Jetpack Paging3 库对同一页面进行多次调用答案

【问题标题】：Jetpack Paging3 library makes several calls to the same pageJetpack Paging3 库对同一页面进行多次调用
【发布时间】：2020-11-16 15:50:15
【问题描述】：

我正在尝试在codelab 之后使用房间数据库作为事实来源和 RemoteMediator 来实现 Jetpack 分页 3 库。该应用程序查询 google book api，但由于某种原因，当我执行搜索时，它会多次调用同一页面。例如，当我在不滚动的情况下搜索火时，我在日志中得到了这个：

D/BooksRepository: new search: fire
D/BooksRemoteMediator: title: fire, page: 0
I/okhttp.OkHttpClient: --> GET https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=0
I/okhttp.OkHttpClient: <-- 200 https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=0 (648ms, unknown-length body)
D/BooksRemoteMediator: title: fire, page: 1
I/okhttp.OkHttpClient: --> GET https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=1
I/okhttp.OkHttpClient: <-- 200 https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=1 (608ms, unknown-length body)
D/BooksRemoteMediator: title: fire, page: 0
I/okhttp.OkHttpClient: --> GET https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=0
I/okhttp.OkHttpClient: <-- 200 https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=0 (629ms, unknown-length body)
D/BooksRemoteMediator: title: fire, page: 1
I/okhttp.OkHttpClient: --> GET https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=1
I/okhttp.OkHttpClient: <-- 200 https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=1 (843ms, unknown-length body)
D/BooksRemoteMediator: title: fire, page: 0
I/okhttp.OkHttpClient: --> GET https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=0
I/okhttp.OkHttpClient: <-- 200 https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=0 (527ms, unknown-length body)
D/BooksRemoteMediator: title: fire, page: 1
I/okhttp.OkHttpClient: --> GET https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=1
I/okhttp.OkHttpClient: <-- 200 https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=1 (734ms, unknown-length body)
D/BooksRemoteMediator: title: fire, page: 2
I/okhttp.OkHttpClient: --> GET https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=2
I/okhttp.OkHttpClient: <-- 200 https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=2 (783ms, unknown-length body)
D/BooksRemoteMediator: title: fire, page: 3
I/okhttp.OkHttpClient: --> GET https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=3
I/okhttp.OkHttpClient: <-- 200 https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=3 (769ms, unknown-length body)
D/BooksRemoteMediator: title: fire, page: 2
I/okhttp.OkHttpClient: --> GET https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=2
I/okhttp.OkHttpClient: <-- 200 https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=2 (521ms, unknown-length body)
D/BooksRemoteMediator: title: fire, page: 3
I/okhttp.OkHttpClient: --> GET https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=3
I/okhttp.OkHttpClient: <-- 200 https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=3 (549ms, unknown-length body)
D/BooksRemoteMediator: title: fire, page: 2
I/okhttp.OkHttpClient: --> GET https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=2
I/okhttp.OkHttpClient: <-- 200 https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=2 (966ms, unknown-length body)
D/BooksRemoteMediator: title: fire, page: 3
I/okhttp.OkHttpClient: --> GET https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=3
I/okhttp.OkHttpClient: <-- 200 https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=3 (673ms, unknown-length body)
D/BooksRemoteMediator: title: fire, page: 4
I/okhttp.OkHttpClient: --> GET https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=4
I/okhttp.OkHttpClient: <-- 200 https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=4 (634ms, unknown-length body)
D/BooksRemoteMediator: title: fire, page: 5
I/okhttp.OkHttpClient: --> GET https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=5
I/okhttp.OkHttpClient: <-- 200 https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=5 (604ms, unknown-length body)
D/BooksRemoteMediator: title: fire, page: 4
I/okhttp.OkHttpClient: --> GET https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=4
I/okhttp.OkHttpClient: <-- 200 https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=4 (632ms, unknown-length body)
D/BooksRemoteMediator: title: fire, page: 3
I/okhttp.OkHttpClient: --> GET https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=3
I/okhttp.OkHttpClient: <-- 200 https://www.googleapis.com/books/v1/volumes?q=intitle%3Afire&key=AIzaSyBxHmT9nFCp9n2uOHkS3Gcq2OO3zbxaMrw&maxResults=40&startIndex=3 (602ms, unknown-length body)

