Using an existing Q_PROPERTY to animate QGraphicsItem inheriting QObject

I have 2 classes, one MainWindow in which we draw in a QGraphicsView a circle (which intend to become a button !) created thanks to an other class. The class MyCircle inherits from QObject and QGraphicsItem since I want to make animation.

My issue is the following :

My goal is first to make a simple animation on my drawing : make it smaller then it goes back to the original size. So I suppose I should use the property geometry, already existing in the QObject class.

To do this I write in my MainWindow.ccp

animationBoutonRondTaille = new QPropertyAnimation(roundButton, "geometry");
animationBoutonRondTaille->setDuration(1000);
animationBoutonRondTaille->setKeyValueAt(0, QRect(-100, -100, 200, 200));
animationBoutonRondTaille->setKeyValueAt(0.5, QRect(-80,-80,160,160));
animationBoutonRondTaille->setKeyValueAt(1, QRect(-100, -100, 200, 200));
animationBoutonRondTaille -> start();

If I don't include

class MyCircle : public QObject, public QGraphicsItem
{
    Q_OBJECT
    Q_PROPERTY(QRect geometry READ geometry WRITE setGeometry)
/.../
}

I got the following error message :

QPropertyAnimation: you're trying to animate a non-existing property geometry of your QObject

But if I do, i got this one :

'class MyCircle' has no member named 'geometry'/'setgeometry'

What is the purpose of inheriting QObject if I have to define all by myself the geometry property ?

Hope you can help me, and sorry if my question is vague, it's the first for me so I don't really know what you expect.

Thanks a lot if you take time to answer.

You have to implement the geometry and setGeometry methods, the Q_PROPERTY are used to call functions using the setProperty("geometry", some_value) and property("geometry") functions that are used internally in QPropertyAnimation, as well as in the Setter function (in this case setGeometry) you must call update() to update the graph.

mycircle.h

#ifndef MYCIRCLE_H
#define MYCIRCLE_H

#include <QGraphicsItem>
#include <QObject>
#include <QPainter>
#include <QStyleOptionGraphicsItem>

class MyCircle : public QObject, public QGraphicsItem
{
    Q_OBJECT
    Q_PROPERTY(QRect geometry READ geometry WRITE setGeometry)
    Q_INTERFACES(QGraphicsItem)
public:
    explicit MyCircle(QObject *parent = nullptr);

    QRect geometry() const;
    void setGeometry(const QRect &value);

    QRectF boundingRect() const;
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option,QWidget *widget);

private:

    QRect rect;
};

#endif // MYCIRCLE_H

mycircle.cpp

#include "mycircle.h"

MyCircle::MyCircle(QObject *parent) : QObject(parent)
{
    rect = QRect(0, 0, 100, 100);
}

QRect MyCircle::geometry() const
{
    return rect;
}

void MyCircle::setGeometry(const QRect &value)
{
    if(rect!=value){
        rect = value;
        update();
    }
}

QRectF MyCircle::boundingRect() const
{
    return QRectF(rect);
}


void MyCircle::paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
           QWidget *widget)
{
    Q_UNUSED(option)
    Q_UNUSED(widget)
    painter->setBrush(QBrush(Qt::red));
    painter->drawEllipse(rect);
}

A complete example can be found here