- Qt 6.4
- Qt SCXML
- Qt SCXML Examples
- Qt SCXML Traffic Light Example (Dynamic)
- Qt 6.4.3 Reference Documentation

# Qt SCXML Traffic Light Example (Dynamic)

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

Traffic Light Example (Dynamic) demonstrates how to connect to the active properties of a state in a dynamically loaded state machine.

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.

#### Dynamically Loading the State Machine

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

To .pro when using qmake:

     QT += widgets scxml

To CMakeLists.txt when using cmake:

     find_package(Qt6 REQUIRED COMPONENTS Core Gui Scxml Widgets)

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

We dynamically create the state machine in trafficlight-widgets-dynamic.cpp:

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

     #include <QApplication>
     #include <QIODevice>
     #include <QTextStream>

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

         QScxmlStateMachine *machine = QScxmlStateMachine::fromFile(
                     QStringLiteral(":statemachine.scxml"));
         if (!machine->parseErrors().isEmpty()) {
             QTextStream errs(stderr, QIODevice::WriteOnly);
             const auto errors = machine->parseErrors();
             for (const QScxmlError &error : errors) {
                 errs << error.toString();
             }

             return -1;
         }

And then instantiate it:

         TrafficLight widget(machine);
         widget.show();
         machine->setParent(&widget);
         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