QGraphicsView mouse events

[caduel]

add

setRect(0,0,70,70);

setRect(0,0,70,70);

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to your constructor.
The boundingRect is not enough...

[high_flyer]

thanks!
You are correct!

But then I don't understand the docs.
In the docs it says:

To write your own graphics item, you first create a subclass of QGraphicsItem, and then start by implementing its two pure virtual public functions: boundingRect(), which returns an estimate of the area painted by the item, and paint(), which implements the actual painting.

why is boundingRect() pure virtual if setRect() must be called as well?

Hmm. oh well, no time for that now, I need to work - big thanks!

[high_flyer]

hmm.... this is more complicated it seems.
The setRect() was needed because it was a QGraphicsRectItem subclass.
But QGraphicsItem subclasses, or for example QGraphicsPathItem do not have the setRect() method.
And with these items I am back to my problem....

Ideas?

EDIT: I figured it out - following the analogy of the QGraphicsRectItem, then for QGraphicsPathItem setPath() must be called, or setXxxx() depending on the subclass specialisation.
This bit should be added to the docs I think...

[wysota]

If you reimplement boundingRect() and shape() you shouldn't need to use setXXXX methods. Maybe you forgot about the shape()? The base implementation should simply call boundingRect() (it's reimplemented in subclasses probably causing your problem).

[sarbh20ss]

hey I am facing the same problem...
I have subclassed QGraphicsView, QGraphicsScene, QGraphicsItem.
I am adding this scene object to view, and item object to scene. I am implementing mouse press events for view subclass and item subclass. Item object's flags are also set.
When I click on item object only view's press event gets called. I want to check if the item is clicked or not through press event of item but that is not getting called.
How do I solve this problem?

Thanks for help in advance.

[Cruz]

The very same problem here. Reimplented shape(), it never gets called.

[wysota]

The very same problem here. Reimplented shape(), it never gets called.

Did you remember to make it const? If you are using C++11 you can make sure the prototype is correct by using the "override" specifier.

[Cruz]

I did make it const, but I doubt this problem has anything to do with shape().

In my case I discovered that I had an error in the boundingRect() definition. My bounding box had an offset to the item, so that the item would not report mouse events when the mouse was moving over it simply because the bounding box was somewhere else. Now that I fixed it, the mouse events do get delivered to the graphics item correctly. I can even remove the shape implementation (which does nothing else than the standard implementation anyways) and it still works.

The one thing that remains to solve is that all three, the graphics item, the graphics scene, and the graphics view, all seem to receive the mouse events simultaneously. I was expecting in this situation that the graphics item consumes the event and it doesn't get propagated all the way up to the view. Is my expectation wrong, or am I not doing something right? Here is the code for the relevant places:

class GraphicsViewWidget : public QGraphicsView
{
    Q_OBJECT
 
    GraphicsViewWidget(QWidget *parent = 0);
    ~GraphicsViewWidget();
 
protected:
    void mousePressEvent(QMouseEvent *event);
    void mouseReleaseEvent(QMouseEvent *event);
    void mouseMoveEvent(QMouseEvent *event);
};
 
void GraphicsViewWidget::mousePressEvent(QMouseEvent *event)
{
    qDebug() << "view press event";
    QGraphicsView::mousePressEvent(event);
}
 
void GraphicsViewWidget::mouseMoveEvent(QMouseEvent *event)
{
    qDebug() << "view mouse move";
    QGraphicsView::mouseMoveEvent(event);
}
 
void GraphicsViewWidget::mouseReleaseEvent(QMouseEvent *event)
{
    qDebug() << "view mouse release";
    QGraphicsView::mouseReleaseEvent(event);
}
 
 
 
class MyScene : public QGraphicsScene
{
    Q_OBJECT
 
    ComState comState;
 
public:
    MyScene (QWidget *parent = 0);
    ~MyScene (){};
 
    void mousePressEvent(QGraphicsSceneMouseEvent *event);
    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event);
    void mouseMoveEvent(QGraphicsSceneMouseEvent *event);
};
 
 
 
MyScene::MyScene (QWidget *parent)
    : QGraphicsScene(parent)
{
    setSceneRect(-100, -100, 200, 200);
 
    addItem(&comState);
    comState.setFlags(QGraphicsItem::ItemIsMovable);
}
 
void MyScene::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
    qDebug() << "scene mouse press";
    QGraphicsScene::mousePressEvent(event);
}
 
void MyScene::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
    qDebug() << "scene mouse move" << mouseGrabberItem();
    QGraphicsScene::mouseMoveEvent(event);
}
 
void MyScene::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
    qDebug() << "scene mouse release";
    QGraphicsScene::mouseReleaseEvent(event);
}
 
