xref: /paging/paging-common/src/commonTest/kotlin/androidx/paging/RemoteMediatorAccessorTest.kt

/*
 * Copyright 2019 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package androidx.paging

import androidx.kruth.assertThat
import androidx.paging.LoadType.APPEND
import androidx.paging.LoadType.PREPEND
import androidx.paging.LoadType.REFRESH
import androidx.paging.PagingSource.LoadResult.Page.Companion.COUNT_UNDEFINED
import androidx.paging.RemoteMediator.InitializeAction.LAUNCH_INITIAL_REFRESH
import androidx.paging.RemoteMediator.InitializeAction.SKIP_INITIAL_REFRESH
import androidx.paging.RemoteMediatorMock.LoadEvent
import androidx.paging.TestPagingSource.Companion.LOAD_ERROR
import androidx.paging.internal.AtomicBoolean
import kotlin.test.Test
import kotlin.test.assertEquals
import kotlin.test.fail
import kotlinx.coroutines.ExperimentalCoroutinesApi
import kotlinx.coroutines.delay
import kotlinx.coroutines.launch
import kotlinx.coroutines.test.StandardTestDispatcher
import kotlinx.coroutines.test.TestScope
import kotlinx.coroutines.test.UnconfinedTestDispatcher
import kotlinx.coroutines.test.advanceTimeBy
import kotlinx.coroutines.test.advanceUntilIdle
import kotlinx.coroutines.test.runCurrent
import kotlinx.coroutines.test.runTest

@OptIn(ExperimentalCoroutinesApi::class, ExperimentalPagingApi::class)
class RemoteMediatorAccessorTest {
    private val testScope = TestScope(UnconfinedTestDispatcher())
    private var mockStateId = 0

    // creates a unique state using the anchor position to be able to do equals check in assertions
    private fun createMockState(anchorPosition: Int? = mockStateId++): PagingState<Int, Int> {
        return PagingState(
            pages = listOf(),
            anchorPosition = anchorPosition,
            config = PagingConfig(10),
            leadingPlaceholderCount = COUNT_UNDEFINED
        )
    }

    @Test
    fun requestLoadIfRefreshAllowed_noop() =
        testScope.runTest {
            val remoteMediator =
                RemoteMediatorMock(loadDelay = 100).apply {
                    initializeResult = SKIP_INITIAL_REFRESH
                }
            val remoteMediatorAccessor = createAccessor(remoteMediator)
            val pagingState = PagingState<Int, Int>(listOf(), null, PagingConfig(1), 0)

            remoteMediatorAccessor.requestRefreshIfAllowed(pagingState)
            advanceUntilIdle()
            assertThat(remoteMediator.newLoadEvents).isEmpty()
        }

    @Test
    fun requestLoadIfRefreshAllowed_simple() =
        testScope.runTest {
            val remoteMediator =
                RemoteMediatorMock(loadDelay = 100).apply {
                    initializeResult = LAUNCH_INITIAL_REFRESH
                }
            val remoteMediatorAccessor = createAccessor(remoteMediator)
            val pagingState = PagingState<Int, Int>(listOf(), null, PagingConfig(1), 0)

            remoteMediatorAccessor.allowRefresh()
            remoteMediatorAccessor.requestRefreshIfAllowed(pagingState)
            advanceUntilIdle()
            assertThat(remoteMediator.newLoadEvents)
                .containsExactly(LoadEvent(loadType = REFRESH, state = pagingState))

            // allowRefresh should only allow one successful request to go through.
            remoteMediatorAccessor.requestRefreshIfAllowed(pagingState)
            advanceUntilIdle()
            assertThat(remoteMediator.newLoadEvents).isEmpty()
        }

    @Test
    fun requestLoadIfRefreshAllowed_retry() =
        testScope.runTest {
            val remoteMediator =
                RemoteMediatorMock(loadDelay = 100).apply {
                    initializeResult = LAUNCH_INITIAL_REFRESH
                }
            val remoteMediatorAccessor = createAccessor(remoteMediator)
            val pagingState = PagingState<Int, Int>(listOf(), null, PagingConfig(1), 0)

            remoteMediator.loadCallback = { _, _ ->
                RemoteMediator.MediatorResult.Error(Exception())
            }

            remoteMediatorAccessor.allowRefresh()
            remoteMediatorAccessor.requestRefreshIfAllowed(pagingState)
            advanceUntilIdle()
            assertThat(remoteMediator.newLoadEvents)
                .containsExactly(LoadEvent(loadType = REFRESH, state = pagingState))

            remoteMediator.loadCallback = { _, _ ->
                RemoteMediator.MediatorResult.Success(endOfPaginationReached = false)
            }

            remoteMediatorAccessor.retryFailed(pagingState)
            advanceUntilIdle()
            assertThat(remoteMediator.newLoadEvents)
                .containsExactly(LoadEvent(loadType = REFRESH, state = pagingState))
        }

    @Test
    fun requestLoad_queuesBoundaryBehindRefresh() =
        testScope.runTest {
            val remoteMediator = RemoteMediatorMock(loadDelay = 100)
            val remoteMediatorAccessor = createAccessor(remoteMediator)
            val firstState = createMockState()
            val secondState = createMockState()

            remoteMediatorAccessor.requestLoad(REFRESH, firstState)
            advanceTimeBy(50) // Start remote refresh, but do not let it finish.
            assertThat(remoteMediator.newLoadEvents).containsExactly(LoadEvent(REFRESH, firstState))
            assertThat(remoteMediatorAccessor.state.value)
                .isEqualTo(
                    LoadStates(
                        refresh = LoadState.Loading,
                        prepend = LoadState.NotLoading.Incomplete,
                        append = LoadState.NotLoading.Incomplete
                    )
                )