我的实现是这样的：

存储库：

class BooksRepository(private val service: BookService, private val database: BooksDatabase) {
    companion object {
        private const val NETWORK_PAGE_SIZE = 40
    }

    fun getSearchResultStream(
        title: String = "",
        author: String = "",
        publisher: String = "",
        isbn: String = ""
    ): Flow<PagingData<Book>> {
        Timber.d("new search: $title")
        val dbQuery = "%${title.replace(' ', '%')}%"
        val pagingSourceFactory =  { database.bookDao.getBooks(dbQuery, author, publisher)}

        return Pager(
            config = PagingConfig(
                pageSize = NETWORK_PAGE_SIZE,
                enablePlaceholders = false
            ),
            remoteMediator = BooksRemoteMediator(title, author, publisher, isbn, apiKey, service, database),
            pagingSourceFactory = pagingSourceFactory
        ).flow
    }
}

远程调解器：

private const val BOOKS_STARTING_PAGE_INDEX = 0
@OptIn(ExperimentalPagingApi::class)
class BooksRemoteMediator(
    private val title: String?,
    private val author: String?,
    private val publisher: String?,
    private val isbn: String?,
    private val key: String,
    private val service: BookService,
    private val booksDatabase: BooksDatabase
) : RemoteMediator<Int, Book>() {

    override suspend fun load(loadType: LoadType, state: PagingState<Int, Book>): MediatorResult {
        val page = when (loadType) {
            LoadType.REFRESH -> {
                val remoteKeys = getRemoteKeyClosestToCurrentPosition(state)
                remoteKeys?.nextKey?.minus(1) ?: BOOKS_STARTING_PAGE_INDEX
            }
            LoadType.PREPEND -> {
                val remoteKeys = getRemoteKeyForFirstItem(state)
                if (remoteKeys == null) {
                    // The LoadType is PREPEND so some data was loaded before,
                    // so we should have been able to get remote keys
                    // If the remoteKeys are null, then we're an invalid state and we have a bug
                    throw InvalidObjectException("Remote key and the prevKey should not be null")
                }
                // If the previous key is null, then we can't request more data
                val prevKey = remoteKeys.prevKey
                if (prevKey == null) {
                    return MediatorResult.Success(endOfPaginationReached = true)
                }
                remoteKeys.prevKey
            }
            LoadType.APPEND -> {
                val remoteKeys = getRemoteKeyForLastItem(state)
                if (remoteKeys == null || remoteKeys.nextKey == null) {
                    throw InvalidObjectException("Remote key should not be null for $loadType")
                }
                remoteKeys.nextKey
            }
        }

        Timber.d("title: $title, page: $page")
        val sb = StringBuilder()
        if (!title.isNullOrBlank()) sb.append("$TITLE$title+")
        if (!author.isNullOrBlank()) sb.append("$AUTHOR$author+")
        if (!publisher.isNullOrBlank()) sb.append("$PUBLISHER$publisher+")
        if (!isbn.isNullOrBlank()) sb.append("$ISBN$isbn+")
        sb.setLength(sb.length - 1)
        val apiQuery = sb.toString()

        try {
            val apiResponse = service.searchBooks(apiQuery, key, state.config.pageSize, page)

            val books = apiResponse.items
            val endOfPaginationReached = books.isEmpty()
            booksDatabase.withTransaction {
                // clear all tables in the database
                if (loadType == LoadType.REFRESH) {
                    booksDatabase.remoteKeysDao.clearRemoteKeys()
                    booksDatabase.bookDao.clearBooks()
                }
                val prevKey = if (page == BOOKS_STARTING_PAGE_INDEX) null else page - 1
                val nextKey = if (endOfPaginationReached) null else page + 1
                val keys = books.map {
                    RemoteKeys(bookId = it.id, prevKey = prevKey, nextKey = nextKey)
                }
                booksDatabase.remoteKeysDao.insertAll(keys)
                booksDatabase.bookDao.insert(books)
            }
            return MediatorResult.Success(endOfPaginationReached = endOfPaginationReached)
        } catch (exception: IOException) {
            return MediatorResult.Error(exception)
        } catch (exception: HttpException) {
            return MediatorResult.Error(exception)
        }
    }

