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Mohit Sarveiya Unit Testing Kotlin Channels & Flows www.twitter.com/heyitsmohit www.codingwithmohit.com

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Unit Testing Channels & Flows ● Run Blocking Test ● Flow Test Cases ● Flow Assertions with Turbine ● View Model Testing ● Testing in Coroutines Lib

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Flow Test Cases

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Flow Test Cases ● Collection ● Run on Dispatcher ● Errors ● Retries Fail ● Retry Succeeds

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GraphQL gRPC REST

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Flow of users

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Flow of users Repository Service

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class UsersRepository(val apiService: ApiService) { fun getUsers(): Flow { return apiService.getUsers() .filter { it.location "== "NYC" } .map { it.name } } }

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class UsersRepository(val apiService: ApiService) { fun getUsers(): Flow { return apiService.getUsers() .filter { it.location "== "NYC" } .map { it.name } } }

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class UsersRepository(val apiService: ApiService) { fun getUsers(): Flow { return apiService.getUsers() .filter { it.location "== "NYC" } .map { it.name } } }

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class UsersRepository(val apiService: ApiService) { fun getUsers(): Flow { return apiService.getUsers() .filter { it.location "== "NYC" } .map { it.name } } } Flow of Users

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class UsersRepository(val apiService: ApiService) { fun getUsers(): Flow { return apiService.getUsers() .filter { it.location "== "NYC" } .map { it.name } } } Filter users

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class UsersRepository(val apiService: ApiService) { fun getUsers(): Flow { return apiService.getUsers() .filter { it.location "== "NYC" } .map { it.name } } } Map user name

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class UsersRepository(val apiService: ApiService) { fun getUsers(): Flow { return apiService.getUsers() .filter { it.location "== "NYC" } .map { it.name } } } How do we test this Flow?

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Kotlin/kotlinx.coroutines Code ! Issues Pull Requests kotlinx-coroutines-core 2,093 commits 4c28f942 13 hours ago Kotlinx-coroutines-debug Kotlinx-coroutines-test reactive js benchmarks

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Kotlin/kotlinx.coroutines Code ! Issues Pull Requests kotlinx-coroutines-core 2,093 commits 4c28f942 13 hours ago Kotlinx-coroutines-debug Kotlinx-coroutines-test reactive js benchmarks Testing Module

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Kotlin/kotlinx.coroutines Code ! Issues Pull Requests README.md Module Kotlin-coroutines-test Test utilities for kotlinx.coroutines dependencies { testImplementation 'org.jetbrains.kotlinx:kotlinx-coroutines-test:1.3.9' }

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@Test fun `should get users`() { }

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@Test fun `should get users`() = runBlockingTest { } Creates a coroutine

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@Test fun `should get users`() = runBlockingTest { } Test Scope

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@Test fun `should get users`() = runBlockingTest { } Test Scope + Test Dispatcher

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@Test fun `should get users`() = runBlockingTest { } Test Scope + Test Dispatcher What is executed in this coroutine?

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@Test fun `should get users`() = runBlockingTest { } Test Scope + Test Dispatcher

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@Test fun `should get users`() = runBlockingTest { } Test Scope + Test Dispatcher

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fun runBlockingTest(testBody) { val scope = TestCoroutineScope(safeContext) val deferred = scope.async { scope.testBody() } deferred.getCompletionExceptionOrNull()"?.let { throw it } scope.cleanupTestCoroutines() if (hasActiveJobs()) { throw UncompletedCoroutinesError("Test finished with active jobs: $endingJobs") } } Our test to run

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fun runBlockingTest(testBody) { val scope = TestCoroutineScope(safeContext) val deferred = scope.async { scope.testBody() } deferred.getCompletionExceptionOrNull()"?.let { throw it } scope.cleanupTestCoroutines() if (hasActiveJobs()) { throw UncompletedCoroutinesError("Test finished with active jobs: $endingJobs") } } Test Coroutine Scope

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fun runBlockingTest(testBody) { val scope = TestCoroutineScope(safeContext) val deferred = scope.async { scope.testBody() } deferred.getCompletionExceptionOrNull()"?.let { throw it } scope.cleanupTestCoroutines() if (hasActiveJobs()) { throw UncompletedCoroutinesError("Test finished with active jobs: $endingJobs") } } Wrap your test in Deferred

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fun runBlockingTest(testBody) { val scope = TestCoroutineScope(safeContext) val deferred = scope.async { scope.testBody() } deferred.getCompletionExceptionOrNull()"?.let { throw it } scope.cleanupTestCoroutines() if (hasActiveJobs()) { throw UncompletedCoroutinesError("Test finished with active jobs: $endingJobs") } } Run your test!

