Traffic Light Example▲

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In this example we write a TrafficLightWidget class. The traffic light has three lights: Red, yellow and green. The traffic light transitions from one light to another (red to yellow to green to yellow to red again) at certain intervals.

 
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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;
};

The LightWidget class represents a single light of the traffic light. It provides an on property and two slots, turnOn() and turnOff(), to turn the light on and off, respectively. The widget paints itself in the color that's passed to the constructor.

 
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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;
};

The TrafficLightWidget class represents the visual part of the traffic light; it's a widget that contains three lights arranged vertically, and provides accessor functions for these.

 
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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()));