[Qt] Window got active or Focus event

WindowActive.cpp

bool MainWindow::event(QEvent * e) // overloading event(QEvent*) method of QMainWindow 
{
    switch(e->type())
    {
        // ...

        case QEvent::WindowActivate :
            // gained focus
            break ;

        case QEvent::WindowDeactivate :
            // lost focus
            break ;
        // ...
    } ;
    return QMainWindow::event(e) ;
}