>
CustomGPolygon::CustomGPolygon(QPolygonF poly, QObject *parent):QGraphicsPolygonItem(poly)
{
menu.addAction("Copy", this, SLOT(copyPolygon()));
menu.addAction("Delete", this, SLOT(deletePolygon()));
connect(this, SIGNAL(duplicatePoly(QPolygonF)), parent, SLOT(drawPolygon(QPolygonF)));
}
CustomGPolygon::~CustomGPolygon()
{}
void CustomGPolygon::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
if(event->buttons() & Qt::LeftButton)
{
QGraphicsItem::mouseMoveEvent(event);
QPolygonF poly = this->polygon();
QPointF edgePoint(0,0);
for(int i = 0; i<poly.size(); i++){
if(poly.at(i).x() > edgePoint.x() && poly.at(i).y() > edgePoint.y())
{
edgePoint.setX(poly.at(i).x());
edgePoint.setY(poly.at(i).y());
}
}
this->className->setPos(edgePoint);
}
}
void CustomGPolygon::deletePolygon()
{
delete this;
}
void CustomGPolygon::copyPolygon()
{
QPolygonF poly = this->polygon();
emit duplicatePoly(poly);
}
To Draw one of these polygons onto my QGraphicsScene, I use the following function in my mainwindow.cpp:
void MainWindow::drawPolygon(const QPolygonF &poly)
{
CustomGPolygon *objectPt = new CustomGPolygon(poly, this);
objectPt->setPen(pen);
objectPt->setFlag(QGraphicsItem::ItemIsMovable);
scene->addItem(objectPt);
objectPt->className = textItem;
map->drawing = false;
}
When I drag this drawn polygon I need the co-ordinates of the vectors within the boundingRect to update - which at the moment, they are not doing.
I have tried adding these flags to solve the problem:
objectPt->setFlag(QGraphicsItem::ItemSendsGeometryChanges);
objectPt->setFlag(QGraphicsItem::ItemSendsScenePositionChanges);
However the problem remained

Answer 1

The QPolygonF set in the item is not about the coordinates of the scene but about the coordinates of the item, so moving the item will not change the QPolygonF. It is similar to the position of our face: If roads move with respect to the world but not with respect to ourselves. So if you want to get the polygon with respect to the scene you will have to make a conversion using the mapToScene() method. On the other hand if you want to track the position of the item then you should not use mouseMoveEvent() but itemChange().
On the other hand your calculation of the point is incorrect, what you should compare is the distance based on some metric, for example the Euclidean distance, since for example with your logic if the polygon is in position with negative coordinates then the edgePoint will always be ( 0,0).
Considering the above, the solution is:
#include <QtWidgets>
class CustomGPolygon: public QObject, public QGraphicsPolygonItem{
Q_OBJECT
public:
CustomGPolygon(QPolygonF poly, QObject *parent=nullptr):
QObject(parent), QGraphicsPolygonItem(poly), className(nullptr){
setFlag(QGraphicsItem::ItemIsMovable);
setFlag(QGraphicsItem::ItemSendsGeometryChanges);
menu.addAction("Copy", this, &CustomGPolygon::copyPolygon);
menu.addAction("Delete", this, &CustomGPolygon::deletePolygon);
// if(parent)
// connect(this, &CustomGPolygon:: SIGNAL(duplicatePoly(QPolygonF)), parent, SLOT(drawPolygon(QPolygonF)));
}
~CustomGPolygon(){}
QGraphicsTextItem *getClassName() const{return className;}
void setClassName(QGraphicsTextItem *value){className = value;}
protected:
QVariant itemChange(GraphicsItemChange change, const QVariant &value){
if(change == GraphicsItemChange::ItemPositionChange && !polygon().isEmpty()){
QPolygonF p = mapToScene(polygon());
QPointF edgePoint = *std::max_element(p.begin(), p.end(),
[](const QPointF & x, const QPointF & y) -> bool
{
return QVector2D(x).length() > QVector2D(y).length();
});
if(className)
className->setPos(edgePoint);
}
return QGraphicsPolygonItem::itemChange(change, value);
}
private Q_SLOTS:
void deletePolygon(){delete this;}
void copyPolygon(){
QPolygonF poly = mapToScene(polygon());
Q_EMIT duplicatePoly(poly);
}
Q_SIGNALS:
void duplicatePoly(QPolygonF);
private:
QGraphicsTextItem *className;
QMenu menu;
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QGraphicsScene scene;
QGraphicsView view(&scene);
QPolygonF poly;
poly << QPointF(0, 0) << QPointF(100, 0) << QPointF(100, 100);
CustomGPolygon *item = new CustomGPolygon(poly);
QGraphicsTextItem *textItem = new QGraphicsTextItem("Stack Overflow");
scene.addItem(textItem);
scene.addItem(item);
item->setClassName(textItem);
view.show();
view.resize(640, 480);
return a.exec();
}
#include "main.moc"