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

# SCXML Traffic Light (Simple, QML)

A Qt Quick application that uses a compiled state machine to implement a simplified traffic light.

Traffic Light 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 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.

#### 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 PRIVATE
         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 state machine as follows:

         TrafficLightStateMachine {
             id: stateMachine
             running: true
         }

#### 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:

             Light {
                 anchors.top: parent.top
                 anchors.horizontalCenter: parent.horizontalCenter
                 color: "red"
                 visible: stateMachine.red || stateMachine.redGoingGreen
             }

             Light {
                 anchors.centerIn: parent
                 color: "yellow"
                 visible: stateMachine.yellow || stateMachine.blinking
             }

             Light {
                 anchors.bottom: parent.bottom
                 anchors.horizontalCenter: parent.horizontalCenter
                 color: "green"
                 visible: stateMachine.green
             }
         }

Example project @ code.qt.io