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void showPopup()
{
    bool checked = pButtonShowHideDropDownMenu->isChecked();

    QPoint pos = pButtonShowHideDropDownMenu->mapToGlobal(QPoint(0,
                                                  pButtonApply->height()));
    dropDownMenu->move(pos);

    dropDownMenu->setVisible(checked);
}

pButtonShowHideDropDownMenu->setChecked(false);

pButtonShowHideDropDownMenu->setChecked(false);

class UDropDownMenu: public QWidget
{
    Q_OBJECT

public:
    UDropDownMenu(QWidget *parent = 0)
    {
        setWindowFlags(Qt::Popup);

        gridLayout = new QGridLayout();

        setLayout(gridLayout);
    }

private:
    QGridLayout *gridLayout;

signals:
    void iHide();

protected:
    void paintEvent(QPaintEvent *event)
    {
        QWidget::paintEvent(event);

        QPainter painter(this);

        QPen pen;
        pen.setWidth(2);
        painter.setPen(pen);

        painter.drawRect(rect());
    }

    void hideEvent(QHideEvent *)
    {
        emit iHide();
    }
};

class UComboBoxButton: public QWidget
{
    Q_OBJECT   

public:
    UComboBoxButton(QWidget *parent = 0);

private:
    void createGUI();

private:
    QPushButton *pButtonShowHideDropDownMenu;
    UDropDownMenu *dropDownMenu;

public slots:
    void showPopup();

private slots:
    void dropDownMenuHide();
};

/// PUBLIC
UComboBoxButton::UComboBoxButton(QWidget *parent)
    : QWidget(parent)
{
    createGUI();
}

/// PRIVATE
void UComboBoxButton::createGUI()
{
    QHBoxLayout *hBoxLayoutMain = new QHBoxLayout();

    dropDownMenu = new UDropDownMenu();
    connect(dropDownMenu, SIGNAL(iHide()),
            this, SLOT(dropDownMenuHide()));

    pButtonShowHideDropDownMenu = new QPushButton();
    pButtonShowHideDropDownMenu->setCheckable(true);
    pButtonShowHideDropDownMenu->setFixedSize(20, 20);

    connect(pButtonShowHideDropDownMenu, SIGNAL(clicked()),
            this, SLOT(showPopup()));

    hBoxLayoutMain->addWidget(pButtonShowHideDropDownMenu);
    hBoxLayoutMain->setSpacing(0);

    setLayout(hBoxLayoutMain);
}

/// PUBLIC SLOTS
void UComboBoxButton::showPopup()
{
    QPushButton *pButton = pButtonShowHideDropDownMenu;    

    QPoint pos = pButton->mapToGlobal(QPoint(0,
                                                  pButton->height()));

    dropDownMenu->move(pos);

    bool checked = pButton->isChecked();
    dropDownMenu->setVisible(checked);
}

/// PRIVATE SLOTS
void UComboBoxButton::dropDownMenuHide()
{
    QPushButton *pButton = pButtonShowHideDropDownMenu;
    pButton->setChecked(false);
}

bool checked = pButton->isChecked(); // всегда равняется true