            // Queue a boundary requests, but it should not launch since refresh is running.
            remoteMediatorAccessor.requestLoad(PREPEND, firstState)
            remoteMediatorAccessor.requestLoad(APPEND, firstState)
            assertThat(remoteMediator.newLoadEvents).isEmpty()
            assertThat(remoteMediatorAccessor.state.value)
                .isEqualTo(
                    LoadStates(
                        refresh = LoadState.Loading,
                        prepend = LoadState.Loading,
                        append = LoadState.Loading
                    )
                )

            // Queue more boundary requests, but with an updated PagingState.
            remoteMediatorAccessor.requestLoad(PREPEND, secondState)
            remoteMediatorAccessor.requestLoad(APPEND, secondState)
            assertThat(remoteMediator.newLoadEvents).isEmpty()
            assertThat(remoteMediatorAccessor.state.value)
                .isEqualTo(
                    LoadStates(
                        refresh = LoadState.Loading,
                        prepend = LoadState.Loading,
                        append = LoadState.Loading
                    )
                )

            // Now wait until all queued requests finish running
            advanceUntilIdle()
            assertThat(remoteMediator.newLoadEvents)
                .containsExactly(
                    LoadEvent(PREPEND, secondState),
                    LoadEvent(APPEND, secondState),
                )
            assertThat(remoteMediatorAccessor.state.value)
                .isEqualTo(
                    LoadStates(
                        refresh = LoadState.NotLoading.Incomplete,
                        prepend = LoadState.NotLoading.Incomplete,
                        append = LoadState.NotLoading.Incomplete
                    )
                )
        }

    @Test
    fun requestLoad_cancelledBoundaryRetriesAfterRefresh() =
        testScope.runTest {
            val remoteMediator =
                RemoteMediatorMock(loadDelay = 100).apply {
                    initializeResult = SKIP_INITIAL_REFRESH
                }
            val remoteMediatorAccessor = createAccessor(remoteMediator)
            val firstState = createMockState()

            // Launch boundary calls, but do not let them finish.
            remoteMediatorAccessor.requestLoad(PREPEND, firstState)
            // Single runner should prevent append from triggering, but it should still be queued.
            remoteMediatorAccessor.requestLoad(APPEND, firstState)
            advanceTimeBy(50)
            assertThat(remoteMediator.newLoadEvents)
                .containsExactly(
                    LoadEvent(PREPEND, firstState),
                )

            // Launch refresh, which should cancel running boundary calls
            remoteMediatorAccessor.requestLoad(REFRESH, firstState)
            advanceTimeBy(50)
            assertThat(remoteMediator.newLoadEvents).containsExactly(LoadEvent(REFRESH, firstState))

            // Let refresh finish, retrying cancelled boundary calls
            advanceUntilIdle()
            assertThat(remoteMediator.newLoadEvents)
                .containsExactly(
                    LoadEvent(PREPEND, firstState),
                    LoadEvent(APPEND, firstState),
                )
        }

    @Test
    fun requestLoad_queuesBoundaryAfterRefreshFails() =
        testScope.runTest {
            val firstState = createMockState()
            val secondState = createMockState()
            val remoteMediator =
                RemoteMediatorMock(loadDelay = 100).apply {
                    initializeResult = RemoteMediator.InitializeAction.LAUNCH_INITIAL_REFRESH
                    loadCallback = { loadType, state ->
                        // Only error out on first refresh.
                        if (loadType == REFRESH && state == firstState) {
                            RemoteMediator.MediatorResult.Error(throwable = LOAD_ERROR)
                        } else {
                            RemoteMediator.MediatorResult.Success(endOfPaginationReached = false)
                        }
                    }
                }
            val remoteMediatorAccessor = createAccessor(remoteMediator)

            // Queue up some remote boundary calls, which will not run immediately because they
            // depend on refresh.
            remoteMediatorAccessor.requestLoad(PREPEND, firstState)
            remoteMediatorAccessor.requestLoad(APPEND, firstState)

            // Trigger refresh, letting it fail.
            remoteMediatorAccessor.requestLoad(REFRESH, firstState)
            advanceUntilIdle()
            // Boundary calls should be queued, but not started.
            assertThat(remoteMediator.newLoadEvents)
                .containsExactly(
                    LoadEvent(REFRESH, firstState),
                )
            // Although boundary calls are queued, they should not trigger or update LoadState since
            // they are waiting for refresh to succeed.
            assertThat(remoteMediatorAccessor.state.value)
                .isEqualTo(
                    LoadStates(
                        refresh = LoadState.Error(LOAD_ERROR),
                        prepend = LoadState.NotLoading.Incomplete,
                        append = LoadState.NotLoading.Incomplete
                    )
                )

            // Let refresh finish, triggering queued boundary calls.
            remoteMediatorAccessor.retryFailed(secondState)
            advanceUntilIdle()
            assertThat(remoteMediator.newLoadEvents)
                .containsExactly(
                    LoadEvent(REFRESH, secondState),
                    LoadEvent(PREPEND, firstState),
                    LoadEvent(APPEND, firstState),
                )
            assertThat(remoteMediatorAccessor.state.value)
                .isEqualTo(
                    LoadStates(
                        refresh = LoadState.NotLoading.Incomplete,
                        prepend = LoadState.NotLoading.Incomplete,
                        append = LoadState.NotLoading.Incomplete
                    )
                )
        }

    @Test
    fun requestLoad_refreshEndOfPaginationReachedClearsBoundaryCalls() =
        testScope.runTest {
            val remoteMediator =
                RemoteMediatorMock(loadDelay = 100).apply {
                    initializeResult = RemoteMediator.InitializeAction.LAUNCH_INITIAL_REFRESH
                    loadCallback = { _, _ ->
                        RemoteMediator.MediatorResult.Success(endOfPaginationReached = true)
                    }
                }
            val remoteMediatorAccessor = createAccessor(remoteMediator)
            val firstState = createMockState()

            // Queue up some remote boundary calls, which will not run immediately because they
            // depend on refresh.
            remoteMediatorAccessor.requestLoad(PREPEND, firstState)
            remoteMediatorAccessor.requestLoad(APPEND, firstState)

