Kotlin Flow State Machine

StateMachine.kt

fun <T> Flow<T>.stateMachine(configuration: StateChanges<T>.() -> Unit): Flow<StateChanges<T>> = flow {
    var oldState: T? = null
    distinctUntilChanged().collect { value ->
        emit(StateChanges(oldState, value))
        oldState = value
    }
}.onEach {
    configuration(it)
}

data class StateChanges<T>(val oldState: T? = null, val newState: T) {

    @FlowPreview
    @ExperimentalCoroutinesApi
    inline fun <reified R : T> onEnter(enterBlock: () -> Unit) {
        if (oldState !is R && newState is R) {
            enterBlock()
        }
    }

    @FlowPreview
    @ExperimentalCoroutinesApi
    inline fun <reified R : T> onReenter(enterBlock: () -> Unit) {
        if (oldState is R && newState is R) {
            enterBlock()
        }
    }

    @FlowPreview
    @ExperimentalCoroutinesApi
    inline fun <reified R : T> onExit(enterBlock: () -> Unit) {
        if (oldState is R && newState !is R) {
            enterBlock()
        }
    }
}