SCXML Traffic Light (Static, QML)

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

Traffic Light 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 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.

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 REQUIRED COMPONENTS Core Gui Qml Scxml)
target_link_libraries(trafficlight-qml-static PRIVATE
    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 state machine as follows:

import TrafficLightApplication

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

    Lights {
        id: lights
        stateMachine: TrafficLightStateMachine {
            running: true
        }

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: [
        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 }
        }
    ]

Example project @ code.qt.io

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