Qt 5.13 | Qt SCXML | Qt SCXML Traffic Light Example (Dynamic)

Qt 5.13.2 Reference Documentation

# Qt SCXML Traffic Light Example (Dynamic)

A widget-based application that uses a dynamically loaded state machine to implement a traffic light.

Traffic Light Example (Dynamic) demonstrates how to connect to the active properties of a state in a dynamically loaded state machine.

The UI is created using Qt Widgets.

#### Running the Example

To run the example from Qt Creator, open the Welcome mode and select the example from Examples. For more information, visit Building and Running an Example.

#### Dynamically Loading the State Machine

We link against the Qt SCXML module by adding the following line to the example .pro file:

      QT += widgets scxml

We dynamically create the state machine in trafficlight-widgets-dynamic.cpp:

      #include "../trafficlight-common/trafficlight.h"

      #include <QApplication>
      #include <QTextStream>

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

          QScxmlStateMachine *machine = QScxmlStateMachine::fromFile(
                      QStringLiteral(":statemachine.scxml"));
          if (!machine->parseErrors().isEmpty()) {
              QTextStream errs(stderr, QIODevice::WriteOnly);
              const auto errors = machine->parseErrors();
              for (const QScxmlError &error : errors) {
                  errs << error.toString();
              }

              return -1;
          }

And then instantiate it:

          TrafficLight widget(machine);
          widget.show();
          machine->setParent(&widget);
          machine->start();

          return app.exec();
      }

#### Connecting to States

In the SCXML file, we specify states for each light: red, yellow, and green. In the `<onentry>` element, we specify the event to send when entering the state and the delay in seconds before sending the event. In the `<transition>` element, we specify the event that triggers the transition to the state specified by the `target` attribute:

              <state id="red">
                  <onentry>
                      <send event="startGoingGreen" delay="3s"/>
                  </onentry>
                  <transition event="startGoingGreen" target="redGoingGreen"/>
              </state>

              <state id="yellow" initial="greenGoingRed">
                  <state id="redGoingGreen">
                      <onentry>
                          <send event="goGreen" delay="1s"/>
                      </onentry>
                      <transition event="goGreen" target="green"/>
                  </state>

                  <state id="greenGoingRed">
                      <onentry>
                          <send event="goRed" delay="1s"/>
                      </onentry>
                      <transition event="goRed" target="red"/>
                  </state>
              </state>

              <state id="green">
                  <onentry>
                      <send event="startGoingRed" delay="3s"/>
                  </onentry>
                  <transition event="startGoingRed" target="greenGoingRed"/>
              </state>

We connect to the states as follows:

          machine->connectToState(QStringLiteral("red"),
                                  widget->redLight(), &LightWidget::switchLight);
          machine->connectToState(QStringLiteral("redGoingGreen"),
                                  widget->redLight(), &LightWidget::switchLight);
          machine->connectToState(QStringLiteral("yellow"),
                                  widget->yellowLight(), &LightWidget::switchLight);
          machine->connectToState(QStringLiteral("blinking"),
                                  widget->yellowLight(), &LightWidget::switchLight);
          machine->connectToState(QStringLiteral("green"),
                                  widget->greenLight(), &LightWidget::switchLight);

Example project @ code.qt.io