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A Qt Quick application that uses a dynamically loaded state machine to implement a traffic light.
Traffic Light QML 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 Quick.
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.
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 PUBLIC Qt6::Core Qt6::Gui Qt6::Qml Qt6::Scxml )
We dynamically create the state machine in the main QML file:
import QtScxml TrafficLight { StateMachineLoader { id: loader source: "qrc:///statemachine.scxml" } stateMachine: loader.stateMachine }
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: [ State { name: "Red" when: stateMachine.red PropertyChanges { target: redLight opacity: 1 } }, State { name: "RedGoingGreen" when: stateMachine.redGoingGreen PropertyChanges { target: redLight opacity: 1 } PropertyChanges { target: yellowLight opacity: 1 } }, State { name: "Yellow" when: stateMachine.yellow || stateMachine.blinking PropertyChanges { target: yellowLight opacity: 1 } }, State { name: "Green" when: stateMachine.green PropertyChanges { target: greenLight opacity: 1 } } ]
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