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fun runBlockingTest(testBody) { val scope = TestCoroutineScope(safeContext) val deferred = scope.async { scope.testBody() } deferred.getCompletionExceptionOrNull()"?.let { throw it } scope.cleanupTestCoroutines() if (hasActiveJobs()) { throw UncompletedCoroutinesError("Test finished with active jobs: $endingJobs") } } Cancel all Jobs in test

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fun runBlockingTest(testBody) { val scope = TestCoroutineScope(safeContext) val deferred = scope.async { scope.testBody() } deferred.getCompletionExceptionOrNull()"?.let { throw it } scope.cleanupTestCoroutines() if (hasActiveJobs()) { throw UncompletedCoroutinesError("Test finished with active jobs: $endingJobs") } } Fail on active coroutines

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fun runBlockingTest(testBody) { val scope = TestCoroutineScope(safeContext) val deferred = scope.async { scope.testBody() } deferred.getCompletionExceptionOrNull()"?.let { throw it } scope.cleanupTestCoroutines() if (activeJobs()).isNotEmpty()) { throw UncompletedCoroutinesError("Test finished with active jobs") } } Setup Run Test Cleanup

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@Test fun `should get users`() = runBlockingTest { }

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class UsersRepository(val apiService: ApiService) { fun getUsers(): Flow { return apiService.getUsers() .filter { it.location "== "NYC" } .map { it.name } } } How do we mock service?

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class UsersRepository(val apiService: ApiService) { fun getUsers(): Flow { return apiService.getUsers() .filter { it.location "== "NYC" } .map { it.name } } } Mock or Create fake

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nhaarman/mockito-kotlin Code ! Issues Pull Requests Mockito-Kotlin A small library that provides helper functions to work with Mockito in Kotlin. maven central 2.2.0

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val apiService = mock { on { getUsers() } doReturn users.asFlow() } @Test fun `should get users`() = runBlockingTest { } Mock API Service

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val apiService = mock { on { getUsers() } doReturn users.asFlow() } @Test fun `should get users`() = runBlockingTest { } Return a Flow of users

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val apiService = mock { on { getUsers() } doReturn users.asFlow() } @Test fun `should get users`() = runBlockingTest { } val users = listOf( User(1, "User 1", "NJ"), User(2, "User 2", "NYC"), User(3, "User 3", "CA"), User(4, "User 4", "NJ"), User(5, "User 5", "NYC") )

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val apiService = mock { on { getUsers() } doReturn users.asFlow() } val usersRepository = UsersRepository(apiService) fun `should get users`() = runBlockingTest { } Inject mock into Repo

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@Test fun `should get users`() = runBlockingTest { val users = usersRepository.getUsers().toList() assertEquals(users, listOf("User 2", "User 5")) } Collect Flow into List

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@Test fun `should get users`() = runBlockingTest { val users = usersRepository.getUsers().toList() assertEquals(users, listOf("User 2", "User 5")) } Assert on returned user names

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@Test fun `should get users`() = runBlockingTest { val users = usersRepository.getUsers().toList() assertEquals(users, listOf("User 2", "User 5")) } Is there a better way to collect from Flows in test?

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cashApp/turbine Code ! Issues Pull Requests Turbine Small testing library for kotlinx.coroutines Flow. testImplementation 'app.cash.turbine:turbine:0.2.1'

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@Test fun `should get users`() = runBlockingTest { val users = usersRepository.getUsers().test { expectItem() shouldBe "User 2" expectItem() shouldBe "User 5" expectComplete() } } Library has test extension on Flow

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@Test fun `should get users`() = runBlockingTest { val users = usersRepository.getUsers().test { expectItem() shouldBe "User 2" expectItem() shouldBe "User 5" expectComplete() } } Read from item from Flow

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@Test fun `should get users`() = runBlockingTest { val users = usersRepository.getUsers().test { expectItem() shouldBe "User 2" expectItem() shouldBe "User 5" expectComplete() } } Assertion

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@Test fun `should get users`() = runBlockingTest { val users = usersRepository.getUsers().test { expectItem() shouldBe "User 2" expectItem() shouldBe "User 5" expectComplete() } } Use expectItem to fetch each item

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@Test fun `should get users`() = runBlockingTest { val users = usersRepository.getUsers().test { expectItem() shouldBe "User 2" expectItem() shouldBe "User 5" expectComplete() } } Verify no more emission by Flow

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@Test fun `should get users`() = runBlockingTest { val users = usersRepository.getUsers().test { expectItem() shouldBe "User 2" expectItem() shouldBe "User 5" expectComplete() } } What if flow emitted more items than 2?

