A state machine library in Kotlin and Swift.
StateMachine
is used in Scarlet
The examples below create a StateMachine
from the following state diagram for matter:
Define states, events and side effects:
sealed class State {
object Solid : State()
object Liquid : State()
object Gas : State()
}
sealed class Event {
object OnMelted : Event()
object OnFroze : Event()
object OnVaporized : Event()
object OnCondensed : Event()
}
sealed class SideEffect {
object LogMelted : SideEffect()
object LogFrozen : SideEffect()
object LogVaporized : SideEffect()
object LogCondensed : SideEffect()
}
Initialize state machine and declare state transitions:
val stateMachine = StateMachine.create<State, Event, SideEffect> {
initialState(State.Solid)
state<State.Solid> {
on<Event.OnMelted> {
transitionTo(State.Liquid, SideEffect.LogMelted)
}
}
state<State.Liquid> {
on<Event.OnFroze> {
transitionTo(State.Solid, SideEffect.LogFrozen)
}
on<Event.OnVaporized> {
transitionTo(State.Gas, SideEffect.LogVaporized)
}
}
state<State.Gas> {
on<Event.OnCondensed> {
transitionTo(State.Liquid, SideEffect.LogCondensed)
}
}
onTransition {
val validTransition = it as? StateMachine.Transition.Valid ?: return@onTransition
when (validTransition.sideEffect) {
SideEffect.LogMelted -> logger.log(ON_MELTED_MESSAGE)
SideEffect.LogFrozen -> logger.log(ON_FROZEN_MESSAGE)
SideEffect.LogVaporized -> logger.log(ON_VAPORIZED_MESSAGE)
SideEffect.LogCondensed -> logger.log(ON_CONDENSED_MESSAGE)
}
}
}
Perform state transitions:
assertThat(stateMachine.state).isEqualTo(Solid)
// When
val transition = stateMachine.transition(OnMelted)
// Then
assertThat(stateMachine.state).isEqualTo(Liquid)
assertThat(transition).isEqualTo(
StateMachine.Transition.Valid(Solid, OnMelted, Liquid, LogMelted)
)
then(logger).should().log(ON_MELTED_MESSAGE)
Adopt StateMachineBuilder
to gain access to the DSL builder methods:
class MyExample: StateMachineBuilder {
... state machine implementation ...
}
Define states, events and side effects:
@StateMachineHashable
enum State {
case solid, liquid, gas
}
@StateMachineHashable
enum Event {
case melt, freeze, vaporize, condense
}
enum SideEffect {
case logMelted, logFrozen, logVaporized, logCondensed
}
Initialize state machine and declare state transitions:
let stateMachine = StateMachine<State, Event, SideEffect> {
initialState(.solid)
state(.solid) {
on(.melt) {
transition(to: .liquid, emit: .logMelted)
}
}
state(.liquid) {
on(.freeze) {
transition(to: .solid, emit: .logFrozen)
}
on(.vaporize) {
transition(to: .gas, emit: .logVaporized)
}
}
state(.gas) {
on(.condense) {
transition(to: .liquid, emit: .logCondensed)
}
}
onTransition {
guard case let .success(transition) = $0, let sideEffect = transition.sideEffect else { return }
switch sideEffect {
case .logMelted: logger.log(Message.melted)
case .logFrozen: logger.log(Message.frozen)
case .logVaporized: logger.log(Message.vaporized)
case .logCondensed: logger.log(Message.condensed)
}
}
}
Perform state transitions:
expect(stateMachine.state).to(equal(.solid))
// When
let transition = try stateMachine.transition(.melt)
// Then
expect(stateMachine.state).to(equal(.liquid))
expect(transition).to(equal(
StateMachine.Transition.Valid(fromState: .solid, event: .melt, toState: .liquid, sideEffect: .logMelted))
)
expect(logger).to(log(Message.melted))
Expand
This information is only applicable to Swift versions older than 5.9
:
Due to Swift enumerations (as opposed to sealed classes in Kotlin), any
State
orEvent
enumeration defined with associated values will require boilerplate implementation forStateMachineHashable
conformance.The easiest way to create this boilerplate is by using the Sourcery Swift code generator along with the AutoStateMachineHashable stencil template provided in this repository. Once the codegen is setup and configured, adopt
AutoStateMachineHashable
instead ofStateMachineHashable
for theState
and/orEvent
enumerations.
StateMachine
is available in Maven Central.
Snapshots of the development version are available in Sonatype's snapshots
repository.
<dependency>
<groupId>com.tinder.statemachine</groupId>
<artifactId>statemachine</artifactId>
<version>0.3.0</version>
</dependency>
implementation 'com.tinder.statemachine:statemachine:0.3.0'
.package(url: "https://github.com/Tinder/StateMachine.git", from: "0.3.0")
pod 'StateMachine', :git => 'https://github.com/Tinder/StateMachine.git'
A composable pattern for pure state machines with effects - Andy Matuschak
Thanks to @nvinayshetty, you can visualize your state machines right in the IDE using the State Arts Intellij plugin.
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