main.cpp Example File

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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:
     virtual void paintEvent(QPaintEvent *)
     {
         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"