Quick Start

State ids and event declaration

Extend the StateId and the Event types.

sealed class TrafficLightStateId : StateId {
    object RED : TrafficLightStateId()
    object YELLOW : TrafficLightStateId()
    object GREEN : TrafficLightStateId()
}

sealed class TrafficLightEvent : Event {
    object TIMER : TrafficLightEvent()
    object SHORT_TIMER : TrafficLightEvent()
}

sealed class PedestrianLightStateId : StateId {
    object WALK : PedestrianLightStateId()
    object WAIT : PedestrianLightStateId()
}

Transition between states

val machine = createMachine {
    initial(RED)
    state(RED) {
        transition(on = TIMER, target = GREEN)
    }
    state(GREEN) {
        transition(on = TIMER, target = YELLOW)
    }
    state(YELLOW) {
        transition(on = TIMER, target = RED)
    }
}

machine.send(TIMER)

assertEquals(GREEN, machine.currentStateId)

Transition between nested states

val machine = createMachine {
    initial(RED)
    state(RED) {
        initial(WALK)
        state(WALK) {
            transition(on = SHORT_TIMER, target = WAIT)
        }
        state(WAIT)
    }
}

machine.send(SHORT_TIMER)

assertEquals(listOf(RED, WAIT), machine.activeStateIds())

Transition between nested state with the internal strategy

With the internal strategy, all events are passed to children before they are passed to the compound state.

val machine = createMachine(strategy = KSStrategyType.Internal) {
    initial(RED)
    state(RED) {
        transition(on = TIMER, target = YELLOW)

        initial(WALK)
        state(WALK) {
            transition(on = TIMER, target = WAIT)
        }
        state(WAIT)
    }
    state(YELLOW)
}

machine.send(TIMER)

assertEquals(listOf(RED, WAIT), machine.activeStateIds())

Parallel state machine

val machine = createMachine(type = Type.Parallel) {
    state(TRAFFIC_LIGHT) {
        initial(RED)
        state(RED) {
            transition(on = TIMER, target = GREEN)
        }
        state(GREEN) {
            transition(on = TIMER, target = RED)
        }
    }
    state(PEDESTRIAN_LIGHT) {
        initial(WAIT)
        state(WAIT) {
            transition(on = TIMER, target = WALK)
        }
        state(WALK) {
            transition(on = TIMER, target = WAIT)
        }
    }
}

machine.send(TIMER)

assertEquals(
    listOf(TRAFFIC_LIGHT, GREEN, PEDESTRIAN_LIGHT, WALK),
    machine.activeStateIds()
)

Listen for transitions

val machine = createMachine {
    initial(RED)
    state(RED)
    state(YELLOW)
    state(GREEN)
}

val listener = MachineTransitionListener { previousActiveStateIds, nextActiveStateIds ->
    print("I'm listening.")
}

machine.subscribe(listener)
machine.unsubscribe(listener)

// OR

machine.onTransition { previousActiveStateIds, nextActiveStateIds ->
    print("I'm listening.")
}

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Last update: July 13, 2021