    private suspend fun getRemoteKeyForLastItem(state: PagingState<Int, Book>): RemoteKeys? {
        // Get the last page that was retrieved, that contained items.
        // From that last page, get the last item
        return state.pages.lastOrNull() { it.data.isNotEmpty() }?.data?.lastOrNull()
            ?.let { book ->
                // Get the remote keys of the last item retrieved
                booksDatabase.remoteKeysDao.remoteKeysBookId(book.id)
            }
    }

    private suspend fun getRemoteKeyForFirstItem(state: PagingState<Int, Book>): RemoteKeys? {
        // Get the first page that was retrieved, that contained items.
        // From that first page, get the first item
        return state.pages.firstOrNull { it.data.isNotEmpty() }?.data?.firstOrNull()
            ?.let { book ->
                // Get the remote keys of the first items retrieved
                booksDatabase.remoteKeysDao.remoteKeysBookId(book.id)
            }
    }

    private suspend fun getRemoteKeyClosestToCurrentPosition(
        state: PagingState<Int, Book>
    ): RemoteKeys? {
        // The paging library is trying to load data after the anchor position
        // Get the item closest to the anchor position
        return state.anchorPosition?.let { position ->
            state.closestItemToPosition(position)?.id?.let { bookId ->
                booksDatabase.remoteKeysDao.remoteKeysBookId(bookId)
            }
        }
    }

}

视图模型：

class BookListViewModel(private val repository: BooksRepository) : ViewModel() {
    private var currentQueryValue: String? = null

    private var currentSearchResult: Flow<PagingData<Book>>? = null

    fun searchRepo(queryString: String): Flow<PagingData<Book>> {
        val lastResult = currentSearchResult
        if (queryString == currentQueryValue && lastResult != null) {
            return lastResult
        }
        currentQueryValue = queryString
        val newResult: Flow<PagingData<Book>> = repository.getSearchResultStream(queryString)
            .cachedIn(viewModelScope)
        currentSearchResult = newResult
        return newResult
    }
}

片段：

class BookListFragment : Fragment() {
...

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        setHasOptionsMenu(true)
        binding = FragmentBookListBinding.inflate(inflater, container, false).apply {
            lifecycleOwner = viewLifecycleOwner
            viewModel = bookListViewModel
        }
        initAdapter()

        val queryString = queryArgs.split(",")
        val query = savedInstanceState?.getString(LAST_SEARCH_QUERY) ?: DEFAULT_QUERY
        search(query)

        setHasOptionsMenu(true)
        return binding.root
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        binding.retryButton.setOnClickListener { adapter.retry() }
    }

    override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        outState.putString(LAST_SEARCH_QUERY, latest)
    }

    private fun search(query: String) {
        // Make sure we cancel the previous job before creating a new one
        searchJob?.cancel()
        searchJob = lifecycleScope.launch {
            bookListViewModel.searchRepo(query).collectLatest{
                adapter.submitData(it)
            }
        }
    }

    private fun initAdapter() {
        binding.rvBooks.adapter = adapter.withLoadStateHeaderAndFooter(
            header = BooksLoadStateAdapter { adapter.retry() },
            footer = BooksLoadStateAdapter { adapter.retry() }
        )

        adapter.addLoadStateListener { loadState ->
            // Only show the list if refresh succeeds.
            binding.rvBooks.isVisible = loadState.source.refresh is LoadState.NotLoading
            // Show loading spinner during initial load or refresh.
            binding.pbLoading.isVisible = loadState.source.refresh is LoadState.Loading
            // Show the retry state if initial load or refresh fails.
            binding.retryButton.isVisible = loadState.source.refresh is LoadState.Error

            // Toast on any error, regardless of whether it came from RemoteMediator or PagingSource
            val errorState = loadState.source.append as? LoadState.Error
                ?: loadState.source.prepend as? LoadState.Error
                ?: loadState.append as? LoadState.Error
                ?: loadState.prepend as? LoadState.Error
            errorState?.let {
                Toast.makeText(
                    context,
                    "\uD83D\uDE28 Wooops ${it.error}",
                    Toast.LENGTH_LONG
                ).show()
            }
        }
    }

