Kotlin Finite State Machine

Kotlin library for creating finite state machines.

It is an abstract machine that can be in exactly one of a finite number of states at any given time. The FSM can change from one state to another in response to some external inputs; the change from one state to another is called a transition. An FSM is defined by a list of its states, its initial state, and the conditions for each transition.
— Wikipedia - Finite-state machine

Sample Usage

sealed class Energy {
    object Kinetic : Energy()
    object Potential : Energy()
}

sealed class EnergyTransition {
    object Store : EnergyTransition()
    object Release : EnergyTransition()
}

val stateMachine = StateMachine(
        Potential,
        Potential onEvent Release::class resultsIn Kinetic,
        Kinetic onEvent Store::class resultsIn Potential
)

// Get the current state, will initially return `Potential`
machine.state

// Move the machine to `Kinetic`
machine.transition(Release)

Samples

Samples intend to show various ways the library might be used.

• Dungeon - MUD style application allowing a player to explore a randomly generated dungeon
• Stopwatch - Simple stopwatch that counts from zero at one second intervals until stopped.

Install

Kfin is now a Kotlin Multiplatform project supporting js, jvm, and jvmRx platforms.

kfin - includes base state machine implementation

compile "com.ToxicBakery.kfinstatemachine:kfin-<platform>:4.+"

Graph - includes directed graph implementation which can be used to create large state machines more easily

compile "com.ToxicBakery.kfinstatemachine:graph-<platform>:4.+"

Build

The library depends on gradle for compilation and requires JDK 8 or higher.

./gradlew build

Considerations

This library originally took a classical approach using a directed graph for state and callbacks for listening to state and transition changes. Thanks to a talk given by Ray Ryan from Square, I created an implementation generally following their state machine API and reworked my examples to fit. The exact slide can be found here and the talk in its whole is worth the watch.