Event filter cannot intercept wheel event from QScrollArea

Question

Here is the event filter:

bool ListenerClass::eventFilter(QObject *obj, QEvent *event) {
    std::cout << "Got event type " << event->type() << std::endl;
    return false;
}

Here is how I have installed it in the constructor of a QScrollArea

this->listenerObj = new ListenerClass(this);
this->setAttribute(Qt::WA_Hover);
this->setAttribute(Qt::WA_NoMousePropagation, false);
this->installEventFilter(this->listenerObj);

The above filter works in general since it can intercept events like resize, hover etc. However the wheel events are intercepted only when the edge of scroll range is reached and not throughout the duration of the scroll.

Viewport position    User action         Output
----------------------------------------------------------
Top                  downwards wheel     <<No output>>
Middle               downwards wheel     <<No output>>
Middle               downwards wheel     <<No output>>
Just touch bottom    downwards wheel     Got event type 31
Bottom (can't        downwards wheel     Got event type 31
 scroll anymore)

How can one intercept wheel events during while the page scrolls?

Answer 1

When the user scrolls the viewport, events happen in the viewport() widget of the scroll area. If the requested scrolling action is possible, the event gets accepted and doesn't propagate to the scroll area itself, so you can't detect it.

To fix this, simply install event filter to the viewport() widget instead.