Qt SCXML Calculator 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 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.

Compiling the State Machine

We link against the Qt SCXML module by adding the following lines to the example's build files.

.pro when using qmake:

QT += widgets scxml

We then specify the state machine to compile:

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

CMakeLists.txt when using cmake:

find_package(Qt6 REQUIRED COMPONENTS Core Gui Scxml Widgets)

target_link_libraries(calculator-widgets PUBLIC
    Qt6::Core
    Qt6::Gui
    Qt6::Scxml
    Qt6::Widgets
)

We then specify the state machine to compile:

qt6_add_statecharts(calculator-widgets
    ../calculator-common/statemachine.scxml
)

The statechart directives STATECHARTS or qt6_add_statecharts invoke the Qt SCXML Compiler, qscxmlc, which is run automatically to generate statemachine.h and statemachine.cpp, which are then added appropriately as headers and sources for compilation.

Instantiating the State Machine

We instantiate the generated CalculatorStateMachine class in the calculator-widgets.cpp file, as follows:

#include "statemachine.h"
#include "mainwindow.h"

#include <QApplication>

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

    CalculatorStateMachine machine;
    MainWindow mainWindow(&machine);

    machine.start();
    mainWindow.show();
    return app.exec();
}

Connecting to Active Properties

After instantiating the state machine, we can connect to the active properties of the states, as follows:

    connect(ui->digit0, &QAbstractButton::clicked, [this] {
        m_machine->submitEvent("DIGIT.0");
    });
    connect(ui->digit1, &QAbstractButton::clicked, [this] {
        m_machine->submitEvent("DIGIT.1");
    });
    connect(ui->digit2, &QAbstractButton::clicked, [this] {
        m_machine->submitEvent("DIGIT.2");
    });

The state machine can notify other code when events occur:

        <transition event="DISPLAY.UPDATE">
            <log label="'result'" expr="short_expr==''?res:short_expr" />
            <send event="updateDisplay">
                <param name="display" expr="short_expr==''?res:short_expr"/>
            </send>
        </transition>

We connect to the updateDisplay event to display the data passed by the events:

    m_machine->connectToEvent(QLatin1String("updateDisplay"), this,
                              [this](const QScxmlEvent &event) {
        const QString display = event.data().toMap()
                .value("display").toString();
        ui->display->setText(display);
    });

Example project @ code.qt.io