Jetpack Paging3 库对同一页面进行多次调用

Question

I am trying to implement Jetpack paging 3 library following thecodelab using room database as source of truth and a RemoteMediator. The app queries the google books api but for some reason when I perform a search it makes several calls to the same page. For example I get this in the log when I search fire without scrolling:

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)

My implementation is like this:

Repository:

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
    }
}

RemoteMediator:

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)
            }
        }
    }

}

ViewModel:

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
    }
}

Fragment:

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"
    }
}

Dao:

@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>
}

Service:

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

It would be great if someone point out what I am doing wrong and help fix this problem. Thank you

Answer 1

I am getting exactly the same error on another API. The version that doesn't use Remote Mediator works fine. When I add Remote Mediator it Refreshes, Appends, Then gets on a Prepend loop.

What I have tried so far:

1. Match the codelabs gradle dependency versions;
2. Placing logs on the viewmodel to see if I was making the request several times.
3. Placing logs on the RemoteMediator class to get the previous, current and next pages and the LoadType that is currently happening

This is what happens without scrolling:

    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

This is how my RemoteMediator Load is:

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)
    }
}

Answer 2

数据库中的primarykey如果来自网络请求可能会导致这个问题

Answer 3

The issue might be with incorrect primary key in your Books table. Can you share what is your

Answer 4

I spent some time with this problem and after reading user9694585 and dunkypie answers, I was able to solve my problem. In my case, I was using the movieDb API, exactly as Doilio Matsinhe, and I discover that I can't rely on the API id for two reasons:

• The remote ids were out of order on the same page. So if I save them on the table as is, I lose the order from the server.
• If the ids are out of order I couldn't reach the real first/last items of the page. I think this part that makes the mediator be in a loop.

So what I ended up doing was making the remote id as an attribute and creating a primary key with autoGenerate = true on movie table:

@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?
   ...
}

My RemoteMediator is very similar to Doilio Matsinhe. So I leave only the methods that were not in his answer:

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)
        }
    }
}

Basically is the one that we have on the codelab but using the movie.remoteId to get the remoteKey.id.