- Qt 6.9
- Qt SCXML
- Qt SCXML Examples
- SCXML Traffic Light (Dynamic, QML)
- Qt 6.9.2 Reference Documentation

# SCXML Traffic Light (Dynamic, QML)

A Qt Quick application that uses a dynamically loaded state machine to implement a traffic light.

Traffic Light QML demonstrates how to connect to the active properties of a state in a dynamically loaded state machine.

The UI 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, see Qt Creator: Tutorial: Build and run.

#### 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 += qml scxml

To CMakeLists.txt when using cmake:

     find_package(Qt6 REQUIRED COMPONENTS Core Gui Qml Scxml)
     target_link_libraries(trafficlight-qml-dynamic PRIVATE
         Qt6::Core
         Qt6::Gui
         Qt6::Qml
         Qt6::Scxml
     )

We dynamically create the state machine in the main QML file:

     import QtScxml
     import TrafficLightApplication

     Window {
         width: lights.width
         height: lights.height
         visible: true

         Lights {
             id: lights
             stateMachine: loader.stateMachine
             // Suppress qmllint warning, dynamic statemachine properties not known at compile-time
             // qmllint disable missing-property
             button.source: stateMachine.working ? "pause.png" : "play.png"
             button.onClicked: stateMachine.submitEvent(stateMachine.working ? "smash" : "repair");
             // qmllint enable missing-property
         }

         StateMachineLoader {
             id: loader
             source: Qt.resolvedUrl("statemachine.scxml")
         }
     }

#### 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:

         states: [
             // Suppress qmllint warning, dynamic statemachine properties not known at compile-time
             // qmllint disable missing-property
             State {
                 name: "Red"
                 when: lights.stateMachine.red

                 PropertyChanges { redLight.opacity: 1 }
             },
             State {
                 name: "RedGoingGreen"
                 when: lights.stateMachine.redGoingGreen

                 PropertyChanges { redLight.opacity: 1 }
                 PropertyChanges { yellowLight.opacity: 1 }
             },
             State {
                 name: "Yellow"
                 when: lights.stateMachine.yellow || lights.stateMachine.blinking

                 PropertyChanges { yellowLight.opacity: 1 }
             },
             State {
                 name: "Green"
                 when: lights.stateMachine.green

                 PropertyChanges { greenLight.opacity: 1 }
             }
             // qmllint enable missing-property
         ]

Example project @ code.qt.io