Partial example for custom Qt Widget in Nuke

MyWidget.cpp

MyWidget::MyWidget(MyKnob* knob) : _knob(knob), _value(knob->_data), _dragging(false)
{
    setFixedSize(60, 30);
    _switchRect = rect();
    _knob->addCallback(WidgetCallback, this);
}

MyWidget::~MyWidget()
{
    if (_knob)
        _knob->removeCallback(WidgetCallback, this);
}

void MyWidget::paintEvent(QPaintEvent* event)
{
    QPainter painter(this);
    painter.setRenderHint(QPainter::Antialiasing);

    // Draw background
    painter.setPen(Qt::NoPen);
    painter.setBrush(_value ? QColor(0, 150, 136) : QColor(120, 120, 120));
    painter.drawRoundedRect(_switchRect, 15, 15);

    // Draw switch
    painter.setBrush(Qt::white);
    int position = _value ? width() - 28 : 2;
    painter.drawEllipse(position, 2, 26, 26);
}

void MyWidget::mousePressEvent(QMouseEvent* event)
{
    if (event->button() == Qt::LeftButton)
    {
        _dragging = true;
        event->accept();
    }
}

void MyWidget::mouseMoveEvent(QMouseEvent* event)
{
    if (_dragging)
    {
        bool newValue = event->pos().x() > width() / 2;
        if (newValue != _value)
        {
            _value = newValue;
            _knob->setValue(_value);
            update();
        }
        event->accept();
    }
}

void MyWidget::mouseReleaseEvent(QMouseEvent* event)
{
    if (event->button() == Qt::LeftButton)
    {
        _dragging = false;
        bool newValue = event->pos().x() > width() / 2;
        _value = newValue;
        _knob->setValue(_value);
        update();
        event->accept();
    }
}

void MyWidget::update()
{
    _value = _knob->_data;
    QWidget::update();
}

void MyWidget::destroy()
{
    _knob = 0;
}

int MyWidget::WidgetCallback(void* closure, Knob::CallbackReason reason)
{
    MyWidget* widget = (MyWidget*)closure;
    assert(widget);
    switch (reason) {
    case Knob::kIsVisible:
    {
        for (QWidget* w = widget->parentWidget(); w; w = w->parentWidget())
            if (qobject_cast<QTabWidget*>(w))
                return widget->isVisibleTo(w);
        return widget->isVisible();
    }

    case Knob::kUpdateWidgets:
        widget->update();
        return 0;

    case Knob::kDestroying:
        widget->destroy();
        return 0;

    default:
        return 0;
    }
}