Qt/C++ - Tutorial 077. QComboBox - ignoring hidden items in the drop-down list when scrolling

QComboBox, Qt, eventFilter

In one of the previous articles, it was shown how to hide some items in the QComboBox drop-down list so that the user could not select them. However, I did not pay attention to the fact that if the user hovers the mouse on the combo box itself and scrolls the mouse wheel, it can select this hidden menu item. Therefore, this behavior should be prohibited.

You can do this with the help of the mouse scrolling event scrolling within this combo box.

Depending on the structure of the program, you can do this in two ways:

1. Inherit from the QComboBox class and override the method wheelEvent(QWheelEvent event* ).
2. Inherit from the QObject class and override the method eventFilter(QObject obj , QEvent * event* )

By itself, the main code of the method will be similar in both cases, there will be different locations of this code. This will be determined by whether you need to create a QComboBox custom class or not.

With inheritance from QComboBox

When inheriting from QComboBox , we override the wheelEvent(QWheelEvent* event) method and check which of the lines are hidden. We will ignore all hidden lines at the event of the scratch and jump to the next available visible line.

void CustomComboBox::wheelEvent(QWheelEvent* event)
{
    int row = this->currentIndex();
    int count = this->count();
    QListView* dropdownList = static_cast<QListView*>(this->view());
 
    do
    {
        event->angleDelta().y() < 0 ? ++row : --row;
        if (row >= 0 && row < count && !dropdownList->isRowHidden(row))
        {
            this->setCurrentIndex(row);
            return;
        }
    }
    while (row >= 0 && row < count);
}

With the filter installed

If you do not want to create a custom QComboBox , but you need this functionality, then you can create a filter in that window (all widgets are inherited from QObject), where your QComboBox is installed.

To do this, we redefine the window method eventFilter(QObject obj , QEvent * event* ).

bool Widget::eventFilter(QObject* obj, QEvent *event)
{
    if (event->type() == QEvent::Wheel)
    {
        QWheelEvent *wheelEvent = static_cast<QWheelEvent*>(event);
        QComboBox* comboBox = static_cast<QComboBox*>(obj);
 
        int row = comboBox->currentIndex();
        int count = comboBox->count();
        QListView* dropdownList = static_cast<QListView*>(comboBox->view());
 
        do
        {
            wheelEvent->angleDelta().y() < 0 ? ++row : --row;
            if (row >= 0 && row < count && !dropdownList->isRowHidden(row))
            {
                comboBox->setCurrentIndex(row);
                return true;
            }
        }
        while (row >= 0 && row < count);
 
        return true;
    }
 
    return QObject::eventFilter(obj, event);
}

and install this filter on the target QComboBox

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Widget)
{
    ui->setupUi(this);
    ui->comboBox->installEventFilter(this); // Установим фильтр
}