SCXML Traffic Light (Dynamic, QML)

A Qt Quick application that uses a dynamically loaded state machine to implement a traffic light.

Traffic Light QML demonstrates how to connect to the active properties of a state in a dynamically loaded state machine.

The UI is created using Qt Quick.

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 lines to the example's build files.

To .pro when using qmake:

QT += qml scxml

To CMakeLists.txt when using cmake:

find_package(Qt6 REQUIRED COMPONENTS Core Gui Qml Scxml)
target_link_libraries(trafficlight-qml-dynamic PRIVATE
    Qt6::Core
    Qt6::Gui
    Qt6::Qml
    Qt6::Scxml
)

We dynamically create the state machine in the main QML file:

import QtScxml
import TrafficLightApplication

Window {
    width: lights.width
    height: lights.height
    visible: true

    Lights {
        id: lights
        stateMachine: loader.stateMachine
        // Suppress qmllint warning, dynamic statemachine properties not known at compile-time
        // qmllint disable missing-property
        button.source: stateMachine.working ? "pause.png" : "play.png"
        button.onClicked: stateMachine.submitEvent(stateMachine.working ? "smash" : "repair");
        // qmllint enable missing-property
    }

    StateMachineLoader {
        id: loader
        source: Qt.resolvedUrl("statemachine.scxml")
    }
}

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:

    states: [
        // Suppress qmllint warning, dynamic statemachine properties not known at compile-time
        // qmllint disable missing-property
        State {
            name: "Red"
            when: lights.stateMachine.red

            PropertyChanges { redLight.opacity: 1 }
        },
        State {
            name: "RedGoingGreen"
            when: lights.stateMachine.redGoingGreen

            PropertyChanges { redLight.opacity: 1 }
            PropertyChanges { yellowLight.opacity: 1 }
        },
        State {
            name: "Yellow"
            when: lights.stateMachine.yellow || lights.stateMachine.blinking

            PropertyChanges { yellowLight.opacity: 1 }
        },
        State {
            name: "Green"
            when: lights.stateMachine.green

            PropertyChanges { greenLight.opacity: 1 }
        }
        // qmllint enable missing-property
    ]

Example project @ code.qt.io

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