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

Qt 5.13.2 Reference Documentation

# Qt SCXML Traffic Light Example (Static)

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

Traffic Light Example (Static) demonstrates how to connect to the active properties of a state in a state machine that is compiled to a class.

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.

#### Compiling the State Machine

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

      QT += qml scxml

We then specify the state machine to compile:

      STATECHARTS = ../trafficlight-common/statemachine.scxml

The Qt SCXML Compiler, `qscxmlc`, is run automatically to generate statemachine.h and statemachine.cpp, and to add them to the `HEADERS` and `SOURCES` variables for compilation.

#### Instantiating the State Machine

We instantiate the generated `TrafficLightStateMachine` class in the trafficlight-widgets-static.cpp file, as follows:

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

      #include <QApplication>

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

          TrafficLightStateMachine machine;
          TrafficLight widget(&machine);
          widget.show();
          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