override wheelEvent for QScrollBar

[ChasW]

I want to override the number of steps per wheel movement to a QScrollBar object that comes from a QWheelEvent.

I have the scrollbar object initialized as a child within a parent widget.

I have managed to handle wheelEvent for the parent widget, but I am confused as to how to do this for the child scrollbar object.

I have read this: http://doc.trolltech.com/4.2/eventsandfilters.html

but I am still not sure how to override it for the scrollbar object.

Any pointers? (pun not intended)

[jacek]

http://doc.trolltech.com/4.2/qapplic...rollLines-prop

[ChasW]

http://doc.trolltech.com/4.2/qapplic...rollLines-prop

Thank you. I did not think to look in the QApplication for this.

It seems that is an application wide setting. Correct me if this is not a valid assumption.

What is the procedure for overriding a specific scrollbar child object wheelEvent and not all scrollbars in the application?

[aamer4yu]

Simply override wheelEvent ( QWheelEvent * event ) for the QScrollBar

[ChasW]

Simply override wheelEvent ( QWheelEvent * event ) for the QScrollBar

That is precisely what I am doing from the parent widget, but it is only catching wheel events forthe parent, not the scrollbar. I realize I am missing something here...

[jpn]

You have two ways;

1. Subclass the scrollbar and override the wheelEvent() for it, instead of the parent
2. Install an event filter on the scrollbar (see the example below)

scrollbar->installEventFilter(this);
 
bool ThisClass::eventFilter(QObject* object, QEvent* event)
{
    if (event->type == QEvent::Wheel)
    {
        // original event
        QWheelEvent* w = static_cast<QWheelEvent*>(event);
 
        // construct a modified version ("2" instead of w->delta())
        QWheelEvent d(w->pos(), w->globalPos(), 2, w->buttons(), w->modifiers(), w->orientation());
 
        // the ugly part comes here; this event filter must be temporarily
        // removed to be able to deliver the modified event to the receiver
        // (otherwise causes an infinite loop)
        object->removeEventFilter(this);
        QApplication::sendEvent(object, &d);
        object->installEventFilter(this);
 
        return true; // filter the original event out
    }
    return false; // pass other events
}

scrollbar->installEventFilter(this);

bool ThisClass::eventFilter(QObject* object, QEvent* event)
{
    if (event->type == QEvent::Wheel)
    {
        // original event
        QWheelEvent* w = static_cast<QWheelEvent*>(event);

        // construct a modified version ("2" instead of w->delta())
        QWheelEvent d(w->pos(), w->globalPos(), 2, w->buttons(), w->modifiers(), w->orientation());

        // the ugly part comes here; this event filter must be temporarily
        // removed to be able to deliver the modified event to the receiver
        // (otherwise causes an infinite loop)
        object->removeEventFilter(this);
        QApplication::sendEvent(object, &d);
        object->installEventFilter(this);

        return true; // filter the original event out
    }
    return false; // pass other events
}

To copy to clipboard, switch view to plain text mode 

Subclassing is definitely the cleaner way to do it. You can set the subclassed scroll bar to any QAbstractScrollArea descendant class.

[aamer4yu]

Originally Posted by aamer4yu
Simply override wheelEvent ( QWheelEvent * event ) for the QScrollBar

I meant to say override for the subclass... sorry I was not clear enough