Disable Scroll in QGraphicsview

[hereiam]

Hi there,

I want to scroll a Scene in QGraphicsView with external contol. So i want to disable scroll event with keys an wheel directly in the QGraphicsView class.

How can I do this?

[Ginsengelf]

Hi, you could install an event filter and catch the key and wheel events there. The documentation for QObject::installEventFilter() shows an example how to "eat" an event.

Ginsengelf

[hereiam]

class MyClass : public QGraphicsScene
{
public:
    MyClass(QObject *parent = nullptr);
    bool eventFilter(QObject *obj, QEvent *event) override;
};
 
MyClass::MyClass(QObject *parent) :
    QGraphicsScene(parent)
{
this->installEventFilter(this);
}
 
 
bool MyClass::eventFilter(QObject *obj, QEvent *event)
{
    if (event->type() == QEvent::GraphicsSceneWheel)
    {
     //   QWheelEvent *wheelEvent = static_cast<QWheelEvent*>(event);
        return true;
    }
    else
    {
        // standard event processing
        return QObject::eventFilter(obj, event);
    }
}

class MyClass : public QGraphicsScene
{
public:
    MyClass(QObject *parent = nullptr);
    bool eventFilter(QObject *obj, QEvent *event) override;
};

MyClass::MyClass(QObject *parent) :
    QGraphicsScene(parent)
{
this->installEventFilter(this);
}


bool MyClass::eventFilter(QObject *obj, QEvent *event)
{
    if (event->type() == QEvent::GraphicsSceneWheel)
    {
     //   QWheelEvent *wheelEvent = static_cast<QWheelEvent*>(event);
        return true;
    }
    else
    {
        // standard event processing
        return QObject::eventFilter(obj, event);
    }
}

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I have installed an event filter in my inherited class "Myclass". But it has no effect.

The wheel event is still active, if I scroll the mouse wheel in the scene.

[d_stranz]

I have installed an event filter in my inherited class "Myclass". But it has no effect.

Your class inherits from QGraphicsScene, which is simply a container for QGraphicsItem instances. It has no visual representation on screen and therefore has no mouse, wheel, or any other input event associated with it.

You probably want to install your event filter on QGraphicsView (and not on a class derived from it, as you have done with your MyClass implementation). It makes no sense to create and install an event filter on an instance of a class itself - if you are going to derive from a class, then simply override the base class methods for the events you want to handle.

The purpose of an event filter is so you can watch events occurring in an instance of one class from a different class. So in your case, you should create the event filter method in your MainWindow class (for example) and install it on your QGraphicsView class.

[hereiam]

I have installed an event filter on QGraphicView but it also has no effect.

class MouseEventFilter : public QObject
{
  Q_OBJECT
 
public:
  MouseEventFilter(QObject *parent = nullptr);
 
protected:
  bool eventFilter(QObject *obj, QEvent *event) override;
};
 
MouseEventFilter::MouseEventFilter(QObject *parent) :
  QObject(parent)
{
}
 
bool MouseEventFilter::eventFilter(QObject *obj, QEvent *event)
{
  // The event is always eaten but the scroll is still active 
  if (event->type() == QEvent::Wheel)
  {
    QWheelEvent *wheelEvent = static_cast<QWheelEvent*>(event);
    qDebug("Ate key press %d", wheelEvent->phase());
    return true;
  }
  else
  {
    // standard event processing
    return QObject::eventFilter(obj, event);
  }
}

class MouseEventFilter : public QObject
{
  Q_OBJECT

public:
  MouseEventFilter(QObject *parent = nullptr);

protected:
  bool eventFilter(QObject *obj, QEvent *event) override;
};

MouseEventFilter::MouseEventFilter(QObject *parent) :
  QObject(parent)
{
}

bool MouseEventFilter::eventFilter(QObject *obj, QEvent *event)
{
  // The event is always eaten but the scroll is still active 
  if (event->type() == QEvent::Wheel)
  {
    QWheelEvent *wheelEvent = static_cast<QWheelEvent*>(event);
    qDebug("Ate key press %d", wheelEvent->phase());
    return true;
  }
  else
  {
    // standard event processing
    return QObject::eventFilter(obj, event);
  }
}

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this->myView = new QGraphicsView(parent);
this->myView->installEventFilter(new MouseEventFilter(this));

this->myView = new QGraphicsView(parent);
this->myView->installEventFilter(new MouseEventFilter(this));

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[d_stranz]

I have installed an event filter on QGraphicView but it also has no effect.

How do you know? Have you run your code in the debugger to see if your event filter is called with any event at all?

Have you disabled the scroll bars in the view (QAbstractScrollArea::setVerticalScrollBarPolicy(), QAbstractScrollArea::setHorizontalScrollBarPolicy())?

[hereiam]

Yes, I have used the debugger. The event is called and it has no effect.
Right, I have disabled both scrollbars.

this->myView = new QGraphicsView(this->myScene);
  this->myView->setViewportUpdateMode(QGraphicsView::BoundingRectViewportUpdate);
  this->myView->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
  this->myView->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
  this->myView->centerOn(0, 0);
  this->myView->setFixedWidth(110);
  this->myView->setDragMode(QGraphicsView::NoDrag);
  this->myView->setFocusPolicy(Qt::NoFocus);
  this->myView->installEventFilter(new MouseEventFilter(this));

this->myView = new QGraphicsView(this->myScene);
  this->myView->setViewportUpdateMode(QGraphicsView::BoundingRectViewportUpdate);
  this->myView->setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
  this->myView->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
  this->myView->centerOn(0, 0);
  this->myView->setFixedWidth(110);
  this->myView->setDragMode(QGraphicsView::NoDrag);
  this->myView->setFocusPolicy(Qt::NoFocus);
  this->myView->installEventFilter(new MouseEventFilter(this));

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