Traffic Light Example

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#include <QtWidgets>

class LightWidget : public QWidget
{
    Q_OBJECT
    Q_PROPERTY(bool on READ isOn WRITE setOn)
public:
    LightWidget(const QColor &color, QWidget *parent = 0)
        : QWidget(parent), m_color(color), m_on(false) {}

    bool isOn() const
        { return m_on; }
    void setOn(bool on)
    {
        if (on == m_on)
            return;
        m_on = on;
        update();
    }

public slots:
    void turnOff() { setOn(false); }
    void turnOn() { setOn(true); }

protected:
    void paintEvent(QPaintEvent *) override
    {
        if (!m_on)
            return;
        QPainter painter(this);
        painter.setRenderHint(QPainter::Antialiasing);
        painter.setBrush(m_color);
        painter.drawEllipse(0, 0, width(), height());
    }

private:
    QColor m_color;
    bool m_on;
};

class TrafficLightWidget : public QWidget
{
public:
    TrafficLightWidget(QWidget *parent = 0)
        : QWidget(parent)
    {
        QVBoxLayout *vbox = new QVBoxLayout(this);
        m_red = new LightWidget(Qt::red);
        vbox->addWidget(m_red);
        m_yellow = new LightWidget(Qt::yellow);
        vbox->addWidget(m_yellow);
        m_green = new LightWidget(Qt::green);
        vbox->addWidget(m_green);
        QPalette pal = palette();
        pal.setColor(QPalette::Background, Qt::black);
        setPalette(pal);
        setAutoFillBackground(true);
    }

    LightWidget *redLight() const
        { return m_red; }
    LightWidget *yellowLight() const
        { return m_yellow; }
    LightWidget *greenLight() const
        { return m_green; }

private:
    LightWidget *m_red;
    LightWidget *m_yellow;
    LightWidget *m_green;
};

QState *createLightState(LightWidget *light, int duration, QState *parent = 0)
{
    QState *lightState = new QState(parent);
    QTimer *timer = new QTimer(lightState);
    timer->setInterval(duration);
    timer->setSingleShot(true);
    QState *timing = new QState(lightState);
    QObject::connect(timing, SIGNAL(entered()), light, SLOT(turnOn()));
    QObject::connect(timing, SIGNAL(entered()), timer, SLOT(start()));
    QObject::connect(timing, SIGNAL(exited()), light, SLOT(turnOff()));
    QFinalState *done = new QFinalState(lightState);
    timing->addTransition(timer, SIGNAL(timeout()), done);
    lightState->setInitialState(timing);
    return lightState;
}

class TrafficLight : public QWidget
{
public:
    TrafficLight(QWidget *parent = 0)
        : QWidget(parent)
    {
        QVBoxLayout *vbox = new QVBoxLayout(this);
        TrafficLightWidget *widget = new TrafficLightWidget();
        vbox->addWidget(widget);
        vbox->setMargin(0);

        QStateMachine *machine = new QStateMachine(this);
        QState *redGoingYellow = createLightState(widget->redLight(), 3000);
        redGoingYellow->setObjectName("redGoingYellow");
        QState *yellowGoingGreen = createLightState(widget->yellowLight(), 1000);
        yellowGoingGreen->setObjectName("yellowGoingGreen");
        redGoingYellow->addTransition(redGoingYellow, SIGNAL(finished()), yellowGoingGreen);
        QState *greenGoingYellow = createLightState(widget->greenLight(), 3000);
        greenGoingYellow->setObjectName("greenGoingYellow");
        yellowGoingGreen->addTransition(yellowGoingGreen, SIGNAL(finished()), greenGoingYellow);
        QState *yellowGoingRed = createLightState(widget->yellowLight(), 1000);
        yellowGoingRed->setObjectName("yellowGoingRed");
        greenGoingYellow->addTransition(greenGoingYellow, SIGNAL(finished()), yellowGoingRed);
        yellowGoingRed->addTransition(yellowGoingRed, SIGNAL(finished()), redGoingYellow);

        machine->addState(redGoingYellow);
        machine->addState(yellowGoingGreen);
        machine->addState(greenGoingYellow);
        machine->addState(yellowGoingRed);
        machine->setInitialState(redGoingYellow);
        machine->start();
    }
};

int main(int argc, char **argv)
{
    QApplication app(argc, argv);

    TrafficLight widget;
    widget.resize(110, 300);
    widget.show();

    return app.exec();
}

#include "main.moc"