            // Trigger refresh and let it mark endOfPaginationReached
            remoteMediatorAccessor.requestLoad(REFRESH, firstState)
            advanceUntilIdle()

            // Ensure boundary calls are not triggered since they should be cleared by
            // endOfPaginationReached from refresh.
            assertThat(remoteMediator.newLoadEvents).containsExactly(LoadEvent(REFRESH, firstState))
            // Although boundary calls are queued, they should not trigger or update LoadState since
            // they are waiting for refresh.
            assertThat(remoteMediatorAccessor.state.value)
                .isEqualTo(
                    LoadStates(
                        refresh = LoadState.NotLoading.Incomplete,
                        prepend = LoadState.NotLoading.Complete,
                        append = LoadState.NotLoading.Complete
                    )
                )
        }

    @Test
    fun load_reportsPrependLoadState() =
        testScope.runTest {
            val emptyState =
                PagingState<Int, Int>(listOf(), null, PagingConfig(10), COUNT_UNDEFINED)
            val remoteMediator = RemoteMediatorMock(loadDelay = 1000)
            val remoteMediatorAccessor = createAccessor(remoteMediator)

            // Assert initial state is NotLoading.Incomplete.
            assertEquals(
                LoadStates.IDLE.copy(prepend = LoadState.NotLoading.Incomplete),
                remoteMediatorAccessor.state.value,
            )

            // Start a PREPEND load.
            remoteMediatorAccessor.requestLoad(
                loadType = PREPEND,
                pagingState = emptyState,
            )

            // Assert state is immediately set to Loading.
            assertEquals(
                LoadStates.IDLE.copy(prepend = LoadState.Loading),
                remoteMediatorAccessor.state.value,
            )

            // Wait for load to finish.
            advanceUntilIdle()

            // Assert state is set to NotLoading.Incomplete.
            assertEquals(
                LoadStates.IDLE.copy(prepend = LoadState.NotLoading.Incomplete),
                remoteMediatorAccessor.state.value,
            )

            // Start a PREPEND load which results in endOfPaginationReached = true.
            remoteMediator.loadCallback = { _, _ ->
                RemoteMediator.MediatorResult.Success(endOfPaginationReached = true)
            }
            remoteMediatorAccessor.requestLoad(
                loadType = PREPEND,
                pagingState = emptyState,
            )

            // Wait for load to finish.
            advanceUntilIdle()

            // Assert state is set to NotLoading.Incomplete.
            assertEquals(
                LoadStates.IDLE.copy(prepend = LoadState.NotLoading.Complete),
                remoteMediatorAccessor.state.value,
            )
        }

    @Test
    fun load_reportsAppendLoadState() =
        testScope.runTest {
            val emptyState =
                PagingState<Int, Int>(listOf(), null, PagingConfig(10), COUNT_UNDEFINED)
            val remoteMediator = RemoteMediatorMock(loadDelay = 1000)
            val remoteMediatorAccessor = createAccessor(remoteMediator)

            // Assert initial state is NotLoading.Incomplete.
            assertEquals(
                LoadStates.IDLE.copy(prepend = LoadState.NotLoading.Incomplete),
                remoteMediatorAccessor.state.value,
            )

            // Start a APPEND load.
            remoteMediatorAccessor.requestLoad(
                loadType = APPEND,
                pagingState = emptyState,
            )

            // Assert state is immediately set to Loading.
            assertEquals(
                LoadStates.IDLE.copy(append = LoadState.Loading),
                remoteMediatorAccessor.state.value,
            )

            // Wait for load to finish.
            advanceUntilIdle()

            // Assert state is set to NotLoading.Incomplete.
            assertEquals(
                LoadStates.IDLE.copy(append = LoadState.NotLoading.Incomplete),
                remoteMediatorAccessor.state.value,
            )

            // Start a APPEND load which results in endOfPaginationReached = true.
            remoteMediator.loadCallback = { _, _ ->
                RemoteMediator.MediatorResult.Success(endOfPaginationReached = true)
            }
            remoteMediatorAccessor.requestLoad(
                loadType = APPEND,
                pagingState = emptyState,
            )

            // Wait for load to finish.
            advanceUntilIdle()

            // Assert state is set to NotLoading.Incomplete.
            assertEquals(
                LoadStates.IDLE.copy(append = LoadState.NotLoading.Complete),
                remoteMediatorAccessor.state.value,
            )
        }

    @Test
    fun load_conflatesPrepend() =
        testScope.runTest {
            val remoteMediator = RemoteMediatorMock(loadDelay = 1000)
            val remoteMediatorAccessor = createAccessor(remoteMediator)

            remoteMediatorAccessor.requestLoad(
                loadType = PREPEND,
                pagingState = PagingState(listOf(), null, PagingConfig(10), COUNT_UNDEFINED)
            )

            remoteMediatorAccessor.requestLoad(
                loadType = PREPEND,
                pagingState = PagingState(listOf(), null, PagingConfig(10), COUNT_UNDEFINED)
            )

            // Assert that exactly one load request was started.
            assertEquals(1, remoteMediator.newLoadEvents.size)

            // Fast-forward time until both load requests jobs complete.
            advanceUntilIdle()

            // Assert that the second load request was skipped since it was launched while the first
            // load request was still running.
            assertEquals(0, remoteMediator.newLoadEvents.size)
        }

    @Test
    fun load_conflatesAppend() =
        testScope.runTest {
            val remoteMediator = RemoteMediatorMock(loadDelay = 1000)
            val remoteMediatorAccessor = createAccessor(remoteMediator)

            remoteMediatorAccessor.requestLoad(
                loadType = APPEND,
                pagingState = PagingState(listOf(), null, PagingConfig(10), COUNT_UNDEFINED)
            )

            remoteMediatorAccessor.requestLoad(
                loadType = APPEND,
                pagingState = PagingState(listOf(), null, PagingConfig(10), COUNT_UNDEFINED)
            )

