Android Paging 3 remoteMediator 使用 Compose 无限加载（APPEND）数据

Question

I'm trying to implement a list with Paging 3 library using RemoteMediator.
Initial loading is OK. Mediator loaded only 3 pages, which is set in 'PageConfig'.
But when scroll the list to trigger extra loading for more data, then Mediator starts loading data infinitely until it returns MediatorResult.Success(endOfPaginationReached = true) (Which means all data in remote was loaded). Even though scrolling is stopped.
I have no idea what makes Mediator keep loading.
I want Mediator to load data only needed along scrolling.

Here is my code:

@OptIn(ExperimentalPagingApi::class)
class PostRemoteMediator(
    private val postApi: ApiInterface,
    private val database: PostDatabase
) : RemoteMediator<Int, Post>() {

    override suspend fun load(loadType: LoadType, state: PagingState<Int, Post>): MediatorResult {
        return try {
            val userId = when (loadType) {
                LoadType.REFRESH -> {
                    logd(">> loadType.REFRESH")
                    STARTING_USER_ID
                }
                LoadType.PREPEND -> {
                    return MediatorResult.Success(endOfPaginationReached = true)
                }
                LoadType.APPEND -> {
                    logd(">> loadType.APPEND")

                    val lastItem = state.lastItemOrNull()
                        ?: return MediatorResult.Success(endOfPaginationReached = true)

                    lastItem.userId + 1
                }
            }

            logd(">> load data with userId = $userId")
            val response = postApi.getUserPosts(userId)

            database.withTransaction {
                if (loadType == LoadType.REFRESH) {
                    database.postsDao().clearAll()
                }

                database.postsDao().insertAll(response?.body() ?: emptyList())
            }

            MediatorResult.Success(
                endOfPaginationReached = response.body().isNullOrEmpty()
            )
        } catch (e: IOException) {
            MediatorResult.Error(e)
        } catch (e: HttpException) {
            MediatorResult.Error(e)
        }
    }
}

@OptIn(ExperimentalPagingApi::class)
class PostRepositoryImpl @Inject constructor(
    private val remoteApi: ApiInterface,
    private val database: PostDatabase
) : PostRepository {
    override fun getUserPosts(): Flow<PagingData<Post>> {
        return Pager(
            config = PagingConfig(
                pageSize = 1
            ),
            remoteMediator = PostRemoteMediator(
                remoteApi,
                database
            )
        ) {
            // returns all data in table as PagingSource<Int, Post>
            database.postsDao().getPosts()
        }.flow
    }
}

@HiltViewModel
class PostViewModel @Inject constructor(
    private val postRepository: PostRepository
) : ViewModel() {
    private val TAG = PostViewModel::class.simpleName

    val postFlow: Flow<PagingData<Post>> = postRepository.getUserPosts().cachedIn(viewModelScope)
}

This is UI code:

@Composable
fun PostList(postsFlow: Flow<PagingData<Post>>) {
    val posts = postsFlow.collectAsLazyPagingItems()

    LazyColumn(contentPadding = PaddingValues(horizontal = 8.dp)) {
        items(posts, key = { it.id }) { post ->
            post?.also { PostItem(userId = it.userId, id = it.id, content = it.body) }
        }

        posts.apply {
            when {
                loadState.mediator?.refresh is LoadState.Loading -> {
                    item { LoadingView(modifier = Modifier.fillParentMaxSize()) }
                }
                loadState.mediator?.append is LoadState.Loading -> {
                    item { LoadingView(modifier = Modifier.wrapContentHeight()) }
                }
                loadState.mediator?.refresh is LoadState.Error -> {
                    val error = loadState.refresh as LoadState.Error
                    item { ErrorView(error.error.localizedMessage!!, modifier = Modifier.fillParentMaxSize()) { retry() } }
                }
                loadState.mediator?.append is LoadState.Error -> {
                    val error = loadState.append as LoadState.Error
                    item { ErrorView(error.error.localizedMessage!!, modifier = Modifier.wrapContentHeight()) { retry() } }
                }
            }
        }
    }
}

Thanks for any answer

Answer 1

Increase the pageSize when creating the PageConfig.

According to the documentation:

Should be several times the number of visible items onscreen. Configuring your page size depends on how your data is being loaded and used. Smaller page sizes improve memory usage, latency, and avoid GC churn. Larger pages generally improve loading throughput, to a point (avoid loading more than 2MB from SQLite at once, since it incurs extra cost). If you're loading data for very large, social-media style cards that take up most of a screen, and your database isn't a bottleneck, 10-20 may make sense. If you're displaying dozens of items in a tiled grid, which can present items during a scroll much more quickly, consider closer to 100.