    private fun initSearch(menu: Menu) {
        val searchItem = menu.findItem(R.id.action_search)
        val searchView = searchItem.actionView as android.widget.SearchView
        searchView.setOnQueryTextListener(object : SearchView.OnQueryTextListener, android.widget.SearchView.OnQueryTextListener {
            override fun onQueryTextSubmit(query: String?): Boolean {
                try {
                    latest = query!!
                    updateBookListFromInput(query)
                } catch (e: Exception) {
                    Timber.d(e)
                }
                return false
            }

            override fun onQueryTextChange(newText: String?): Boolean {
                return false
            }
        })
        lifecycleScope.launch {
            adapter.loadStateFlow
                // Only emit when REFRESH LoadState changes.
                .distinctUntilChangedBy { it.refresh }
                // Only react to cases where REFRESH completes i.e., NotLoading.
                .filter { it.refresh is LoadState.NotLoading }
                .collect { binding.rvBooks.scrollToPosition(0) }
        }
    }

    private fun updateBookListFromInput(query: String?) {
        query?.trim().let {
            if (!it.isNullOrEmpty()) {
                search(it.toString())
            }
        }
    }


    override fun onCreateOptionsMenu(menu: Menu, inflater: MenuInflater) {
        inflater.inflate(R.menu.book_list_menu, menu)
        initSearch(menu)
        _menu = menu
        val recentList: ArrayList<String> = SpUtil.getQueryList(requireContext())
        var recentMenu: MenuItem? = null
        for (item in recentList) {
            recentMenu = menu.add(Menu.NONE, recentList.indexOf(item), Menu.NONE, item)
        }
    }

    companion object {
        private const val LAST_SEARCH_QUERY: String = "last_search_query"
        private const val DEFAULT_QUERY = "Fishing"
    }
}

道：

@Dao
interface BooksDao {
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    suspend fun insert(books: List<Book>)

    @Query("DELETE FROM books")
    suspend fun clearBooks()

    @Query("SELECT * FROM books WHERE (title LIKE :title) OR (authors LIKE :author) " +
            "OR (publisher LIKE :publisher) ORDER BY title ASC")
    fun getBooks(title: String, author: String = "", publisher: String = ""): PagingSource<Int, Book>
}

服务：

interface BookService {
    @GET("volumes")
    suspend fun searchBooks(
        @Query("q") query: String,
        @Query("key") apiKey: String,
        @Query("maxResults") max: Int,
        @Query("startIndex") page: Int
    ): BookSearchResponse
}

如果有人指出我做错了什么并帮助解决这个问题，那就太好了。谢谢

【问题讨论】：

嗨。您设法解决了这个问题吗？

标签： android android-paging android-paging-library

【解决方案1】：

我花了一些时间来解决这个问题，在阅读了 user9694585 和 dunkypie 的答案后，我能够解决我的问题。就我而言，我使用的是 movieDb API，与 Doilio Matsinhe 完全一样，但我发现我不能依赖 API id 有两个原因：

远程 ID 在同一页面上出现故障。因此，如果我将它们按原样保存在桌子上，我将失去来自服务器的订单。
如果 ID 有问题，我无法访问页面的真正第一个/最后一个项目。我认为这部分使中介处于循环状态。

所以我最终做的是将远程 id 作为属性并在电影表上使用 autoGenerate = true 创建一个主键：

@Entity(tableName = "movie")    
data class MovieTable(
    @PrimaryKey(autoGenerate = true)
    var id: Long = 0,
    var remoteId: Long = 0,
    ...
)

@Dao
interface RemoteKeysDao {
   ...
    @Query("SELECT * FROM remote_keys WHERE movieId = :movieId")
    suspend fun remoteKeyId(movieId: Long): RemoteKeyTable?
   ...
}

我的 RemoteMediator 与 Doilio Matsinhe 非常相似。所以我只留下他答案中没有的方法：

private suspend fun getRemoteKeyForLastItem(state: PagingState<Int, MovieTable>): RemoteKeyTable? {
    return state.pages
        .lastOrNull {
            it.data.isNotEmpty()
        }?.data?.lastOrNull()
        ?.let { movie ->
            remoteKeyLocalDataSource.remoteKeyId(movie.remoteId)
        }
}

private suspend fun getRemoteKeyForFirstItem(state: PagingState<Int, MovieTable>): RemoteKeyTable? {
    return state.pages
        .firstOrNull {
            it.data.isNotEmpty()
        }?.data?.firstOrNull()
        ?.let { movie ->
            remoteKeyLocalDataSource.remoteKeyId(movie.remoteId)
        }
}

private suspend fun getRemoteKeyClosestToCurrentPosition(
    state: PagingState<Int, MovieTable>
): RemoteKeyTable? {
    return state.anchorPosition?.let { position ->
        state.closestItemToPosition(position)?.remoteId?.let { id ->
            remoteKeyLocalDataSource.remoteKeyId(id)
        }
    }
}

