SCXML Traffic Light (Dynamic, Widgets)

A widget-based application that uses a dynamically loaded state machine to implement a traffic light.

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The Qt Company Ltd.

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Publié le 14 février 2024 - Mis à jour le 14 février 2024

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SCXML Traffic Light (Dynamic, Widgets)▲

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

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.

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 += widgets scxml

To CMakeLists.txt when using cmake:

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find_package(Qt6 REQUIRED COMPONENTS Core Gui Scxml Widgets)

target_link_libraries(trafficlight-widgets-dynamic PRIVATE
    Qt6::Core
    Qt6::Gui
    Qt6::Scxml
    Qt6::Widgets
)

We dynamically create the state machine in trafficlight-widgets-dynamic.cpp:

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#include "../trafficlight-common/trafficlight.h"

#include &lt;QtWidgets/qapplication.h&gt;
#include &lt;QtCore/qiodevice.h&gt;
#include &lt;QtCore/qtextstream.h&gt;

using namespace Qt::Literals::StringLiterals;

int main(int argc, char **argv)
{
    QApplication app(argc, argv);

    QScxmlStateMachine *machine = QScxmlStateMachine::fromFile(u":statemachine.scxml"_s);

    if (!machine-&gt;parseErrors().isEmpty()) {
        QTextStream errs(stderr, QIODevice::WriteOnly);
        const auto errors = machine-&gt;parseErrors();
        for (const QScxmlError &amp;error : errors) {
            errs &lt;&lt; error.toString();
        }
        return -1;
    }

And then instantiate it:

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    TrafficLight widget(machine);
    widget.show();
    machine-&gt;setParent(&amp;widget);
    machine-&gt;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:

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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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    machine-&gt;connectToState(u"red"_s, widget-&gt;redLight(), &amp;LightWidget::switchLight);
    machine-&gt;connectToState(u"redGoingGreen"_s, widget-&gt;redLight(), &amp;LightWidget::switchLight);
    machine-&gt;connectToState(u"yellow"_s, widget-&gt;yellowLight(), &amp;LightWidget::switchLight);
    machine-&gt;connectToState(u"blinking"_s, widget-&gt;yellowLight(), &amp;LightWidget::switchLight);
    machine-&gt;connectToState(u"green"_s, widget-&gt;greenLight(), &amp;LightWidget::switchLight);

Example project▲

Example project @ code.qt.io

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