At the very 1st scroll it is laggy while using paging library 3 [238227615]

Assigned

Bug

Status Update

No update yet.

Description

al...@gmail.com

created issue #1

Jul 7, 2022 10:13AM

Component used: Paging3 Version used: 3.1.0 Devices/Android versions reproduced on: Pixel4a API 31.

Sample project can be found here : https://github.com/alirezaeiii/Paging3-Sample

At the very 1st scroll it is laggy, and I can not figure out why. Would you please help out?

private const val STARTING_PAGE_INDEX = 1

class ShowPagingSource(
    private val service: ShowService
) : PagingSource<Int, Show>() {

    override suspend fun load(params: LoadParams<Int>): LoadResult<Int, Show> {
        val page = params.key ?: STARTING_PAGE_INDEX
        return try {
            val response = service.fetchShowList(page)
            LoadResult.Page(
                data = response,
                prevKey = if (page == STARTING_PAGE_INDEX) null else page - 1,
                nextKey = if (response.isEmpty()) null else page + 1
            )
        } catch (exception: IOException) {
            LoadResult.Error(exception)
        } catch (exception: HttpException) {
            LoadResult.Error(exception)
        }
    }
}

In my repository layer :

@Singleton
class ShowRepositoryImpl @Inject constructor(private val service: ShowService) : ShowRepository {

    override val fetchResultStream: Flow<PagingData<Show>> = Pager(
        config = PagingConfig(pageSize = NETWORK_PAGE_SIZE),
        pagingSourceFactory = { ShowPagingSource(service) }
    ).flow


    companion object {
        const val NETWORK_PAGE_SIZE = 245
    }
}

And in the ViewModel class :

@HiltViewModel
class ShowViewModel @Inject constructor(
    repository: ShowRepository
) : ViewModel() {

    val pagingDataFlow: Flow<PagingData<Show>> =
        repository.fetchResultStream.cachedIn(viewModelScope)
}

In my Activity I submit the data in recyclerView as follow :

lifecycleScope.launch {
            viewModel.pagingDataFlow.collectLatest(showAdapter::submitData)
        }

Is there something missed? why scrolling is laggy in 1st scroll?

Comments

cl...@google.com <cl...@google.com> #2Jul 8, 2022 10:18PM

Hello, can you please attach a sample that reproduces your error?

al...@gmail.com <al...@gmail.com> #3Jul 9, 2022 07:41AM

I’ve attached two screen recordings demonstrating the mediator's behavior during the first and second launches after installation. Additionally, https://gist.github.com/yatiksihag01/20106aa4855bd438e3eb982034d4bb7d is link to a GitHub Gist containing the code for my remote mediator file for your reference.

Message last modified on Jul 9, 2022 06:08PM

cl...@google.com <cl...@google.com> #4Jul 13, 2022 06:33PM

Based on first_launch_after_installation.mp4, your paging state is indeed showing null for nextKey, so it makes sense that Paging concluded there are no more items to load.

So I think the question is why the PagingState contains only an empty page and null values. My guess is either

RemoteMediator REFRESH actually returned empty data - what is the returned allNewsItems value on initial REFRESH?
RemoteMediator REFRESH successfully loaded data and inserted it into database, but Paging hasn't processed that yet by the time Append is triggered, so PagingState is empty.

The second case doesn't seem likely though.

If you attach an executable app, I can look into this further.

Otherwise, you may gain more insight through Paging logs with adb shell setprop log.tag.Paging VERBOSE. You can also try implementing RemoteKeys in this codelab which addresses possible races between Refresh and subsequent loads.

Message last modified on Jul 13, 2022 06:34PM

al...@gmail.com <al...@gmail.com> #5Jul 20, 2022 02:42PM

Hey! I ran into this same issue. It is in fact the second case you mentioned and is documented here.

I've published a repo reproducing the issue and filed a ticket proposing how this could be changed.

du...@google.com <du...@google.com> #6Jul 20, 2022 05:40PM

1 - run submitData on a background thread throws : java.lang.IllegalStateException: Method addObserver must be called on the main thread.

This doesn't seem related to Paging, what did you try exactly?

al...@gmail.com <al...@gmail.com> #7Jul 20, 2022 07:17PM

I tried :

lifecycleScope.launch(Dispatchers.IO) {
     viewModel.pagingDataFlow.collectLatest(showAdapter::submitData)
}

du...@google.com <du...@google.com> #8Jul 20, 2022 07:24PM

Is your ViewModel lazily initialized? That must be done on main thread. Perhaps you can try:

val pagingDataFlow = viewModel.pagingDataFlow
lifecycleScope.launch(Dispatchers.IO) {
  pagingDataFlow.collectLatest(showAdapter::submitData)
}

al...@gmail.com <al...@gmail.com> #9Jul 20, 2022 07:43PM

I tried your suggestion and it worked, but I do see the lag again. Here is the System trace :

https://drive.google.com/file/d/16lSaorvbVQB0oAl6Kl2r9NhNlevRX3U8/view?usp=drive_web (trace.perfetto-trace)