Kotlin Finite-state machine
This work is licensed under Apache License 2.0
This is a small implementation of an FSM in Kotlin.
Resources
Getting Started
The state machine implementation supports events triggering transitions from one state to another while performing an optional action as well as entry and exit actions.
Features
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Event driven state machine.
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External and internal transitions
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State entry and exit actions.
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Default state actions.
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Default entry and exit actions.
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Determine allowed events.
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Multiple state maps with push / pop transitions
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Automatic transitions
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Externalisation of state.
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Typed event parameters and return values.
Todo
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✓ Multiple state maps
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✓ Push / pop transitions
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✓ Automatic transitions
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✓ Externalisation of state
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✓ Typed event parameters
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✓ Typed event return values
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✓ Simple Visualization
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✓ Detail Visualization
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✓ Gradle Plugin for Visualization
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✓ Timeouts
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✓ Corountines
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❏ Different type of contexts for Nested statemaps
Quick Tutorial
This is the classic turnstile FSM model from [SMC](http://smc.sourceforge.net/)
Simple turnstile example
Assume we and to manage the state on a simple lock.
We want to ensure that the lock() function is only called when the lock is not locked and we want unlock() to be called when locked.
Then we use the DSL to declare a definition of a statemachine matching the diagram:
State Diagram
State Table
| Start State | Event | End State | Action |
|---|---|---|---|
LOCKED |
PASS |
LOCKED |
alarm |
LOCKED |
COIN |
UNLOCKED |
unlock |
UNLOCKED |
PASS |
LOCKED |
lock |
UNLOCKED |
COIN |
UNLOCKED |
returnCoin |
Context class
class Turnstile(var locked: Boolean = true) {
fun unlock() {
assert(locked) { "Cannot unlock when not locked" }
println("Unlock")
locked = false
}
fun lock() {
assert(!locked) { "Cannot lock when locked" }
println("Lock")
locked = true
}
fun alarm() {
println("Alarm")
}
fun returnCoin() {
println("Return coin")
}
override fun toString(): String {
return "Turnstile(locked=$locked)"
}
}Enums for States and Events
We declare 2 enums, one for the possible states and one for the possible events.
enum class TurnstileStates {
LOCKED,
UNLOCKED
}
enum class TurnstileEvents {
COIN,
PASS
}Packaged definition and execution
class TurnstileFSM(turnstile: Turnstile) {
private val fsm = definition.create(turnstile)
fun coin() = fsm.sendEvent(TurnstileEvents.COIN)
fun pass() = fsm.sendEvent(TurnstileEvents.PASS)
companion object {
private val definition = stateMachine(
TurnstileStates.values().toSet(),
TurnstileEvents::class,
Turnstile::class
) {
initialState {
if (locked)
TurnstileStates.LOCKED
else
TurnstileStates.UNLOCKED
}
default {
onEntry { startState, targetState, _ ->
println("entering:$startState -> $targetState for $this")
}
// default transition will invoke alarm
action { state, event, _ ->
println("Default action for state($state) -> event($event) for $this")
alarm()
}
onExit { startState, targetState, _ ->
println("exiting:$startState -> $targetState for $this")
}
}
// when current state is LOCKED
whenState(TurnstileStates.LOCKED) {
// external transition on COIN to UNLOCKED state
onEvent(TurnstileEvents.COIN to TurnstileStates.UNLOCKED) {
unlock()
}
}
// when current state is UNLOCKED
whenState(TurnstileStates.UNLOCKED) {
// internal transition on COIN
onEvent(TurnstileEvents.COIN) {
returnCoin()
}
// external transition on PASS to LOCKED state
onEvent(TurnstileEvents.PASS to TurnstileStates.LOCKED) {
lock()
}
}
}.build()
}
}With this definition we are saying:
When the state is LOCKED and on a COIN event then transition to UNLOCKED and execute the lambda which is treated
as a member of the context { unlock() }
When the state is LOCKED and on event PASS we perform the action alarm() without changing the end state.
Usage
Then we instantiate the FSM and provide a context to operate on:
val turnstile = Turnstile()
val fsm = TurnstileFSM(turnstile)Now we have a context that is independent of the FSM.
Sending events may invoke actions:
// State state is LOCKED
fsm.coin()
// Expect unlock action end state is UNLOCKED
fsm.pass()
// Expect lock() action and end state is LOCKED
fsm.pass()
// Expect alarm() action and end state is LOCKED
fsm.coin()
// Expect unlock() and end state is UNLOCKED
fsm.coin()
// Expect returnCoin() and end state is UNLOCKEDThis model means the FSM can be instantiated as needed if the context has values that represent the state. The idea is that the context will properly maintain it’s internal state.
The FSM can derive the formal state from the value(s) of properties of the context.
The Documentation contains more detail on creating finite state machine implementations.
The documentation contains examples for:
Repository
Use this repository for SNAPSHOT builds. Releases are on Maven Central
repositories {
maven {
url 'https://oss.sonatype.org/content/groups/public'
}
}Dependencies
KMP Projects
The dependency used in common modules.
dependencies {
implementation 'io.jumpco.open:kfsm:1.6.1'
}JVM Projects
dependencies {
implementation 'io.jumpco.open:kfsm-jvm:1.6.1'
}KotlinJS Projects
dependencies {
implementation 'io.jumpco.open:kfsm-js:1.6.1'
}Kotlin/Native Projects using LinuxX64
dependencies {
implementation 'io.jumpco.open:kfsm-linuxX64:1.6.1'
}Kotlin/Native Projects using MinGW64
dependencies {
implementation 'io.jumpco.open:kfsm-mingwX64:1.6.1'
}Kotlin/Native Projects using macOS
dependencies {
implementation 'io.jumpco.open:kfsm-macosX64:1.6.1'
}Simple Visualisation
By using the StateMachineDefinition created by the DSL we do not have access to the details of guard expressions or actions.
For more detailed visualization use kfsm-io.jumpco.open.kfsm.viz
@Test
fun produceVisualizationTurnstileFSM() {
println("== TurnStile")
val visualization = visualize(TurnstileFSM.definition)
File("generated", "turnstile.plantuml").writeText(plantUml(visualization))
}Plantuml
Questions:
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Should entry / exit actions receive state or event as arguments?
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Should default actions receive state or event as arguments?
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Is there a more elegant way to define States and Events using sealed classes?
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Are any features missing from the implementation?