            // Assert that exactly one load request was started.
            assertEquals(1, remoteMediator.newLoadEvents.size)

            // Fast-forward time until both load requests jobs complete.
            advanceUntilIdle()

            // Assert that the second load request was skipped since it was launched while the first
            // load request was still running.
            assertEquals(0, remoteMediator.newLoadEvents.size)
        }

    @Test
    fun load_conflatesRefresh() =
        testScope.runTest {
            val remoteMediator = RemoteMediatorMock(loadDelay = 1000)
            val remoteMediatorAccessor = createAccessor(remoteMediator)

            remoteMediatorAccessor.requestLoad(
                loadType = REFRESH,
                pagingState = PagingState(listOf(), null, PagingConfig(10), COUNT_UNDEFINED)
            )

            remoteMediatorAccessor.requestLoad(
                loadType = REFRESH,
                pagingState = PagingState(listOf(), null, PagingConfig(10), COUNT_UNDEFINED)
            )

            // Assert that exactly one load request was started.
            assertEquals(1, remoteMediator.newLoadEvents.size)

            // Fast-forward time until both load requests jobs complete.
            advanceUntilIdle()

            // Assert that the second load request was skipped since it was launched while the first
            // load request was still running.
            assertEquals(0, remoteMediator.newLoadEvents.size)
        }

    @Test
    fun load_concurrentInitializeJobCancelsBoundaryJobs() =
        testScope.runTest {
            val emptyState =
                PagingState<Int, Int>(listOf(), null, PagingConfig(10), COUNT_UNDEFINED)
            val remoteMediator =
                object : RemoteMediatorMock(loadDelay = 1000) {
                    var loading = AtomicBoolean(false)

                    override suspend fun load(
                        loadType: LoadType,
                        state: PagingState<Int, Int>
                    ): MediatorResult {
                        if (!loading.compareAndSet(false, true)) fail("Concurrent load")

                        return try {
                            super.load(loadType, state)
                        } finally {
                            loading.set(false)
                        }
                    }
                }
            val remoteMediatorAccessor = createAccessor(remoteMediator)

            remoteMediatorAccessor.requestLoad(loadType = PREPEND, pagingState = emptyState)

            remoteMediatorAccessor.requestLoad(loadType = APPEND, pagingState = emptyState)

            // Start prependJob and appendJob, but do not let them finish.
            advanceTimeBy(500)

            // Assert that only the PREPEND RemoteMediator.load() call was made.
            assertEquals(listOf(LoadEvent(PREPEND, emptyState)), remoteMediator.newLoadEvents)

            // Start refreshJob
            remoteMediatorAccessor.requestLoad(loadType = REFRESH, pagingState = emptyState)

            // Give prependJob enough time to be cancelled and refresh started due to higher
            // priority
            advanceTimeBy(500)

            assertEquals(listOf(LoadEvent(REFRESH, emptyState)), remoteMediator.newLoadEvents)
            // assert that all of them are in loading state as we don't know if refresh will succeed
            // if refresh fails, we would retry append / prepend
            assertEquals(
                LoadStates(
                    refresh = LoadState.Loading,
                    append = LoadState.Loading,
                    prepend = LoadState.Loading
                ),
                remoteMediatorAccessor.state.value
            )

            // Wait for all outstanding / queued jobs to finish.
            advanceUntilIdle()

            // Assert all outstanding / queued jobs finished.
            assertEquals(
                LoadStates(
                    refresh = LoadState.NotLoading.Incomplete,
                    append = LoadState.NotLoading.Incomplete,
                    prepend = LoadState.NotLoading.Incomplete
                ),
                remoteMediatorAccessor.state.value
            )

            // Queued boundary requests should be triggered, even though they are out-of-date.
            assertThat(remoteMediator.newLoadEvents)
                .containsExactly(
                    LoadEvent(PREPEND, emptyState),
                    LoadEvent(APPEND, emptyState),
                )
        }

    @Test
    fun load_concurrentBoundaryJobsRunsSerially() =
        testScope.runTest {
            val emptyState =
                PagingState<Int, Int>(listOf(), null, PagingConfig(10), COUNT_UNDEFINED)
            val remoteMediator =
                object : RemoteMediatorMock(loadDelay = 1000) {
                    var loading = AtomicBoolean(false)

                    override suspend fun load(
                        loadType: LoadType,
                        state: PagingState<Int, Int>
                    ): MediatorResult {
                        if (!loading.compareAndSet(false, true)) fail("Concurrent load")

                        return try {
                            super.load(loadType, state)
                        } finally {
                            loading.set(false)
                        }
                    }
                }

            val remoteMediatorAccessor = createAccessor(remoteMediator)

            remoteMediatorAccessor.requestLoad(loadType = PREPEND, pagingState = emptyState)

            remoteMediatorAccessor.requestLoad(loadType = APPEND, pagingState = emptyState)

            // Assert that only one job runs due to second job joining the first before starting.
            assertEquals(1, remoteMediator.newLoadEvents.size)

            // Advance some time, but not enough to finish first load.
            advanceTimeBy(500)
            assertEquals(0, remoteMediator.newLoadEvents.size)

            // Assert that second job starts after first finishes.
            advanceTimeBy(500)
            runCurrent()
            assertEquals(1, remoteMediator.newLoadEvents.size)

            // Allow second job to finish.
            advanceTimeBy(1000)
        }

    @Test
    fun ignoreAppendPrependWhenRefreshIsRequired() {
        val remoteMediatorMock =
            RemoteMediatorMock().apply {
                initializeResult = RemoteMediator.InitializeAction.LAUNCH_INITIAL_REFRESH
            }
        val accessor = testScope.createAccessor(remoteMediatorMock)
        accessor.requestLoad(APPEND, createMockState())
        accessor.requestLoad(PREPEND, createMockState())
        testScope.advanceUntilIdle()
        assertThat(remoteMediatorMock.loadEvents).isEmpty()
    }

