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:
      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"