sealed class MainActivityViewState(val loading: Boolean, val message: String?, val errorMessage: String?) {
  class Success(val name: String): MainActivityViewState(false, name, null)
  class Loading : MainActivityViewState(true, null, null)
  class Error(errorMessage: String) : MainActivityViewState(false, null, errorMessage)
}

repository.loadPerson("1")
            .map { personData -> MainActivityViewState.Success(personData.name) }
            .onErrorReturn { error -> MainActivityViewState.Error(error.localizedMessage) }
            .startWith { MainActivityViewState.Loading() }
            .subscribe({ state ->
              view.render(state)
            }, { throwable ->
              throw IllegalStateException()
            })