    @Test
    fun allowAppendPrependWhenRefreshIsNotRequired() {
        val remoteMediatorMock =
            RemoteMediatorMock().apply { initializeResult = SKIP_INITIAL_REFRESH }
        val accessor = testScope.createAccessor(remoteMediatorMock)

        val appendState = createMockState(1)
        val prependState = createMockState(2)
        accessor.requestLoad(APPEND, appendState)
        accessor.requestLoad(PREPEND, prependState)
        testScope.advanceUntilIdle()
        assertThat(remoteMediatorMock.loadEvents)
            .containsExactly(LoadEvent(APPEND, appendState), LoadEvent(PREPEND, prependState))
    }

    @Test
    fun ignoreAppendPrependBeforeRefresh() {
        val remoteMediatorMock =
            RemoteMediatorMock(loadDelay = 100).apply {
                initializeResult = RemoteMediator.InitializeAction.LAUNCH_INITIAL_REFRESH
            }
        val testScope = TestScope(StandardTestDispatcher())
        val accessor = testScope.createAccessor(remoteMediatorMock)

        val refreshState = createMockState()
        accessor.requestLoad(REFRESH, refreshState)
        // no requests yet since scope is not triggered
        assertThat(remoteMediatorMock.loadEvents).isEmpty()

        // advance enough to trigger the request, not enough to complete
        testScope.advanceTimeBy(50)
        // these should be ignored
        accessor.requestLoad(REFRESH, createMockState())

        testScope.advanceTimeBy(1)
        val appendState = createMockState()
        accessor.requestLoad(APPEND, appendState)

        val prependState = createMockState()
        testScope.advanceTimeBy(1)
        accessor.requestLoad(PREPEND, prependState)

        assertThat(remoteMediatorMock.newLoadEvents)
            .containsExactly(LoadEvent(REFRESH, refreshState))

        // now advance enough that we can accept append prepend
        testScope.advanceUntilIdle()
        // queued append/prepend should be executed afterwards.
        assertThat(remoteMediatorMock.newLoadEvents)
            .containsExactly(
                LoadEvent(APPEND, appendState),
                LoadEvent(PREPEND, prependState),
            )

        val otherPrependState = createMockState()
        val otherAppendState = createMockState()
        accessor.requestLoad(PREPEND, otherPrependState)
        accessor.requestLoad(APPEND, otherAppendState)

        testScope.advanceTimeBy(50)
        assertThat(remoteMediatorMock.newLoadEvents)
            .containsExactly(LoadEvent(PREPEND, otherPrependState))
        // while prepend running, any more requests should be ignored
        accessor.requestLoad(PREPEND, createMockState())
        testScope.advanceTimeBy(10)
        assertThat(remoteMediatorMock.newLoadEvents).isEmpty()

        testScope.advanceTimeBy(41)
        assertThat(remoteMediatorMock.newLoadEvents)
            .containsExactly(LoadEvent(APPEND, otherAppendState))
        accessor.requestLoad(APPEND, createMockState())
        // while append running, any more requests should be ignored
        accessor.requestLoad(APPEND, createMockState())
        testScope.advanceUntilIdle()
        assertThat(remoteMediatorMock.newLoadEvents).isEmpty()

        // now the work is done, we can add more
        val newAppendState = createMockState()
        accessor.requestLoad(APPEND, newAppendState)
        testScope.advanceUntilIdle()
        assertThat(remoteMediatorMock.newLoadEvents)
            .containsExactly(LoadEvent(APPEND, newAppendState))
    }

    @Test
    fun dropAppendPrependIfRefreshIsTriggered() {
        val remoteMediatorMock =
            RemoteMediatorMock(loadDelay = 100).apply { initializeResult = SKIP_INITIAL_REFRESH }
        val accessor = testScope.createAccessor(remoteMediatorMock)
        val initialAppend = createMockState()
        accessor.requestLoad(APPEND, initialAppend)
        testScope.advanceTimeBy(50)
        assertThat(remoteMediatorMock.newLoadEvents)
            .containsExactly(LoadEvent(APPEND, initialAppend))
        // now before that append finishes, trigger a refresh
        val newRefresh = createMockState()
        accessor.requestLoad(REFRESH, newRefresh)
        testScope.advanceTimeBy(10)
        // check that we immediately get the new refresh because we'll cancel the append
        assertThat(remoteMediatorMock.newLoadEvents).containsExactly(LoadEvent(REFRESH, newRefresh))
        assertThat(remoteMediatorMock.incompleteEvents)
            .containsExactly(LoadEvent(APPEND, initialAppend))
    }

    @Test
    fun loadEvents() {
        val remoteMediatorMock =
            RemoteMediatorMock(loadDelay = 100).apply { initializeResult = SKIP_INITIAL_REFRESH }
        val accessor = testScope.createAccessor(remoteMediatorMock)

        // Initial state
        assertThat(accessor.state.value).isEqualTo(LoadStates.IDLE)

        // Append request should go through since it doesn't require refresh
        val firstAppendState = createMockState()
        accessor.requestLoad(APPEND, firstAppendState)
        testScope.advanceTimeBy(40)
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.NotLoading.Incomplete,
                    prepend = LoadState.NotLoading.Incomplete,
                    append = LoadState.Loading
                )
            )
        assertThat(remoteMediatorMock.newLoadEvents)
            .containsExactly(LoadEvent(APPEND, firstAppendState))