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@Test fun `should get users`() = runBlockingTest { val users = usersRepository.getUsers().test { expectItem() shouldBe "User 2" expectItem() shouldBe "User 5" expectComplete() } } UserRepoTests.kt Run: UserRepoTest should get users app.cash.turbine.AssertionError: Expected complete but found Item(User 6) One more item in Flow

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@Test fun `should get users`() = runBlockingTest { val users = usersRepository.getUsers().test { expectItem() shouldBe "User 2" expectItem() shouldBe "User 5" expectItem() shouldBe "User 6” expectComplete() } } UserRepoTests.kt Run: UserRepoTest should get users Add expect item

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@Test fun `should get users`() = runBlockingTest { val users = usersRepository.getUsers().test { expectItem() shouldBe "User 2" expectItem() shouldBe "User 5" expectComplete() } } What if we didn’t expect complete?

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@Test fun `should get users`() = runBlockingTest { val users = usersRepository.getUsers().test { expectItem() shouldBe "User 2" expectItem() shouldBe "User 5" } } UserRepoTests.kt Run: UserRepoTest should get users app.cash.turbine.AssertionError: Unconsumed events found: - Complete Need to assert flow finished

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@Test fun `should get users`() = runBlockingTest { val users = usersRepository.getUsers().test { expectItem() shouldBe "User 2" expectItem() shouldBe "User 5" expectComplete() } } UserRepoTests.kt Run: should get users UserRepoTest

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@Test fun `should get users`() = runBlockingTest { val users = usersRepository.getUsers().test { expectItem() shouldBe "User 2" expectItem() shouldBe "User 5" expectComplete() } } UserRepoTests.kt Run: should get users UserRepoTest How does this work?

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Flow to test How Turbine Works

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Channel (Unlimited) How Turbine Works

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Channel (Unlimited) sealed class Event { object Complete data class Error(""...) data class Item(val value: T) } Event How Turbine Works

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Channel (Unlimited) data class Item(val value: T): Event() How Turbine Works

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Channel (Unlimited) data class Item(val value: T): Event() How Turbine Works

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Channel (Unlimited) data class Item(val value: T): Event() How Turbine Works

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Channel (Unlimited) object Complete: Event() How Turbine Works

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Channel (Unlimited) Exception data class Error(""...): Event() How Turbine Works

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Channel (Unlimited) How do I query items in channels? How Turbine Works

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interface FlowTurbine { val timeout: Duration fun expectNoEvents() suspend fun expectItem(): T fun expectError(): Throwable suspend fun expectComplete() } API to verify emissions How Turbine Works

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interface FlowTurbine { val timeout: Duration fun expectNoEvents() suspend fun expectItem(): T fun expectError(): Throwable suspend fun expectComplete() } Verify duration of emission How Turbine Works

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interface FlowTurbine { val timeout: Duration fun expectNoEvents() suspend fun expectItem(): T fun expectError(): Throwable suspend fun expectComplete() } Verify no emissions How Turbine Works

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interface FlowTurbine { val timeout: Duration fun expectNoEvents() suspend fun expectItem(): T fun expectError(): Throwable suspend fun expectComplete() } Verify item emitted How Turbine Works

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interface FlowTurbine { val timeout: Duration fun expectNoEvents() suspend fun expectItem(): T fun expectError(): Throwable suspend fun expectComplete() } Verify error occurred How Turbine Works

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interface FlowTurbine { val timeout: Duration fun expectNoEvents() suspend fun expectItem(): T fun expectError(): Throwable suspend fun expectComplete() } Verify all items emitted How Turbine Works

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interface FlowTurbine { val timeout: Duration fun expectNoEvents() suspend fun expectItem(): T fun expectError(): Throwable suspend fun expectComplete() } How Turbine Works

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How Turbine Works suspend fun Flow.test(""...) { coroutineScope { } Create a coroutine

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How Turbine Works suspend fun Flow.test(""...) { coroutineScope { val events = Channel>(UNLIMITED) } Channel of events

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How Turbine Works suspend fun Flow.test(""...) { coroutineScope { val events = Channel>(UNLIMITED) launch { } }

