Simple MVI coroutine example

SimpleMvi.kt

interface MviViewModel<in I, out VS, out SE> {
    val states: Flow<VS>
    val sideEffects: Flow<SE>
    fun send(intention: I)
}

class SimpleViewModel(scope: CoroutineScope) : MviViewModel<Intention, State, SideEffect> {

    private val _states = MutableStateFlow(0) // this is an always have an initial state example
    private val _sideEffects = Channel<SideEffect>(capacity = Channel.UNLIMITED) // we don't want the actor coroutine to ever suspend, in case there is no UI consuming

    override val states: Flow<Int> = _states
    override val sideEffects: Flow<SideEffect> = _sideEffects.receiveAsFlow()

    // actor is an ongoing coroutine, so there can be no race conditions for state confined to it
    // it processes incoming intentions one by one to build a new state
    // means you can launch this actor with the default dispatcher to maximize cpu core usage
    private val actor = scope.actor<Intention>(
        capacity = Channel.UNLIMITED // unlimited capacity is needed so `actor.offer(..)` never returns `false`
    ) {
        // `StateFlow` requires an initial state - but you could use the upcoming `.shareIn(..)` operator to not require an initial state
        // also, any additional state you share across processing `Intention`s can be stored here, though a single state object is :thumbs up
        var currentState: State = _states.value
        
        for (intention: Intention in channel) { // every intention sent through `actor.offer` emits here
            // simple example, but you could add logging for every intention sent through here, as well as every state change
            currentState = when (intention) {
                Intention.Increment -> currentState + 1
                Intention.Decrement -> currentState - 1
            }

            // update `Flow<State>`
            _states.value = currentState
        }
    }

    override fun send(intention: Intention) {
        actor.offer(intention) // not in a suspending context, so no `.send(..)` available
    }
}

typealias State = Int
typealias SideEffect = Unit

sealed class Intention {
    object Increment : Intention()
    object Decrement : Intention()
}