        // Trigger refresh, cancelling remote append
        val firstRefreshState = createMockState()
        accessor.requestLoad(REFRESH, firstRefreshState)
        testScope.advanceTimeBy(1)
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Loading,
                    prepend = LoadState.NotLoading.Incomplete,
                    append = LoadState.Loading
                )
            )
        // advance enough to complete refresh
        testScope.advanceUntilIdle()
        // assert that we receive refresh, and append is retried since it was cancelled
        assertThat(remoteMediatorMock.newLoadEvents)
            .containsExactly(
                LoadEvent(REFRESH, firstRefreshState),
                LoadEvent(APPEND, firstAppendState),
            )
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.NotLoading.Incomplete,
                    prepend = LoadState.NotLoading.Incomplete,
                    append = LoadState.NotLoading.Incomplete
                )
            )

        val appendState = createMockState()
        accessor.requestLoad(APPEND, appendState)
        val prependState = createMockState()
        accessor.requestLoad(PREPEND, prependState)
        testScope.advanceTimeBy(50)
        // both states should be set to loading even though prepend is not really running
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.NotLoading.Incomplete,
                    prepend = LoadState.Loading,
                    append = LoadState.Loading
                )
            )
        assertThat(remoteMediatorMock.newLoadEvents).containsExactly(LoadEvent(APPEND, appendState))
        // advance enough to trigger prepend
        testScope.advanceTimeBy(51)
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.NotLoading.Incomplete,
                    prepend = LoadState.Loading,
                    append = LoadState.NotLoading.Incomplete
                )
            )
        assertThat(remoteMediatorMock.newLoadEvents)
            .containsExactly(LoadEvent(PREPEND, prependState))
        testScope.advanceUntilIdle()
        val exception = Throwable()
        remoteMediatorMock.loadCallback = { type, _ ->
            if (type == PREPEND) {
                RemoteMediator.MediatorResult.Error(exception)
            } else {
                null
            }
        }
        accessor.requestLoad(APPEND, createMockState())
        accessor.requestLoad(PREPEND, createMockState())
        testScope.advanceUntilIdle()
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.NotLoading.Incomplete,
                    prepend = LoadState.Error(exception),
                    append = LoadState.NotLoading.Incomplete
                )
            )
        // now complete append, a.k.a. endOfPaginationReached
        remoteMediatorMock.loadCallback = { type, _ ->
            if (type == APPEND) {
                RemoteMediator.MediatorResult.Success(endOfPaginationReached = true)
            } else {
                null
            }
        }
        accessor.requestLoad(APPEND, createMockState())
        testScope.advanceUntilIdle()
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.NotLoading.Incomplete,
                    prepend = LoadState.Error(exception),
                    append = LoadState.NotLoading.Complete
                )
            )
        // clear events
        remoteMediatorMock.newLoadEvents
        // another append request should just be ignored
        accessor.requestLoad(APPEND, createMockState())
        testScope.advanceUntilIdle()
        assertThat(remoteMediatorMock.newLoadEvents).isEmpty()
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.NotLoading.Incomplete,
                    prepend = LoadState.Error(exception),
                    append = LoadState.NotLoading.Complete
                )
            )
        val refreshState = createMockState()
        accessor.requestLoad(REFRESH, refreshState)
        testScope.advanceTimeBy(50)
        // prepend error state is still present
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Loading,
                    prepend = LoadState.Error(exception),
                    append = LoadState.NotLoading.Complete
                )
            )
        testScope.advanceUntilIdle()
        // if refresh succeeds, it will clear the error state for refresh
        assertThat(accessor.state.value).isEqualTo(LoadStates.IDLE)
        assertThat(remoteMediatorMock.newLoadEvents)
            .containsExactly(LoadEvent(REFRESH, refreshState))
    }

    @Test
    fun retry_refresh() {
        val remoteMediatorMock =
            RemoteMediatorMock(loadDelay = 100).apply { initializeResult = SKIP_INITIAL_REFRESH }
        val exception = Exception()
        val accessor = testScope.createAccessor(remoteMediatorMock)
        remoteMediatorMock.loadCallback = { loadType, _ ->
            delay(100)
            if (loadType == REFRESH) {
                RemoteMediator.MediatorResult.Error(exception)
            } else {
                null
            }
        }
        val firstRefreshState = createMockState()
        accessor.requestLoad(REFRESH, firstRefreshState)
        assertThat(remoteMediatorMock.newLoadEvents)
            .containsExactly(LoadEvent(REFRESH, firstRefreshState))
        testScope.advanceUntilIdle()
        assertThat(accessor.state.value.refresh).isEqualTo(LoadState.Error(exception))
        val retryState = createMockState()
        accessor.retryFailed(retryState)
        testScope.advanceUntilIdle()
        assertThat(remoteMediatorMock.newLoadEvents).containsExactly(LoadEvent(REFRESH, retryState))
    }

    @Test
    fun failedRefreshShouldNotAllowAppendPrependIfRefreshIsRequired() {
        val remoteMediatorMock =
            RemoteMediatorMock(loadDelay = 100).apply {
                initializeResult = RemoteMediator.InitializeAction.LAUNCH_INITIAL_REFRESH
            }
        val exception = Exception()
        val accessor = testScope.createAccessor(remoteMediatorMock)
        remoteMediatorMock.loadCallback = { _, _ ->
            delay(100)
            RemoteMediator.MediatorResult.Error(exception)
        }
        val initialState = createMockState()
        accessor.requestLoad(REFRESH, initialState)
        // even though we are sending append prepend, they won't really trigger since refresh will
        // fail

        // ensure that we didn't set append/prepend to loading when refresh is required
        assertThat(accessor.state.value)
            .isEqualTo(LoadStates.IDLE.modifyState(REFRESH, LoadState.Loading))
        testScope.advanceUntilIdle()
        // make sure only refresh has happened
        assertThat(remoteMediatorMock.newLoadEvents)
            .containsExactly(LoadEvent(REFRESH, initialState))
        assertThat(accessor.state.value)
            .isEqualTo(LoadStates.IDLE.modifyState(REFRESH, LoadState.Error(exception)))
    }

