Qt SCXML Traffic Light QML Example (Simple)▲

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Traffic Light QML Example (Simple) 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 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.

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 state machine as follows:

 
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    TrafficLightStateMachine {
        id: stateMachine
        running: true
    }

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