Tracking mouse move in QGraphicsScene class

Question

I subclassed QGraphicsScene and added method mouseMoveEvent to handle mouse move event. I created a ruler on top of GraphicsView and have the ruler tracking mouse movement. In the QGraphicsScene::mousemoveEvent I calls mouseMoveEvent of the ruler widget explcitely. The purpose is to have the ruler knows that the current mouse position.

Now it seems that QGraphicsScene::mousemoveEvent is not called when I move the mouse. However, I can get it to work if I press the left mouse button and move it while holding the button. This is not what I'd like to see; I'd like this method is called whenever I place the mouse over the view and move the mouse.

Is there any workaround?

Answer 1

As stated in the QGraphicsView documentation, the view is responsible for translating mouse and keyboard events into scene events and propagating that to the scene:

You can interact with the items on the scene by using the mouse and keyboard. QGraphicsView translates the mouse and key events into scene events, (events that inherit QGraphicsSceneEvent,), and forward them to the visualized scene.

Since mouse move events only occur when a button is pressed by default, you need to setMouseTracking(true) on the view for the move events to generated in the first place, so that it can forward those to the scene.
Alternatively, if you don't need the translation to scene coordinates, you could reimplement the mouseMoveEvent in the view directly rather than in your scene. But in this case, make sure you call the base class QGraphicsView::mouseMoveEvent in your implementation, so that hover events are properly generated for the items in your scene.

Answer 2

I've been asking, and in some place found some useful information, and testing write this:

tgs.cpp:

#include <QtGui>
#include "tgs.h"
#define _alto  300
#define _ancho 700
#include <QGraphicsSceneMouseEvent>

TGs::TGs(QObject *parent)
    :QGraphicsScene(parent)
{ // Constructor of Scene
    this->over = false;
}

void TGs::drawBackground(QPainter *painter, const QRectF &rect)
{

#define adjy 30
#define adjx 30

    int j = 0;
    int alto = 0;

    QPen pen;
    pen.setWidth(1);
    pen.setBrush(Qt::lightGray);
    painter->setPen(pen);   

    painter->drawText(-225, 10, this->str);
    alto = _alto;  // 50 + 2

    for(int i = 0; i < alto; ++i)
    {
        j = i * adjy - 17;

        painter->drawLine(QPoint(-210, j), QPoint(_ancho, j));
    }

    for(int i = 0; i < 300; ++i)
    {
        j = i * adjx - 210;

        painter->drawLine(QPoint(j, 0), QPoint(j, _ancho * 2));
    }
}

void TGs::mouseMoveEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
    QString string = QString("%1, %2")
               .arg(mouseEvent->scenePos().x())
              .arg(mouseEvent->scenePos().y()); // Update the cursor position text
    this->str = string;
    this->update();
}

void TGs::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
    this->update();
}

void TGs::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
    this->update();
}

tgs.h:

#ifndef TGS_H
#define TGS_H

#include <QObject>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QGraphicsTextItem>

QT_BEGIN_NAMESPACE

class QGraphicsSceneMouseEvent;
class QMenu;
class QPointF;
class QGraphicsLineItem;
class QFont;
class QGraphicsTextItem;
class QColor;

QT_END_NAMESPACE

class TGs : public QGraphicsScene
{
public:
    TGs(QObject *parent = 0);

public slots:
    void drawBackground(QPainter *painter, const QRectF &rect);
    void mouseMoveEvent(QGraphicsSceneMouseEvent * mouseEvent);
    void mousePressEvent(QGraphicsSceneMouseEvent *event);
    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event);

    bool over;
    QString str;
    QGraphicsTextItem cursor;
};

#endif // TGS_H