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 lines to the example's build files.

.pro when using qmake:

QT += qml scxml

We then specify the state machine to compile:

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

CMakeLists.txt when using cmake:

find_package(Qt6 COMPONENTS Core Gui Qml Scxml)

target_link_libraries(trafficlight-qml-static PUBLIC
    Qt6::Core
    Qt6::Gui
    Qt6::Qml
    Qt6::Scxml
)

We then specify the state machine to compile:

qt6_add_statecharts(trafficlight-qml-static
    ../trafficlight-common/statemachine.scxml
)

The statechart directives STATECHARTS or qt6_add_statecharts invoke the Qt SCXML Compiler, qscxmlc, which is run automatically to generate statemachine.h and statemachine.cpp, which are then added appropriately as headers and sources 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

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