- Qt 6.4
- Qt SCXML
- Qt SCXML Examples
- Qt SCXML Calculator QML Example
- Qt 6.4.3 Reference Documentation

# Qt SCXML Calculator QML Example

A Qt Quick application that implements the Calculator example presented in the SCXML Specification.

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 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 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

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: (event)=> 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