RemoteMediator

Defines a set of callbacks used to incrementally load data from a remote source into a local source wrapped by a PagingSource, e.g., loading data from network into a local db cache.

A RemoteMediator is registered by passing it to Pager's constructor.

RemoteMediator allows hooking into the following events:

• Stream initialization

• REFRESH signal driven from UI

• PagingSource returns a LoadResult which signals a boundary condition, i.e., the most recent LoadResult.Page in the PREPEND or APPEND direction has LoadResult.Page.prevKey or LoadResult.Page.nextKey set to null respectively.

import androidx.paging.ExperimentalPagingApi
import androidx.paging.LoadType
import androidx.paging.PagingState
import androidx.paging.RemoteMediator
import androidx.paging.samples.shared.ExampleBackendService
import androidx.paging.samples.shared.RoomDb
import androidx.paging.samples.shared.User
import androidx.room.withTransaction

/**
 * Sample RemoteMediator for a DB + Network based PagingData stream, which triggers network
 * requests to fetch additional items when a user scrolls to the end of the list of items stored
 * in DB.
 *
 * This sample loads a list of [User] items from an item-keyed Retrofit paginated source. This
 * source is "item-keyed" because we're loading the next page using information from the items
 * themselves (the ID param) as a key to fetch more data.
 */
@OptIn(ExperimentalPagingApi::class)
class ExampleRemoteMediator(
    private val query: String,
    private val database: RoomDb,
    private val networkService: ExampleBackendService
) : RemoteMediator<Int, User>() {
    val userDao = database.userDao()

    override suspend fun initialize(): InitializeAction {
        val cacheTimeout = TimeUnit.HOURS.convert(1, TimeUnit.MILLISECONDS)
        return if (System.currentTimeMillis() - userDao.lastUpdated() >= cacheTimeout) {
            // Cached data is up-to-date, so there is no need to re-fetch from network.
            InitializeAction.SKIP_INITIAL_REFRESH
        } else {
            // Need to refresh cached data from network; returning LAUNCH_INITIAL_REFRESH here
            // will also block RemoteMediator's APPEND and PREPEND from running until REFRESH
            // succeeds.
            InitializeAction.LAUNCH_INITIAL_REFRESH
        }
    }

    override suspend fun load(
        loadType: LoadType,
        state: PagingState<Int, User>
    ): MediatorResult {
        return try {
            // The network load method takes an optional `after=<user.id>` parameter. For every
            // page after the first, we pass the last user ID to let it continue from where it
            // left off. For REFRESH, pass `null` to load the first page.
            val loadKey = when (loadType) {
                LoadType.REFRESH -> null
                // In this example, we never need to prepend, since REFRESH will always load the
                // first page in the list. Immediately return, reporting end of pagination.
                LoadType.PREPEND -> return MediatorResult.Success(endOfPaginationReached = true)
                LoadType.APPEND -> {
                    val lastItem = state.lastItemOrNull()

                    // We must explicitly check if the last item is `null` when appending,
                    // since passing `null` to networkService is only valid for initial load.
                    // If lastItem is `null` it means no items were loaded after the initial
                    // REFRESH and there are no more items to load.
                    if (lastItem == null) {
                        return MediatorResult.Success(endOfPaginationReached = true)
                    }

                    lastItem.id
                }
            }

            // Suspending network load via Retrofit. This doesn't need to be wrapped in a
            // withContext(Dispatcher.IO) { ... } block since Retrofit's Coroutine CallAdapter
            // dispatches on a worker thread.
            val response = networkService.searchUsers(query = query, after = loadKey)

            database.withTransaction {
                if (loadType == LoadType.REFRESH) {
                    userDao.deleteByQuery(query)
                }

                // Insert new users into database, which invalidates the current
                // PagingData, allowing Paging to present the updates in the DB.
                userDao.insertAll(response.users)
            }

