- Qt 6.4
- Qt SCXML
- Qt SCXML Examples
- Qt SCXML Traffic Light Example (Static)
- Qt 6.4.2 Reference Documentation

# Qt SCXML Traffic Light Example (Static)

A widget-based application that uses a compiled state machine to implement a traffic light.

Traffic Light Example (Static) 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 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 += qml scxml

We then specify the state machine to compile:

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

##### CMakeLists.txt when using cmake:

     find_package(Qt6 REQUIRED COMPONENTS Core Gui Qml Scxml)

     target_link_libraries(trafficlight-qml-static PUBLIC
         Qt6::Core
         Qt6::Gui
         Qt6::Qml
         Qt6::Scxml
     )

We then specify the state machine to compile:

     qt6_add_statecharts(trafficlight-qml-static
         ../trafficlight-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 `TrafficLightStateMachine` class in the trafficlight-widgets-static.cpp file, as follows:

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

     #include <QApplication>

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

         TrafficLightStateMachine machine;
         TrafficLight widget(&machine);
         widget.show();
         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:

             <state id="red">
                 <onentry>
                     <send event="startGoingGreen" delay="3s"/>
                 </onentry>
                 <transition event="startGoingGreen" target="redGoingGreen"/>
             </state>

             <state id="yellow" initial="greenGoingRed">
                 <state id="redGoingGreen">
                     <onentry>
                         <send event="goGreen" delay="1s"/>
                     </onentry>
                     <transition event="goGreen" target="green"/>
                 </state>

                 <state id="greenGoingRed">
                     <onentry>
                         <send event="goRed" delay="1s"/>
                     </onentry>
                     <transition event="goRed" target="red"/>
                 </state>
             </state>

             <state id="green">
                 <onentry>
                     <send event="startGoingRed" delay="3s"/>
                 </onentry>
                 <transition event="startGoingRed" target="greenGoingRed"/>
             </state>

We connect to the states as follows:

         machine->connectToState(QStringLiteral("red"),
                                 widget->redLight(), &LightWidget::switchLight);
         machine->connectToState(QStringLiteral("redGoingGreen"),
                                 widget->redLight(), &LightWidget::switchLight);
         machine->connectToState(QStringLiteral("yellow"),
                                 widget->yellowLight(), &LightWidget::switchLight);
         machine->connectToState(QStringLiteral("blinking"),
                                 widget->yellowLight(), &LightWidget::switchLight);
         machine->connectToState(QStringLiteral("green"),
                                 widget->greenLight(), &LightWidget::switchLight);

Example project @ code.qt.io