Make QSpinBox react to mouse wheel events when cursor is not over it

问题

I am using Qt 5.3.2 with Qt Creator 3.2.1 with MinGW 4.8.2 on Windows 7. I have a QSpinBox and can change its value with the mouse wheel only if the mouse is over the QSpinBox. If the mouse is not over the QSpinBox, scrolling the mouse wheel has no effect, even though the QSpinBox still has focus. What do I need to do to be able to change values in the QSpinBox that has focus with the mouse wheel even if the mouse is not hovering over it? Setting mouseTracking to true does not have that effect.

回答1:

Use eventFilter to do this. Install it on your mainWindow:

bool MainWindow::eventFilter(QObject *obj, QEvent *event)
{
        if (obj == this && event->type() == QEvent::Wheel)
        {
            QWheelEvent *wheelEvent = static_cast<QWheelEvent *>(event);
            if(wheelEvent->delta() > 0)
                ui->spinBox->setValue(ui->spinBox->value() + 1);
            else
                ui->spinBox->setValue(ui->spinBox->value() - 1);
        }
}

It is just example, so you can improve it as you want.

Or use this:

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

        if (obj == this && event->type() == QEvent::Wheel)
        {
            QApplication::sendEvent(ui->spinBox,event);
        }
}

In this example, when you detect wheel event, you send it to your spinbox.

But don't forget

protected:
bool eventFilter(QObject *obj, QEvent *event);//in header

and

qApp->installEventFilter(this);//in constructor

As DmitrySazonov recommended. We will detect wheelEvents when our spinBox in focus, when spinBox losed focus, we don't react on wheel(other widgets react normal). We do this in one eventFilter. To do this provide new bool variable. For example:

private:
bool spin;//in header

Initialize it in constructor:

spin = false;

And your eventFilter should be.

    bool MainWindow::eventFilter(QObject *obj, QEvent *event)
    {
        if(obj == ui->spinBox && event->type() == QEvent::FocusIn)
            spin = true;


        if(spin)
        {
            if (obj == this && event->type() == QEvent::Wheel)
            {
                QApplication::sendEvent(ui->spinBox,event);
            }
        }

        if(obj == ui->spinBox && event->type() == QEvent::FocusOut)
            spin = false;
    }

Or do just this, without additional variable:

if (obj == this && event->type() == QEvent::Wheel)
{
    if(ui->spinBox->hasFocus())
        QApplication::sendEvent(ui->spinBox,event);
}

回答2:

I did not mention it in the question but I have more that one QSpinBox and testing them all seems sub-optimal, so I need a generic message forwarder. Based on the Chernobyl's code I made my own version of the message filter:

bool MainWindow::eventFilter(QObject *obj, QEvent *event){
    if (obj == this && event->type() == QEvent::Wheel)
    {
        auto focusWidget = QApplication::focusWidget();
        if (focusWidget){
            qApp->removeEventFilter(this);
            QApplication::sendEvent(focusWidget, event);
            qApp->installEventFilter(this);
            return true;
        }
    }
    return false;
}

This forwards all QWheelEvents to the QWidget with the focus. One could also add other events that need to be forwarded.

The qApp->removeEventFilter and qApp->installEventFilter inside the event filter is the only way I found that prevents the event filter calling itself when scrolling on the main window causing a stack overflow (condition focusWidget != this does not help). There isprobably a way to prevent the infinite recursion without reinstalling the event filter on every QWheelEvent.

来源：https://stackoverflow.com/questions/26041471/make-qspinbox-react-to-mouse-wheel-events-when-cursor-is-not-over-it

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qspinbox