QGraphicsview mouserelease event does not register

Question

I am trying to get the scene coordinates of a both a mousepress and a mouserelease event by means of an event filter. It works fine for the mousepress, but the mouserelease seems not to be registering.

What am I missing here?

Please note I am using Qt Creator 2.2.0(based on Qt 4.7.4)

class mouseEater : public QObject{
// Q_OBJECT
protected:
 bool eventFilter(QObject *obj, QEvent *event);
};
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

mouseEater *eater = new mouseEater();

QGraphicsScene *scene;
scene = new QGraphicsScene();
ui->view->installEventFilter(eater);
ui->view->setScene(scene);
}

bool mouseEater::eventFilter(QObject *obj, QEvent *event){

QMouseEvent *mEvent = static_cast<QMouseEvent *>(event);

QPoint point = mEvent->pos();

cout<<"type:"<<event->type()<<endl;

switch(event->type()){
case QEvent::MouseButtonPress:

    cout<<"mousepress at "<<point.x()<<","<<point.y()<<endl;
    return true;
    break;
case QEvent::MouseButtonRelease:
    cout<<"mouserelease at "<<point.x()<<","<<point.y()<<endl;
    return true;
    break;
default:
    //standard event processing
    return QObject::eventFilter(obj, event);
    break;
}
return 0;
}

sam-w · Accepted Answer · 2011-10-11T13:29:44.567

The quick answer is that QGraphicsView doesn't seem to allow other QObjects to eat its mouseReleaseEvents. I'm not sure why just yet, but I think it's something to do with how mouseReleaseEvent() is reimplemented in QGraphicsView. A little hacking gives a workaround though:

HEADER:

class MainWindow : public QMainWindow
{
  Q_OBJECT

public:
  explicit MainWindow(QWidget *parent = 0);
  ~MainWindow();

protected:
  virtual bool eventFilter(QObject *, QEvent *);

private:
  Ui::MainWindow *ui;
};

class cDerived : public QGraphicsView
{
  Q_OBJECT

public:
  cDerived(QWidget *parent) : QGraphicsView(parent) {}

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

SOURCE:

MainWindow::MainWindow(QWidget *parent) :
  QMainWindow(parent),
  ui(new Ui::MainWindow)
{
  ui->setupUi(this);

  QGraphicsView *View = new cDerived(ui->centralWidget);
  QGraphicsScene *Scene = new QGraphicsScene(ui->centralWidget->rect(), ui->centralWidget);
  View->setScene(Scene);
  View->installEventFilter(this);
}

MainWindow::~MainWindow()
{
  delete ui;
}

bool MainWindow::eventFilter(QObject *obj, QEvent *event)
{
  switch(event->type())
  {
  case QEvent::MouseButtonPress:

      qDebug()<<"eaten mousepress";
      return true;
      break;
  case QEvent::MouseButtonRelease:
      qDebug()<<"eaten mouserelease";
      return true;
      break;
  default:
      //standard event processing
      return QObject::eventFilter(obj, event);
      break;
  }
}

void cDerived::mousePressEvent(QMouseEvent *event)
{
  qDebug()<<"mousepress";
  event->ignore();
}

void cDerived::mouseReleaseEvent(QMouseEvent *event)
{
  qDebug()<<"mouserelease";
  event->ignore();
}

Try commenting out the 'event->ignore()' calls in the reimplemented mouse event catchers and it becomes clear how the mouseReleaseEvent() in QGraphicsView might be going wrong...

If you can reimplement QGraphicsView as a very thin class with just the constructor and mouseReleaseEvent() { event->ignore() } then you should be able to get around this bug (though, while this hack seems to be stable - suggesting that your problem might be a bug in QGraphicsView, without exhaustively looking through the QGraphicsView source we can't be sure this isn't in fact an intended feature and therefore that this hack won't break anything down the line!)

-Sam

score 0 · Answer 2 · edited May 23 '17 at 11:47

0

Use QGraphicsSceneMouseEvent and the likes instead of QMouseEvent. As I found out in my question, QGraphicsScene does not dispatch normal mouse events.

Best regards

edited May 23 '17 at 11:47

Community

1
1

answered Oct 11 '11 at 13:51

Gerstmann

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unfortunately, if I install the event filter on the scene and change the events to the QGrapicsSceneMouseEvent I seem to be unable to get any usefull coordinates. No matter where I click, the coordinates are alway the same.. – Gurba Oct 12 '11 at 06:40
Is your QEvent to QGraphicsSceneMouseEvent cast successfull? What kind of coordinates are you getting? – Gerstmann Oct 12 '11 at 09:12
you have to use setSceneRect to match the view, because it defaults to the qreal thing (-1 -> +1 i think) on both axis' which can be confusing for some. If you NEED it to be in something like that, then you should already know you'll need to do the appropriate conversions but thats another ball of wax.... – osirisgothra Jan 26 '14 at 06:33

QGraphicsview mouserelease event does not register

2 Answers2