Qt SCXML Traffic Light Example (Dynamic)▲

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Traffic Light 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 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 line to the example .pro file:

 
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QT += widgets scxml

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

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

#include <QApplication>
#include <QTextStream>

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

    QScxmlStateMachine *machine = QScxmlStateMachine::fromFile(
                QStringLiteral(":statemachine.scxml"));
    if (!machine->parseErrors().isEmpty()) {
        QTextStream errs(stderr, QIODevice::WriteOnly);
        const auto errors = machine->parseErrors();
        for (const QScxmlError &error : errors) {
            errs << error.toString();
        }

        return -1;
    }

And then instantiate it:

 
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    TrafficLight widget(machine);
    widget.show();
    machine->setParent(&widget);
    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"