基本上是我们在codelab上的那个，但是使用movie.remoteId来获取remoteKey.id。

【讨论】：

这就像一个魅力。但是还有一个额外的信息——在从数据库中获取数据时，确保你在 primary_key 上排序而不是其他字段。否则问题依旧
@AnkurAggarwal 这就是限制。这意味着我们必须小心地按照我们打算检索它的顺序插入我们的数据。您是否碰巧知道这是否被视为错误并计划修复？

【解决方案2】：

如果数据库中的primarykey来自网络请求，可能会导致这个问题

【讨论】：

【解决方案3】：

问题可能在于您的 Books 表中的主键不正确。你能分享一下你是什么

【讨论】：

【解决方案4】：

我在另一个 API 上遇到了完全相同的错误。不使用 Remote Mediator 的版本可以正常工作。当我添加 Remote Mediator 时，它会刷新、追加，然后进入 Prepend 循环。

到目前为止我所尝试的：

匹配codelabs gradle依赖版本；
在视图模型上放置日志以查看我是否多次发出请求。
将日志放在RemoteMediator 类上以获取上一页、当前页和下一页以及当前正在发生的LoadType

这是不滚动的情况：

    2020-07-26 17:43:56.859 327-327/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator: Current Page: 1 and LoadType = REFRESH
    2020-07-26 17:43:58.279 327-385/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator$load: Prev Key: null Next Key: 2
    2020-07-26 17:43:58.503 327-327/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator: Current Page: 2 and LoadType = APPEND
    2020-07-26 17:44:00.185 327-375/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator$load: Prev Key: 1 Next Key: 3
    2020-07-26 17:44:00.303 327-327/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator: Current Page: 1 and LoadType = PREPEND
    2020-07-26 17:44:00.575 327-375/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator$load: Prev Key: null Next Key: 2
    2020-07-26 17:44:00.601 327-327/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator: Current Page: 2 and LoadType = APPEND
    2020-07-26 17:44:00.776 327-327/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator: Current Page: 1 and LoadType = PREPEND
    2020-07-26 17:44:01.056 327-385/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator$load: Prev Key: null Next Key: 2
    2020-07-26 17:44:01.185 327-327/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator: Current Page: 1 and LoadType = PREPEND
    2020-07-26 17:44:01.336 327-386/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator$load: Prev Key: null Next Key: 2
    2020-07-26 17:44:01.630 327-327/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator: Current Page: 1 and LoadType = PREPEND
    2020-07-26 17:44:01.830 327-385/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator$load: Prev Key: null Next Key: 2
    2020-07-26 17:44:01.913 327-327/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator: Current Page: 1 and LoadType = PREPEND
    2020-07-26 17:44:02.444 327-387/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator$load: Prev Key: null Next Key: 2
    2020-07-26 17:44:02.567 327-327/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator: Current Page: 1 and LoadType = PREPEND
    2020-07-26 17:44:02.716 327-385/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator$load: Prev Key: null Next Key: 2
    2020-07-26 17:44:02.810 327-327/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator: Current Page: 1 and LoadType = PREPEND
    2020-07-26 17:44:02.983 327-387/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator$load: Prev Key: null Next Key: 2
    2020-07-26 17:44:03.070 327-327/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator: Current Page: 1 and LoadType = PREPEND
    2020-07-26 17:44:03.239 327-385/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator$load: Prev Key: null Next Key: 2
    2020-07-26 17:44:03.348 327-327/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator: Current Page: 1 and LoadType = PREPEND
    2020-07-26 17:44:03.521 327-387/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator$load: Prev Key: null Next Key: 2
    2020-07-26 17:44:03.665 327-327/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator: Current Page: 1 and LoadType = PREPEND
    2020-07-26 17:44:04.102 327-385/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator$load: Prev Key: null Next Key: 2
    2020-07-26 17:44:04.232 327-327/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator: Current Page: 1 and LoadType = PREPEND
    2020-07-26 17:44:04.476 327-386/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator$load: Prev Key: null Next Key: 2
    2020-07-26 17:44:04.577 327-327/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator: Current Page: 1 and LoadType = PREPEND
    2020-07-26 17:44:05.857 327-387/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator$load: Prev Key: null Next Key: 2
    2020-07-26 17:44:06.570 327-327/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator: Current Page: 1 and LoadType = PREPEND
    2020-07-26 17:44:06.932 327-386/com.doiliomatsinhe.mymovies D/MoviesRemoteMediator$load: Prev Key: null Next Key: 2

