gistfile1.cpp

class FocusInFilter : public QObject
{
    Q_OBJECT

public:

    FocusInFilter(QObject *parent = nullptr) : QObject(parent)
    {}

signals:

    void focussed();

protected:

    bool eventFilter(QObject *obj, QEvent *event)
    {
        if(event->type() == QEvent::FocusIn)
        {
            emit focussed();
        }

        return QObject::eventFilter(obj, event);
    }

private:

};

------

focus_filter = new FocusInFilter(this);
connect(focus_filter, SIGNAL(focussed()), this, SLOT(doStuff()));

auto widget_children = this->findChildren<QWidget*>();
foreach(QWidget *child, widget_children)
{
    child->installEventFilter(focus_filter);
}