 
 
class ComState : public QGraphicsItem
{
public:
    double vx;
    double vy;
 
public:
    ComState(QGraphicsItem *parent = 0);
    ~ComState(){};
 
    QRectF boundingRect() const;
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);
 
    void mousePressEvent(QGraphicsSceneMouseEvent *event);
    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event);
    void mouseMoveEvent(QGraphicsSceneMouseEvent *event);
};
 
 
 
ComState::ComState(QGraphicsItem *parent)
    : QGraphicsItem(parent)
{
    vx = 0;
    vy = 0;
}
 
QRectF ComState::boundingRect() const
{
    return QRectF(-0.1, -0.1, 0.2, 0.2);
}
 
void ComState::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
    qDebug() << "item mouse press";
}
 
void ComState::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
    qDebug() << "item mouse move";
}
 
void ComState::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
    qDebug() << "item mouse release";
}

class GraphicsViewWidget : public QGraphicsView
{
    Q_OBJECT

    GraphicsViewWidget(QWidget *parent = 0);
    ~GraphicsViewWidget();

protected:
    void mousePressEvent(QMouseEvent *event);
    void mouseReleaseEvent(QMouseEvent *event);
    void mouseMoveEvent(QMouseEvent *event);
};

void GraphicsViewWidget::mousePressEvent(QMouseEvent *event)
{
    qDebug() << "view press event";
    QGraphicsView::mousePressEvent(event);
}

void GraphicsViewWidget::mouseMoveEvent(QMouseEvent *event)
{
    qDebug() << "view mouse move";
    QGraphicsView::mouseMoveEvent(event);
}

void GraphicsViewWidget::mouseReleaseEvent(QMouseEvent *event)
{
    qDebug() << "view mouse release";
    QGraphicsView::mouseReleaseEvent(event);
}



class MyScene : public QGraphicsScene
{
    Q_OBJECT

    ComState comState;

public:
    MyScene (QWidget *parent = 0);
    ~MyScene (){};

    void mousePressEvent(QGraphicsSceneMouseEvent *event);
    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event);
    void mouseMoveEvent(QGraphicsSceneMouseEvent *event);
};



MyScene::MyScene (QWidget *parent)
    : QGraphicsScene(parent)
{
    setSceneRect(-100, -100, 200, 200);

    addItem(&comState);
    comState.setFlags(QGraphicsItem::ItemIsMovable);
}

void MyScene::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
    qDebug() << "scene mouse press";
    QGraphicsScene::mousePressEvent(event);
}

void MyScene::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
    qDebug() << "scene mouse move" << mouseGrabberItem();
    QGraphicsScene::mouseMoveEvent(event);
}

void MyScene::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
    qDebug() << "scene mouse release";
    QGraphicsScene::mouseReleaseEvent(event);
}



class ComState : public QGraphicsItem
{
public:
    double vx;
    double vy;

public:
    ComState(QGraphicsItem *parent = 0);
    ~ComState(){};

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

    void mousePressEvent(QGraphicsSceneMouseEvent *event);
    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event);
    void mouseMoveEvent(QGraphicsSceneMouseEvent *event);
};



ComState::ComState(QGraphicsItem *parent)
    : QGraphicsItem(parent)
{
    vx = 0;
    vy = 0;
}

QRectF ComState::boundingRect() const
{
    return QRectF(-0.1, -0.1, 0.2, 0.2);
}

void ComState::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
    qDebug() << "item mouse press";
}

void ComState::mouseMoveEvent(QGraphicsSceneMouseEvent *event)
{
    qDebug() << "item mouse move";
}

void ComState::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
{
    qDebug() << "item mouse release";
}

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[wysota]

You are calling the base class implementation so the view forwards events to the scene which forwards them to the item.

[Cruz]

I thought the logic was the other way around. The most specific item is called first, which would be the graphics item, and then the event is propagated up until some object accepts it.

But anyhow, I HAVE to call the base implementation in order to use the functionality provided by the Graphics View Framework. And I also have to remimplement the mouse events in the view and the scene to do custom things. So how can I for example detect the the item is being dragged and not the view, if view and item both react to the mouse move event?

[wysota]

I thought the logic was the other way around. The most specific item is called first, which would be the graphics item, and then the event is propagated up until some object accepts it.

You click on the view so it is the first one to get the event.

But anyhow, I HAVE to call the base implementation in order to use the functionality provided by the Graphics View Framework. And I also have to remimplement the mouse events in the view and the scene to do custom things. So how can I for example detect the the item is being dragged and not the view, if view and item both react to the mouse move event?

Store additional information somewhere that you have already handled the event and check that information on every level (view, scene, item) of event handling.