这就是我的 RemoteMediator 负载的方式：

override suspend fun load(loadType: LoadType, state: PagingState<Int, DatabaseMovie>): MediatorResult {
    val page = when (loadType) {
        LoadType.REFRESH -> {
            val remoteKeys = getRemoteKeyClosestToCurrentPosition(state)
            Timber.d("REFRESH-> ${remoteKeys?.prevKey} -> ${remoteKeys?.nextKey} ")
            remoteKeys?.nextKey?.minus(1) ?: MOVIES_LIST_STARTING_PAGE
        }
        LoadType.PREPEND -> {
            val remoteKeys = getRemoteKeyForFirstItem(state)
            if (remoteKeys == null) {
                throw InvalidObjectException("Remote key and the prevKey should not be null")
            }
            // If the previous key is null, then we can't request more data
            if (remoteKeys.prevKey == null) {
                return MediatorResult.Success(endOfPaginationReached = true)
            }
            Timber.d("PREPEND-> ${remoteKeys.prevKey} -> ${remoteKeys.nextKey} ")
            remoteKeys.prevKey

        }

        LoadType.APPEND -> {
            val remoteKeys = getRemoteKeyForLastItem(state)
            if (remoteKeys?.nextKey == null) {
                throw InvalidObjectException("Remote key should not be null for $loadType")
            }
            Timber.d("APPEND-> ${remoteKeys.prevKey} -> ${remoteKeys.nextKey} ")
            remoteKeys.nextKey
        }
    }

    Timber.d("Page= $page, LoadType= $loadType")
    return try {
        val response = if (category != null && language != null) {
            service.getMovies(category, SECRET_KEY, language, page) } else {
            throw InvalidObjectException("Query Parameters can't be NULL")
        }
        val movies = response.results
        val endOfPaginationReached = movies.isEmpty()

        database.withTransaction {
            if (loadType == LoadType.REFRESH) {
                database.moviesDao.clearMovies()
                database.remoteKeysDao.clearRemoteKeys()
            }

            val prevKey = if (page == MOVIES_LIST_STARTING_PAGE) null else page - 1
            val nextKey = if (endOfPaginationReached) null else page + 1
            Timber.d("PrevKey =$prevKey, NextKey=$nextKey")
            val keys = movies.map {
                RemoteKeys(
                    movieId = it.id,
                    prevKey = prevKey,
                    nextKey = nextKey
                )
            }
            database.moviesDao.insertAllMovies(*movies.asDatabaseModel())
            database.remoteKeysDao.insertAll(keys)

        }

        MediatorResult.Success(endOfPaginationReached = endOfPaginationReached)
    } catch (exception: IOException) {
        MediatorResult.Error(exception)
    } catch (exception: HttpException) {
        MediatorResult.Error(exception)
    }
}

【讨论】：

你最终是否在PagingSource.LoadResult.Page 和RemoteMediator.load() 中同时返回prevKey 中prevKey 的null 键和RemoteMediator.load() 中PREPEND 中的MediatorResult.Success(endOfPaginationReached = true)`？
嗨@dlam，是的，我是。我已在帖子中附加了我的 PREPEND 方法
我正在使用 TheMovieDB 的 API
您还需要在PagingSource.load 的PagingSource.LoadResult.Page 结果中返回null 以获取prevKey。你能附上你的PagingSource.load 实现吗？
我没有使用PagingSource，我已经切换到RemoteMediator 我已经附上了它的加载实现。