SCXML Traffic Light (Dynamic, QML) | Qt SCXML (original) (raw)

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, see Qt Creator: Tutorial: Build and run.

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:

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](qml-qtscxml-statemachineloader.html) {
    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](qml-qtquick-state.html) {
        name: "Red"
        when: lights.stateMachine.red

        [PropertyChanges](qml-qtquick-propertychanges.html) { redLight.opacity: 1 }
    },
    [State](qml-qtquick-state.html) {
        name: "RedGoingGreen"
        when: lights.stateMachine.redGoingGreen

        [PropertyChanges](qml-qtquick-propertychanges.html) { redLight.opacity: 1 }
        [PropertyChanges](qml-qtquick-propertychanges.html) { yellowLight.opacity: 1 }
    },
    [State](qml-qtquick-state.html) {
        name: "Yellow"
        when: lights.stateMachine.yellow || lights.stateMachine.blinking

        [PropertyChanges](qml-qtquick-propertychanges.html) { yellowLight.opacity: 1 }
    },
    [State](qml-qtquick-state.html) {
        name: "Green"
        when: lights.stateMachine.green

        [PropertyChanges](qml-qtquick-propertychanges.html) { greenLight.opacity: 1 }
    }
    // qmllint enable missing-property
]

Example project @ code.qt.io

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