Qt SCXML Traffic Light QML Example (Dynamic)

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

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Publié le 26 janvier 2024 - Mis à jour le 26 janvier 2024

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Qt SCXML Traffic Light QML Example (Dynamic)▲

Traffic Light QML Example (Dynamic) 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:

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QT += qml scxml

To CMakeLists.txt when using cmake:

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find_package(Qt6 REQUIRED COMPONENTS Core Gui Qml Scxml)

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

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

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import QtScxml

TrafficLight {
    StateMachineLoader {
        id: loader
        source: "qrc:///statemachine.scxml"
    }

    stateMachine: loader.stateMachine
}

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:

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        &lt;state id="red"&gt;
            &lt;onentry&gt;
                &lt;send event="startGoingGreen" delay="3s"/&gt;
            &lt;/onentry&gt;
            &lt;transition event="startGoingGreen" target="redGoingGreen"/&gt;
        &lt;/state&gt;

        &lt;state id="yellow" initial="greenGoingRed"&gt;
            &lt;state id="redGoingGreen"&gt;
                &lt;onentry&gt;
                    &lt;send event="goGreen" delay="1s"/&gt;
                &lt;/onentry&gt;
                &lt;transition event="goGreen" target="green"/&gt;
            &lt;/state&gt;

            &lt;state id="greenGoingRed"&gt;
                &lt;onentry&gt;
                    &lt;send event="goRed" delay="1s"/&gt;
                &lt;/onentry&gt;
                &lt;transition event="goRed" target="red"/&gt;
            &lt;/state&gt;
        &lt;/state&gt;

        &lt;state id="green"&gt;
            &lt;onentry&gt;
                &lt;send event="startGoingRed" delay="3s"/&gt;
            &lt;/onentry&gt;
            &lt;transition event="startGoingRed" target="greenGoingRed"/&gt;
        &lt;/state&gt;

We connect to the states as follows:

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    states: [
        State {
            name: "Red"
            when: stateMachine.red

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

            PropertyChanges {
                target: redLight
                opacity: 1
            }

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

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

            PropertyChanges {
                target: greenLight
                opacity: 1
            }
        }
    ]

Example project▲

Example project @ code.qt.io

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