Qt SCXML Traffic Light Example (Static)

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

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The Qt Company Ltd.

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Publié le 27 décembre 2024 - Mis à jour le 27 décembre 2024

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Qt SCXML Traffic Light Example (Static)▲

Traffic Light Example (Static) 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 Widgets.

Running the Example▲

To run the example from http://, open the Welcome mode and select the example from Examples. For more information, visit http://.

Compiling the State Machine▲

We link against the Qt SCXML module by adding the following line to the .pro file:

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QT += qml scxml

We then specify the state machine to compile:

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STATECHARTS = ../trafficlight-common/statemachine.scxml

The Qt SCXML Compiler, qscxmlc, is run automatically to generate statemachine.h and statemachine.cpp, and to add them to the HEADERS and SOURCES variables for compilation.

Instantiating the State Machine▲

We instantiate the generated TrafficLightStateMachine class in the trafficlight-widgets-static.cpp file, as follows:

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

#include &lt;QApplication&gt;

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

    TrafficLightStateMachine machine;
    TrafficLight widget(&amp;machine);
    widget.show();
    machine.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(QStringLiteral("red"),
                            widget-&gt;redLight(), &amp;LightWidget::switchLight);
    machine-&gt;connectToState(QStringLiteral("redGoingGreen"),
                            widget-&gt;redLight(), &amp;LightWidget::switchLight);
    machine-&gt;connectToState(QStringLiteral("yellow"),
                            widget-&gt;yellowLight(), &amp;LightWidget::switchLight);
    machine-&gt;connectToState(QStringLiteral("blinking"),
                            widget-&gt;yellowLight(), &amp;LightWidget::switchLight);
    machine-&gt;connectToState(QStringLiteral("green"),
                            widget-&gt;greenLight(), &amp;LightWidget::switchLight);

List of Files▲

Files:

List of Files▲

trafficlight-widgets-static/trafficlight-widgets-static.cpp
trafficlight-widgets-static/trafficlight-widgets-static.pro
trafficlight-widgets-static/trafficlight-widgets-static.qrc

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