Can't add shadow to frameless window

[Dymytr Yovchev]

How to add a full shadow to MainWindow or centralwidget?

I already use this
Not work for me =
and also try different flags:

setAttribute(Qt::WA_TranslucentBackground); //enable MainWindow to be transparent
setMask(geometry());
        this->setWindowFlags(Qt::SplashScreen); //enable MainWindow to be transparent
        this->setWindowFlags( Qt::FramelessWindowHint); //enable MainWindow to be transparent

and also show()
and apply shadow effect

    CustomShadowEffect* shadowEffect = new CustomShadowEffect();
    shadowEffect->setBlurRadius(9.0);
    shadowEffect->setDistance(6.0);
    shadowEffect->setColor(QColor(0, 0, 0, 80));
    this->centralWidget->setAutoFillBackground(true);
    this->centralWidget->setGraphicsEffect(shadowEffect);