Using an existing Q_PROPERTY to animate QGraphicsItem inheriting QObject

Question

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.

score 1 · Answer 1 · answered Jun 02 '17 at 12:08

1

First of all QObject does not have a geometry property. It is a property of QWidget. The error message is telling you that there are no geometry and setGeometry member functions. So you have to provide those methods. See the example in property system docs.

answered Jun 02 '17 at 12:08

thuga

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My mistake in deed ! Thanks for your answer. – TaiZzZ Jun 02 '17 at 13:20

eyllanesc · Accepted Answer · 2017-06-02T12:16:55.133

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

Using an existing Q_PROPERTY to animate QGraphicsItem inheriting QObject

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