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How Turbine Works suspend fun Flow.test(""...) { coroutineScope { val events = Channel>(UNLIMITED) launch { collect { item "-> events.send(Event.Item(item)) } } Store emissions

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How Turbine Works suspend fun Flow.test(""...) { coroutineScope { val events = Channel>(UNLIMITED) launch { try { collect { ""... } } catch { events.send(Event.Error) } } Send error to Channel

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How Turbine Works suspend fun Flow.test( validate: suspend FlowTurbine.() "-> Unit ) { coroutineScope { val events = Channel>(UNLIMITED) launch { try { collect { ""... } } catch { events.send(Event.Error) Lambda block

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How Turbine Works suspend fun Flow.test( validate: suspend FlowTurbine.() "-> Unit ) { coroutineScope { val events = Channel>(UNLIMITED) launch { try { collect { ""... } } catch { events.send(Event.Error) API

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@Test fun `should get users`() = runBlockingTest { val users = usersRepository.getUsers().test { expectItem() shouldBe "User 2" expectItem() shouldBe "User 5" expectComplete() } } Read emissions with API

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Flow Test Cases ● Collection ● Run on Dispatcher ● Errors ● Retries Fail ● Retry Succeeds

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } Run on Dispatcher

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class UsersRepository( val apiService: ApiService, val dispatcher: CoroutineDispatcher ) { fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) } Inject Dispatcher

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class UsersRepository( val apiService: ApiService, val dispatcher: CoroutineDispatcher ) { fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .flowOn(dispatcher) } Use Dispatcher

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class UsersRepository( val apiService: ApiService, val dispatcher: CoroutineDispatcher ) { fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) }

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@Test fun `should get users`() = runBlockingTest { usersRepository.getUsers().test { expectError() } } How do I use a custom dispatcher?

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@Test fun `should get users`() = runBlockingTest { usersRepository.getUsers().test { expectError() } } fun TestCoroutineDispatcher.runBlockingTest(""...)

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val dispatcher = TestCoroutineDispatcher() val usersRepository = UsersRepository(""..., dispatcher) @Test fun `should get users`() = dispatcher.runBlockingTest { usersRepository.getUsers().test { expectError() } }

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val dispatcher = TestCoroutineDispatcher() val usersRepository = UsersRepository(""..., dispatcher) @Test fun `should get users`() = dispatcher.runBlockingTest { usersRepository.getUsers().test { expectError() } }

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val dispatcher = TestCoroutineDispatcher() val usersRepository = UsersRepository(""..., dispatcher) @Test fun `should get users`() = dispatcher.runBlockingTest { usersRepository.getUsers().test { expectError() } }

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Flow Test Cases ● Collection ● Run on Dispatcher ● Errors ● Retries Fail ● Retry Succeeds

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class UsersRepository(val apiService: ApiService) { fun getUsers(): Flow { return apiService.getUsers() .filter { it.location "== "NYC" } .map { it.name } } } Test Error?

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@Test fun `should handle error`() = dispatcher.runBlockingTest { usersRepository.getUsers().test { expectError() } }

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@Test fun `should handle error`() = dispatcher.runBlockingTest { whenever(apiService.getUsers()) usersRepository.getUsers().test { expectError() } } Mock API method

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@Test fun `should handle error`() = dispatcher.runBlockingTest { whenever(apiService.getUsers()) doReturn flow { throw Exception() } usersRepository.getUsers().test { expectError() } } Throw exception

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@Test fun `should handle error`() = dispatcher.runBlockingTest { whenever(apiService.getUsers()) doReturn flow { throw Exception() } usersRepository.getUsers().test { expectError() } } Verify error is thrown

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@Test fun `should handle error`() = dispatcher.runBlockingTest { whenever(apiService.getUsers()) doReturn flow { throw Exception() } usersRepository.getUsers().test { expectError() } } UserRepoTests.kt Run: should handle error UserRepoTest

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@Test fun `should handle error`() = runBlockingTest { whenever(apiService.getUsers()) doReturn flow { throw Exception() } usersRepository.getUsers().test { expectError() } } UserRepoTests.kt Run: should handle error UserRepoTest Try Catch

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class UsersRepository(val apiService: ApiService) { fun getUsers(): Flow = apiService.getUsers() .filter { it.location "== "NYC" } .map { it.name } } Better way to handle errors?

