Not Bug, rather a question

[abhinav272]

Room smartly runs all the queries which return LiveData or Observable on a background thread, then why when returning DataSource.Factory it runs on UI thread and we have to manually run it on a background thread?

[adam-hurwitz]

@abhinav272, Threading is handled on a background thread by the PagedList with the extension function toLiveData for the Paging 2 library.

If you use LivePagedListBuilder to get a LiveData, it will initialize PagedLists on a background thread for you.

If you log the thread in the repository it should show as being on a background thread, and if you log the thread the data is returned on inside the ViewModel, it is returned on the main thread by the Paging 2 library.

SomeRepository.kt

fun initFeed() = flow {
        try {
            Log.v(LOG_TAG, "initFeed: ${Thread.currentThread().name}")
            val tweetsQuery = dao.getAllTweets()
                .toLiveData(
                    pageSize = FEED_PAGEDLIST_SIZE,
                    boundaryCallback = boundaryCallBack
                ).asFlow()
            tweetsQuery.collect { results ->
                emit(success(results))
            }
        } catch (exception: Exception) {
            emit(error(exception.localizedMessage!!, null))
        }
    }

SomeViewModel.kt

private fun initFeed() {
        repository.initFeed().onEach { results ->
            Log.v(LOG_TAG, "initFeed: ${Thread.currentThread().name}")
            _viewState._feed.value = results.data
        }.launchIn(viewModelScope)
}