UserViewReactorMutateReduce.swift

func mutate(action: Action) -> Observable<Mutation> {
  switch action {
    case .follow:
      return UserService.follow()
        .map { Mutation.setFollowing(true) }
        .catchErrorJustReturn(Mutation.setFollowing(false))

    case .unfollow:
      return UserService.unfollow()
        .map { Mutation.setFollowing(false) }
        .catchErrorJustReturn(Mutation.setFollowing(true))
  }
}

func reduce(state: State, mutation: Mutation) -> State {
  var newState = state
  switch mutation {
  case let setFollowing(isFollowing):
    newState.isFollowing = isFollowing
  }
  return newState
}