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inline class Result { fun success(value: T) fun failure(exception: Throwable) } Union to represent success or failure

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fun getUsers(): Flow> = return apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .catch { emit(Result.failure(it)) } Flow emits Results

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .catch { emit(Result.failure(it)) } Success

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .catch { emit(Result.failure(it)) } Emit Result Failure

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@Test fun `should handle error`() = dispatcher.runBlockingTest { whenever(apiService.getUsers()) doReturn flow { throw Exception() } usersRepository.getUsers().test { } }

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@Test fun `should handle error`() = dispatcher.runBlockingTest { whenever(apiService.getUsers()) doReturn flow { throw Exception() } usersRepository.getUsers().test { expectItem().isFailure.shouldBeTrue() expectComplete() } } Verify Result is failure

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@Test fun `should handle error`() = dispatcher.runBlockingTest { whenever(apiService.getUsers()) doReturn flow { throw Exception() } usersRepository.getUsers().test { expectItem().isFailure expectComplete() } } Verify no more emissions

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .catch { emit(Result.failure(it)) } Exception

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .catch { emit(Result.failure(it)) } Emit Result item

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Flow Test Cases ● Collection ● Run on Dispatcher ● Errors ● Retries Fail ● Retry Succeeds

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Exception

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Catch Exception

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Retry?

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Retry 2-times

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Delay for each retry

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Error

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Retry

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Delay 2 seconds

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Try running Flow again

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Retry fails, go to catch

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher)

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) How do we test retries?

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@Test fun `should retry twice`() = dispatcher.runBlockingTest { usersRepository.getUsers().test { advanceTimeBy(2000) advanceTimeBy(2000) expectItem().isFailure.shouldBeTrue() expectComplete() } }

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@Test fun `should retry twice`() = dispatcher.runBlockingTest { whenever(apiService.getUsers()) doReturn flow { throw IOException() } usersRepository.getUsers().test { advanceTimeBy(2000) advanceTimeBy(2000) expectItem().isFailure.shouldBeTrue() expectComplete() } } Throw Exception

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@Test fun `should retry twice`() = dispatcher.runBlockingTest { whenever(apiService.getUsers()) doReturn flow { throw IOException() } usersRepository.getUsers().test { expectItem().isFailure.shouldBeTrue() expectComplete() } } Read from Flow

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@Test fun `should retry twice`() = dispatcher.runBlockingTest { whenever(apiService.getUsers()) doReturn flow { throw IOException() } usersRepository.getUsers().test { expectItem().isFailure.shouldBeTrue() expectComplete() } } Verify Result is Failure

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@Test fun `should retry twice`() = dispatcher.runBlockingTest { whenever(apiService.getUsers()) doReturn flow { throw IOException() } usersRepository.getUsers().test { expectItem().isFailure.shouldBeTrue() expectComplete() } }

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@Test fun `should retry twice`() = dispatcher.runBlockingTest { whenever(apiService.getUsers()) doReturn flow { throw IOException() } usersRepository.getUsers().test { expectItem().isFailure.shouldBeTrue() expectComplete() } } UserRepoTests.kt Run: UserRepoTest should retry twice kotlinx.coroutines.TimeoutCancellationException: Timed out waiting for 1000 ms ! ! Timeout Exception

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Error

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Retry

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Delay 2 seconds

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Advance virtual time forward

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@Test fun `should retry twice`() = dispatcher.runBlockingTest { whenever(apiService.getUsers()) doReturn flow { throw IOException() } usersRepository.getUsers().test { advanceTimeBy(2000) expectItem().isFailure.shouldBeTrue() expectComplete() } } Advance virtual time on dispatcher

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@Test fun `should retry twice`() = dispatcher.runBlockingTest { whenever(apiService.getUsers()) doReturn flow { throw IOException() } usersRepository.getUsers().test { advanceTimeBy(2000) expectItem().isFailure.shouldBeTrue() expectComplete() } } Advance time by 2 seconds

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Retry

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Delay for before 2nd retry

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@Test fun `should retry twice`() = dispatcher.runBlockingTest { whenever(apiService.getUsers()) doReturn flow { throw IOException() } usersRepository.getUsers().test { advanceTimeBy(2000) advanceTimeBy(2000) expectItem().isFailure.shouldBeTrue() expectComplete() } } Advance time forward for 2nd retry

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) 2nd Retry

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) All Retries failed

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@Test fun `should retry twice`() = dispatcher.runBlockingTest { whenever(apiService.getUsers()) doReturn flow { throw IOException() } usersRepository.getUsers().test { advanceTimeBy(2000) advanceTimeBy(2000) expectItem().isFailure.shouldBeTrue() expectComplete() } }