    @Test
    fun failedRefreshShouldAllowAppendPrependIfRefreshIsNotRequired() {
        val remoteMediatorMock =
            RemoteMediatorMock(loadDelay = 100).apply { initializeResult = SKIP_INITIAL_REFRESH }
        val exception = Exception()
        val accessor = testScope.createAccessor(remoteMediatorMock)
        remoteMediatorMock.loadCallback = { loadType, _ ->
            delay(100)
            if (loadType != APPEND) {
                RemoteMediator.MediatorResult.Error(exception)
            } else {
                null // let append succeed
            }
        }
        val initialState = createMockState()
        accessor.requestLoad(REFRESH, initialState)
        accessor.requestLoad(PREPEND, initialState)
        accessor.requestLoad(APPEND, initialState)
        // make sure we optimistically updated append prepend states
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Loading,
                    append = LoadState.Loading,
                    prepend = LoadState.Loading
                )
            )
        testScope.advanceUntilIdle()
        // make sure all requests did happen eventually
        assertThat(remoteMediatorMock.newLoadEvents)
            .containsExactly(
                LoadEvent(REFRESH, initialState),
                LoadEvent(PREPEND, initialState),
                LoadEvent(APPEND, initialState)
            )
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Error(exception),
                    append = LoadState.NotLoading.Incomplete,
                    prepend = LoadState.Error(exception)
                )
            )
    }

    @Test
    fun retry_retryBothAppendAndPrepend() {
        val remoteMediatorMock =
            RemoteMediatorMock(loadDelay = 100).apply { initializeResult = SKIP_INITIAL_REFRESH }
        val exception = Exception()
        val accessor = testScope.createAccessor(remoteMediatorMock)
        remoteMediatorMock.loadCallback = { _, _ ->
            delay(100)
            RemoteMediator.MediatorResult.Error(exception)
        }
        val appendState = createMockState()
        val prependState = createMockState()
        accessor.requestLoad(PREPEND, prependState)
        accessor.requestLoad(APPEND, appendState)
        testScope.advanceUntilIdle()
        assertThat(remoteMediatorMock.newLoadEvents)
            .containsExactly(LoadEvent(PREPEND, prependState), LoadEvent(APPEND, appendState))
        // now retry, ensure both runs
        val retryState = createMockState()
        accessor.retryFailed(retryState)
        // make sure they both move to loading
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.NotLoading.Incomplete,
                    append = LoadState.Loading,
                    prepend = LoadState.Loading
                )
            )
        testScope.advanceUntilIdle()
        // ensure they both got called
        assertThat(remoteMediatorMock.newLoadEvents)
            .containsExactly(LoadEvent(PREPEND, retryState), LoadEvent(APPEND, retryState))
        // make sure new loading states are correct
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.NotLoading.Incomplete,
                    append = LoadState.Error(exception),
                    prepend = LoadState.Error(exception)
                )
            )
    }

    @Test
    fun retry_multipleTriggersOnlyRefresh() {
        val remoteMediator =
            object : RemoteMediatorMock(100) {
                override suspend fun initialize(): InitializeAction {
                    return SKIP_INITIAL_REFRESH
                }
            }
        val exception = Exception()
        remoteMediator.loadCallback = { _, _ ->
            // fail all
            delay(60)
            RemoteMediator.MediatorResult.Error(exception)
        }
        val accessor = testScope.createAccessor(remoteMediator)
        accessor.requestLoad(REFRESH, createMockState())
        accessor.requestLoad(APPEND, createMockState())
        accessor.requestLoad(PREPEND, createMockState())
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Loading,
                    append = LoadState.Loading,
                    prepend = LoadState.Loading
                )
            )
        // let refresh start but don't let it finish
        testScope.advanceUntilIdle()
        // get all errors
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Error(exception),
                    append = LoadState.Error(exception),
                    prepend = LoadState.Error(exception)
                )
            )
        // let requests succeed
        remoteMediator.loadCallback = null
        val retryState = createMockState()
        accessor.retryFailed(retryState)
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Loading,
                    append = LoadState.NotLoading.Incomplete,
                    prepend = LoadState.NotLoading.Incomplete
                )
            )
    }

    @Test
    fun failingRefreshRetriesAppendPrepend_refreshNotRequired() {
        val remoteMediator =
            object : RemoteMediatorMock(100) {
                override suspend fun initialize(): InitializeAction {
                    return SKIP_INITIAL_REFRESH
                }
            }
        val exception = Exception()
        remoteMediator.loadCallback = { type, _ ->
            // only fail for refresh
            if (type == REFRESH) {
                delay(60)
                RemoteMediator.MediatorResult.Error(exception)
            } else {
                null
            }
        }
        val accessor = testScope.createAccessor(remoteMediator)
        accessor.requestLoad(REFRESH, createMockState())
        accessor.requestLoad(APPEND, createMockState())
        accessor.requestLoad(PREPEND, createMockState())
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Loading,
                    append = LoadState.Loading,
                    prepend = LoadState.Loading
                )
            )
        // let refresh start but don't let it finish
        testScope.advanceTimeBy(50)
        // make sure refresh does not revert the append / prepend states
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Loading,
                    append = LoadState.Loading,
                    prepend = LoadState.Loading
                )
            )
        // let refresh fail, it should retry append prepend
        testScope.advanceTimeBy(20)
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Error(exception),
                    append = LoadState.Loading,
                    prepend = LoadState.Loading
                )
            )
        // let the prepend retry start
        testScope.advanceTimeBy(100)
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Error(exception),
                    append = LoadState.NotLoading.Incomplete,
                    prepend = LoadState.Loading
                )
            )
        testScope.advanceUntilIdle()
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Error(exception),
                    append = LoadState.NotLoading.Incomplete,
                    prepend = LoadState.NotLoading.Incomplete
                )
            )
    }

