Не могу вернуть значение из subscribe

Не могу вернуть значение из subscribe

маппер

    internal class StreamToItemMapper : (List<Stream>) -> (List<StreamItem>) {
    private val streamsApi = App.retrofit.create(StreamsApi::class.java)
    private val chatToItemMapper = ChatToItemMapper()

    override fun invoke(streams: List<Stream>): List<StreamItem> {
        return streams.map { stream ->
            StreamItem(
                category = stream.category,
                chats = streamsApi.getStreamTopics(stream.streamId).map { it.topics }.map(chatToItemMapper)
                    .observeOn(AndroidSchedulers.mainThread()).subscribeBy(return)
            )
        }
    }
}

viewModel

        searchSubject
        .subscribeOn(Schedulers.io())
        .distinctUntilChanged()
        .debounce(250, TimeUnit.MILLISECONDS, Schedulers.io())
        .switchMap { searchQuery -> streamsImpl.loadStreams(searchQuery, subscribedFragment) }
        .map(streamToItemMapper)
        .observeOn(AndroidSchedulers.mainThread())
        .doOnSubscribe { _streamsScreenState.postValue(StreamsScreenState.Loading) }
        .subscribeBy(
            onNext = { _streamsScreenState.value = StreamsScreenState.Result(it) },
            onError = { _streamsScreenState.value = StreamsScreenState.Error(it) }
        )
        .addTo(compositeDisposable)