Qt: How do I notify changing mouse coordinates to

Question

I have a little problem with the Qt class QGraphicsScene: To detect the current mouse coordinates I made a new class QGraphicsScenePlus with QGraphicsScene as the base class. I have already redefined the slot function mouseMoveEvent(QGraphicsSceneMouseEvent* event) and the received coordinates seem to be correct. Now I want to notify the parent QMainWindow class, where the QGraphicsScenePlus object is stored, whenever the mouse coordinates change. What is the best way to do this? I already tried to define signals and slots, but it didn't work. The slot function wasn't found during the execution of the program.

Here is the code so far:

qgraphicssceneplus.h

#ifndef QGRAPHICSSCENEPLUS_H
#define QGRAPHICSSCENEPLUS_H

#include <QObject>
#include <QGraphicsScene>
#include <QGraphicsSceneMouseEvent>

class QGraphicsScenePlus : public QGraphicsScene {

public:
    QGraphicsScenePlus(QObject* parent = 0);

public slots:
    void mouseMoveEvent(QGraphicsSceneMouseEvent* event);

public:
    int mx = 0;
    int my = 0;
};

#endif // QGRAPHICSSCENEPLUS_H

qgraphicssceneplus.cpp

#include "qgraphicssceneplus.h"

QGraphicsScenePlus::QGraphicsScenePlus(QObject* parent) : QGraphicsScene(parent) {

}


void QGraphicsScenePlus::mouseMoveEvent(QGraphicsSceneMouseEvent* mouseEvent) {

    mx = mouseEvent->scenePos().x();
    my = mouseEvent->scenePos().y();

    this->update();
}

Answer 1

Comment

I am not sure how you made the above code compiled.

1. Even though you subclass from a QObject, you still need the Q_OBJECT macro to keep meta-object compiler informed:

class QGraphicsScenePlus : public QGraphicsScene {

    Q_OBJECT    // <--- You miss this

public:
    QGraphicsScenePlus(QObject* parent = 0);

2. It's not allowed to assign primitive value in C++ class definition, do it in the constructor instead:

public:
    int mx /*= 0*/;
    int my /*= 0*/;
};

---

Solution

As for your question:

What is the best way to do this? I already tried to define signals and slots, but it didn't work.

The best way is still Signals & Slots.

Code

qgraphicssceneplus.h

class QGraphicsScenePlus : public QGraphicsScene {

    Q_OBJECT

public:
    QGraphicsScenePlus(QObject* parent = 0);

public slots:
    void mouseMoveEvent(QGraphicsSceneMouseEvent* event);

signals:
    void sendCoord(int,int);  // for sending the information of coordinates

public:
    int mx;
    int my;
};

qgraphicssceneplus.cpp

QGraphicsScenePlus::QGraphicsScenePlus(QObject* parent) : QGraphicsScene(parent) {

    mx = 0;
    my = 0;
}

void QGraphicsScenePlus::mouseMoveEvent(QGraphicsSceneMouseEvent* mouseEvent) {

    mx = mouseEvent->scenePos().x();
    my = mouseEvent->scenePos().y();

    emit sendCoord(mx, my);  // emit the signal

    this->update();
}

To catch the signal, define the slot in QMainWindow. For example:

public slots:
    void receiveCoord(int x, int y);

and connect it to the signal of your graphic scene.

Demo