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

import androidx.paging.ExperimentalPagingApi
import androidx.paging.LoadType
import androidx.paging.PagingState
import androidx.paging.RemoteMediator
import androidx.paging.samples.shared.ExampleBackendService
import androidx.paging.samples.shared.RemoteKey
import androidx.paging.samples.shared.RoomDb
import androidx.paging.samples.shared.User
import androidx.room.withTransaction

/**
 * Sample RemoteMediator for a DB + Network based PagingData stream, which triggers network
 * requests to fetch additional items when a user scrolls to the end of the list of items stored
 * in DB.
 *
 * This sample loads a list of [User] via Retrofit from a page-keyed network service using
 * [String] tokens to load pages (each response has a next/previous token), and inserts them
 * into database.
 */
@OptIn(ExperimentalPagingApi::class)
class ExampleRemoteMediator(
    private val query: String,
    private val database: RoomDb,
    private val networkService: ExampleBackendService
) : RemoteMediator<Int, User>() {
    val userDao = database.userDao()
    val remoteKeyDao = database.remoteKeyDao()

    override suspend fun initialize(): InitializeAction {
        val cacheTimeout = TimeUnit.HOURS.convert(1, TimeUnit.MILLISECONDS)
        return if (System.currentTimeMillis() - userDao.lastUpdated() >= cacheTimeout) {
            // Cached data is up-to-date, so there is no need to re-fetch from network.
            InitializeAction.SKIP_INITIAL_REFRESH
        } else {
            // Need to refresh cached data from network; returning LAUNCH_INITIAL_REFRESH here
            // will also block RemoteMediator's APPEND and PREPEND from running until REFRESH
            // succeeds.
            InitializeAction.LAUNCH_INITIAL_REFRESH
        }
    }

    override suspend fun load(
        loadType: LoadType,
        state: PagingState<Int, User>
    ): MediatorResult {
        return try {
            // The network load method takes an optional [String] parameter. For every page
            // after the first, we pass the [String] token returned from the previous page to
            // let it continue from where it left off. For REFRESH, pass `null` to load the
            // first page.
            val loadKey = when (loadType) {
                LoadType.REFRESH -> null
                // In this example, we never need to prepend, since REFRESH will always load the
                // first page in the list. Immediately return, reporting end of pagination.
                LoadType.PREPEND -> return MediatorResult.Success(endOfPaginationReached = true)
                // Query remoteKeyDao for the next RemoteKey.
                LoadType.APPEND -> {
                    val remoteKey = database.withTransaction {
                        remoteKeyDao.remoteKeyByQuery(query)
                    }

                    // We must explicitly check if the page key is `null` when appending,
                    // since `null` is only valid for initial load. If we receive `null`
                    // for APPEND, that means we have reached the end of pagination and
                    // there are no more items to load.
                    if (remoteKey.nextKey == null) {
                        return MediatorResult.Success(endOfPaginationReached = true)
                    }

                    remoteKey.nextKey
                }
            }

            // Suspending network load via Retrofit. This doesn't need to be wrapped in a
            // withContext(Dispatcher.IO) { ... } block since Retrofit's Coroutine CallAdapter
            // dispatches on a worker thread.
            val response = networkService.searchUsers(query, loadKey)

            // Store loaded data, and next key in transaction, so that they're always consistent
            database.withTransaction {
                if (loadType == LoadType.REFRESH) {
                    remoteKeyDao.deleteByQuery(query)
                    userDao.deleteByQuery(query)
                }

                // Update RemoteKey for this query.
                remoteKeyDao.insertOrReplace(RemoteKey(query, response.nextKey))

                // Insert new users into database, which invalidates the current
                // PagingData, allowing Paging to present the updates in the DB.
                userDao.insertAll(response.users)
            }

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

Summary

<Key : Any, Value : Any> RemoteMediator()

open suspend fun initialize(): RemoteMediator.InitializeAction

abstract suspend fun load(loadType: LoadType, state: PagingState<Key, Value>): RemoteMediator.MediatorResult