    @Test
    fun loadMoreRefreshShouldRetryRefresh() {
        // see: b/173438474
        val remoteMediator = RemoteMediatorMock(loadDelay = 100)
        val exception = Exception()
        remoteMediator.loadCallback = { _, _ ->
            delay(60)
            RemoteMediator.MediatorResult.Error(exception)
        }
        val accessor = testScope.createAccessor(remoteMediator)
        val state1 = createMockState()
        accessor.requestLoad(REFRESH, state1)
        assertThat(accessor.state.value.refresh).isEqualTo(LoadState.Loading)
        // run to get the error
        testScope.advanceUntilIdle()
        assertThat(accessor.state.value.refresh).isEqualTo(LoadState.Error(exception))
        // now send another load type refresh, should trigger another load
        remoteMediator.loadCallback = null // let it succeed
        val state2 = createMockState()
        accessor.requestLoad(REFRESH, state2)
        assertThat(accessor.state.value.refresh).isEqualTo(LoadState.Loading)
        testScope.advanceUntilIdle()
        assertThat(accessor.state.value.refresh).isEqualTo(LoadState.NotLoading.Incomplete)
    }

    @Test
    fun loadMoreRefreshShouldRetryRefresh_withAppendPrependErrors() {
        // see: b/173438474
        val remoteMediator = RemoteMediatorMock(loadDelay = 100)
        val exception = Exception()
        remoteMediator.loadCallback = { _, _ ->
            delay(60)
            RemoteMediator.MediatorResult.Error(exception)
        }
        val accessor = testScope.createAccessor(remoteMediator)
        val state1 = createMockState()
        accessor.requestLoad(REFRESH, state1)
        accessor.requestLoad(APPEND, state1)
        accessor.requestLoad(PREPEND, state1)
        // run to get the error
        testScope.advanceUntilIdle()
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Error(exception),
                    prepend = LoadState.Error(exception),
                    append = LoadState.Error(exception),
                )
            )
        // now send another load type refresh, should trigger another load
        remoteMediator.loadCallback = null // let it succeed
        val state2 = createMockState()
        accessor.requestLoad(REFRESH, state2)
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Loading,
                    prepend = LoadState.Error(exception), // keep errors for these for now
                    append = LoadState.Error(exception),
                )
            )
        testScope.advanceUntilIdle()
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.NotLoading.Incomplete,
                    prepend = LoadState.NotLoading.Incomplete, // clear errors
                    append = LoadState.NotLoading.Incomplete,
                )
            )
    }

    @Test
    fun loadMoreRefreshShouldRetryRefresh_withAppendPrependErrors_secondRefreshFails() {
        // see: b/173438474
        val remoteMediator = RemoteMediatorMock(loadDelay = 100)
        val exception1 = Exception("1")
        remoteMediator.loadCallback = { _, _ ->
            delay(60)
            RemoteMediator.MediatorResult.Error(exception1)
        }
        val accessor = testScope.createAccessor(remoteMediator)
        val state1 = createMockState()
        accessor.requestLoad(REFRESH, state1)
        accessor.requestLoad(APPEND, state1)
        accessor.requestLoad(PREPEND, state1)
        // run to get the error
        testScope.advanceUntilIdle()
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Error(exception1),
                    prepend = LoadState.Error(exception1),
                    append = LoadState.Error(exception1),
                )
            )
        // now send another load type refresh, should trigger another load
        val exception2 = Exception("2")
        remoteMediator.loadCallback = { _, _ ->
            delay(60)
            RemoteMediator.MediatorResult.Error(exception2)
        }
        val state2 = createMockState()
        accessor.requestLoad(REFRESH, state2)
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Loading,
                    prepend = LoadState.Error(exception1), // keep errors for these for now
                    append = LoadState.Error(exception1),
                )
            )
        testScope.advanceUntilIdle()
        assertThat(accessor.state.value)
            .isEqualTo(
                LoadStates(
                    refresh = LoadState.Error(exception2),
                    prepend = LoadState.Error(exception1), // these keep their original exceptions
                    append = LoadState.Error(exception1),
                )
            )
    }

    @Test
    fun requireRetry_append() {
        requireRetry(APPEND)
    }

    @Test
    fun requireRetry_prepend() {
        requireRetry(PREPEND)
    }

    private fun requireRetry(loadType: LoadType) {
        // ensure that we don't retry a failed request until a retry arrives.
        val remoteMediator = RemoteMediatorMock(100)
        val exception = Exception()
        remoteMediator.loadCallback = { _, _ ->
            delay(60)
            RemoteMediator.MediatorResult.Error(exception)
        }
        val accessor = testScope.createAccessor(remoteMediator)
        val state1 = createMockState()
        accessor.requestLoad(loadType, state1)
        assertThat(accessor.state.value.get(loadType)).isEqualTo(LoadState.Loading)
        testScope.advanceUntilIdle()
        assertThat(accessor.state.value.get(loadType)).isEqualTo(LoadState.Error(exception))
        assertThat(remoteMediator.newLoadEvents).containsExactly(LoadEvent(loadType, state1))
        // subsequent add calls shouldn't do anything
        accessor.requestLoad(loadType, createMockState())
        assertThat(accessor.state.value.get(loadType)).isEqualTo(LoadState.Error(exception))
        testScope.advanceUntilIdle()
        assertThat(accessor.state.value.get(loadType)).isEqualTo(LoadState.Error(exception))
        assertThat(remoteMediator.newLoadEvents).isEmpty()

        // if we send a retry, then it will work
        remoteMediator.loadCallback = null
        val retryState = createMockState()
        accessor.retryFailed(retryState)
        assertThat(accessor.state.value.get(loadType)).isEqualTo(LoadState.Loading)
        testScope.advanceUntilIdle()
        assertThat(remoteMediator.newLoadEvents).containsExactly(LoadEvent(loadType, retryState))
        assertThat(accessor.state.value.get(loadType)).isEqualTo(LoadState.NotLoading.Incomplete)
    }

    private fun TestScope.createAccessor(
        mediator: RemoteMediatorMock
    ): RemoteMediatorAccessor<Int, Int> {
        val accessor = RemoteMediatorAccessor(scope = this, delegate = mediator)
        TestScope().launch(coroutineContext) { accessor.initialize() }
        return accessor
    }
}