Qt SCXML Calculator QML Example

Calculator uses Qt SCXML to implement the Calculator Example presented in the SCXML Specification.

The state machine is specified in the statemachine.scxml file and compiled into the CalculatorStateMachine class. The user interface 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:

QT += widgets scxml

We then specify the state machine to compile:

STATECHARTS = ../calculator-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 make the generated CalculatorStateMachine class available to QML by registering it as a QML type in the calculator-qml.cpp file:

#include "statemachine.h"

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    qmlRegisterType<CalculatorStateMachine>("CalculatorStateMachine", 1, 0,
                                            "CalculatorStateMachine");

    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/calculator-qml.qml")));
    if (engine.rootObjects().isEmpty())
        return -1;

    return app.exec();
}

To use the CalculatorStateMachine type in a QML file, we import it:

import CalculatorStateMachine 1.0

We instantiate a CalculatorStateMachine and listen to the updateDisplay event. When it occurs, we change the text on the calculator display:

    CalculatorStateMachine {
        id: statemachine
        running: true
        EventConnection {
            events: ["updateDisplay"]
            onOccurred: resultText.text = event.data.display
        }
    }

When users press the calculator buttons, the buttons submit events to the state machine:

        Button {
            id: resultButton
            x: 3 * width
            y: parent.height / 5
            textHeight: y - 2
            fontHeight: 0.4
            width: parent.width / 4
            height: y * 4
            color: pressed ? "#e0b91c" : "#face20"
            text: "="
            onClicked: statemachine.submitEvent("EQUALS")
        }

Example project @ code.qt.io