BaseballStateMachine.kt

class GameStateMachine(
  events: Flowable<Event>,
  gameStateManager: GameStateManager,
  playHarvester: PlayHarvesterImpl
) {

  private var state: GameStateMachineState =
    BatterUpState(GameCoordinator(gameStateManager, playHarvester))

  val viewStates: Flowable<ViewState> = Flowable.just(state.viewState).concatWith(events
    .doOnNext { state = state.consumeEvent(it) }
    .map { state.viewState })
}

sealed class ViewState {
  class BatterUpViewState(
    val batter: Player,
    val organizations: Organizations,
    val playersOnBase: List<PlayerOnBase>
  ) : ViewState()

  class BatterOptionViewState : ViewState()
  class BatterOutViewState : ViewState()
  class BatterOutOptionsViewState : ViewState()
  class BatterOutFieldViewState(val fielders: List<Position>) : ViewState()
  class BatterOnOptionsViewState(val options: List<Option>, val selectedBase: Base) : ViewState()
  class BaseRunnerViewState(val playerOnBase: PlayerOnBase) : ViewState()
  class BaseRunnerOptionsViewState(val options: List<Option>, val playerOnBase: PlayerOnBase): ViewState()
  class AdvanceRunnerViewState(val playerOnBase: PlayerOnBase): ViewState()
}

interface GameStateMachineState {
  val viewState: ViewState
  fun consumeEvent(event: Event): GameStateMachineState

  sealed class Event {
    class BatterClick : Event()
    class BaseClick(val base: Base) : Event()
    class OutClick : Event()
    class OccupiedBaseClick(val base: Base) : Event()
    class OptionsSelected(val option: Option) : Event()
    class OutRecordingBarLeftBtnClick : Event()
    class OutRecordingBarRightBtnClick : Event()
    class PositionInserted(val position: Position) : Event()
  }
}

// Example of a GameStateMachineState
class BaseRunnerState(
  private val playerOnBase: PlayerOnBase,
  private val gameCoordinator: GameCoordinator
) : GameStateMachineState {
  override val viewState: ViewState
    get() = BaseRunnerViewState(playerOnBase)

  override fun consumeEvent(event: Event): GameStateMachineState {
    return when(event) {
      is OutClick -> handleOutClick()
      is BaseClick -> handleBaseClick(event)
      else -> throw IllegalStateException("$this cannot handle $event")
    }
  }

  private fun handleOutClick(): GameStateMachineState {
    gameCoordinator.baseRunnerOut(playerOnBase.player)
    return BatterUpState(gameCoordinator)
  }

  private fun handleBaseClick(baseClick: BaseClick): GameStateMachineState {
    return BaseRunnerOptionsState(gameCoordinator, PlayerOnBase(playerOnBase.player, baseClick.base))
  }
}
  // This function is in a top level presenter. I'm sure there's a better way to do this ugly block.
  private fun renderViewStateChange(old: ViewState, new: ViewState) {
    return when {
      (old is BatterUpViewState && new is BatterOptionViewState) -> old.transitionTo(new)
      (old is BatterOptionViewState && new is BatterOutViewState) -> old.transitionTo(new)
      (old is BatterOutViewState && new is BatterUpViewState) -> old.transitionTo(new)
      (old is BatterOutViewState && new is BatterOutOptionsViewState) -> old.transitionTo(new)
      (old is BatterOutViewState && new is BatterOutFieldViewState) -> old.transitionTo(new)
      (old is BatterOutFieldViewState && new is BatterOutFieldViewState) -> old.transitionTo(new)
      (old is BatterOutFieldViewState && new is BatterOutViewState) -> old.transitionTo(new)
      (old is BatterOutFieldViewState && new is BatterUpViewState) -> old.transitionTo(new)
      (old is BatterOutOptionsViewState && new is BatterUpViewState) -> old.transitionTo(new)
      (old is BatterOptionViewState && new is BatterOnOptionsViewState) -> old.transitionTo(new)
      (old is BatterOnOptionsViewState && new is BatterOnOptionsViewState) -> old.transitionTo(new)
      (old is BatterOnOptionsViewState && new is BatterUpViewState) -> old.transitionTo(new)
      (old is BatterUpViewState && new is BaseRunnerViewState) -> old.transitionTo(new)
      (old is BaseRunnerViewState && new is BatterUpViewState) -> old.transitionTo(new)
      (old is BaseRunnerViewState && new is BaseRunnerOptionsViewState) -> old.transitionTo(new)
      (old is BaseRunnerOptionsViewState && new is BaseRunnerOptionsViewState) -> old.transitionTo(
        new
      )
      (old is BaseRunnerOptionsViewState && new is BatterUpViewState) -> old.transitionTo(new)
      (old is BatterOnOptionsViewState && new is AdvanceRunnerViewState) -> old.transitionTo(new)
      (old is BatterOutOptionsViewState && new is AdvanceRunnerViewState) -> old.transitionTo(new)
      (old is BatterOutViewState && new is AdvanceRunnerViewState) -> old.transitionTo(new)
      (old is AdvanceRunnerViewState && new is AdvanceRunnerViewState) -> old.transitionTo(new)
      (old is AdvanceRunnerViewState && new is BatterUpViewState) -> old.transitionTo(new)

      else -> throw IllegalStateException("Invalid view states: Going from $old to $new")
    }
  }
  
  // Example of one of the `transitionTo` methods.
  private fun BatterUpViewState.transitionTo(newState: BatterOptionViewState) {
    view.hideBatterButton()
    view.showOutButton(this@GamePresenter)
    view.showBases(this@GamePresenter, null)
  }