Qt 5.6 | Qt Widgets | Traffic Light Example

Qt 5.6.2 Reference Documentation

# Traffic Light Example

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.

      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 *) Q_DECL_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.

      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.

      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;
      }

The createLightState() function creates a state that turns a light on when the state is entered, and off when the state is exited. The state uses a timer, and as we shall see the timeout is used to transition from one LightState to another. Here is the statechart for the light state:

      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();
          }
      };

The TrafficLight class combines the TrafficLightWidget with a state machine. The state graph has four states: red-to-yellow, yellow-to-green, green-to-yellow and yellow-to-red. The initial state is red-to-yellow; when the state's timer times out, the state machine transitions to yellow-to-green. The same process repeats through the other states. This is what the statechart looks like:

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

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

          return app.exec();
      }

The main() function constructs a TrafficLight and shows it.

Files:

- statemachine/trafficlight/main.cpp
- statemachine/trafficlight/trafficlight.pro