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Flow Test Cases ● Collection ● Run on Dispatcher ● Errors ● Retries Fail ● Retry Succeeds

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) 2nd Retry Succeeds

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Apply Filter, Map

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var throwError = false whenever(apiService.getUsers()) doReturn flow { if (throwError) { throw IOException() } else { emit(User(1, "User 1", "NYC")) } } Flag to control sending error

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var throwError = false whenever(apiService.getUsers()) doReturn flow { if (throwError) { throw IOException() } else { emit(User(1, "User 1", "NYC")) } } Mock API Service to return Flow

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var throwError = false whenever(apiService.getUsers()) doReturn flow { if (throwError) { throw IOException() } else { emit(User(1, "User 1", "NYC")) } } Throw Error on exception

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var throwError = false whenever(apiService.getUsers()) doReturn flow { if (throwError) { throw IOException() } else { emit(User(1, "User 1", "NYC")) } } Emit User

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@Test fun `should retry succeed`() = dispatcher.runBlockingTest { usersRepository.getUsers().test { advanceTimeBy(2000) throwError = false advanceTimeBy(2000) expectItem().isSuccess.shouldBeTrue() expectComplete() } } Retry

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@Test fun `should retry succeed`() = dispatcher.runBlockingTest { usersRepository.getUsers().test { advanceTimeBy(2000) throwError = false advanceTimeBy(2000) expectItem().isSuccess.shouldBeTrue() expectComplete() } } Flow will return data

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@Test fun `should retry succeed`() = dispatcher.runBlockingTest { usersRepository.getUsers().test { advanceTimeBy(2000) throwError = false advanceTimeBy(2000) expectItem().isSuccess.shouldBeTrue() expectComplete() } } 2nd Retry

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@Test fun `should retry succeed`() = dispatcher.runBlockingTest { usersRepository.getUsers().test { advanceTimeBy(2000) throwError = false advanceTimeBy(2000) expectItem().isSuccess.shouldBeTrue() expectComplete() } } Verify Success

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) 2nd Retry

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher)

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fun getUsers(): Flow> = apiService.getUsers() .filter { it.location "== "NYC" } .map { Result.success(it.name) } .retry(2) { delay(2000) true } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) Operators

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@Test fun `should retry succeed`() = dispatcher.runBlockingTest { usersRepository.getUsers().test { advanceTimeBy(2000) throwError = false advanceTimeBy(2000) expectItem().isSuccess.shouldBeTrue() expectComplete() } }

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Flow Test Cases ● Collection ● Run on Dispatcher ● Errors ● Retries Fail ● Retry Succeeds

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View Model Testing

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Flow of users Repository Service

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View Model Repository

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class UserViewModel(): ViewModel() { val states = MutableStateFlow(State()) fun getData() { usersRepository.getUsers() .onEach { states.value = State(data = it.get()) } .launchIn(viewModelScope) } } data class State(val data: String = "") Jetpack View Model

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class UserViewModel(val usersRepository: UsersRepository): ViewModel() { val states = MutableStateFlow(State()) fun getData() { usersRepository.getUsers() .onEach { states.value = State(data = it.get()) } .launchIn(viewModelScope) } } data class State(val data: String = "") Inject Repo

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class UserViewModel(val usersRepository: UsersRepository): ViewModel() { val states = MutableStateFlow("") fun getData() { usersRepository.getUsers() .onEach { states.value = State(data = it.get()) } .launchIn(viewModelScope) } } State Flow

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class UserViewModel(val usersRepository: UsersRepository): ViewModel() { val states = MutableStateFlow("") fun getData() { usersRepository.getUsers() .onEach { states.value = State(data = it.get()) } .launchIn(viewModelScope) } } Flow of user names

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class UserViewModel(val usersRepository: UsersRepository): ViewModel() { val states = MutableStateFlow("") fun getData() { usersRepository.getUsers() .onEach { states.value = it } .launchIn(viewModelScope) } } Update State flow for each emission

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class UserViewModel(val usersRepository: UsersRepository): ViewModel() { val states = MutableStateFlow("") fun getData() { usersRepository.getUsers() .onEach { states.value = it } .launchIn(viewModelScope) } } Launch coroutine in view module sopce

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class UserViewModel(val usersRepository: UsersRepository): ViewModel() { val states = MutableStateFlow("") fun getData() { usersRepository.getUsers() .onEach { states.value = it } .launchIn(viewModelScope) } }

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val repo = mock() val viewModel = UserViewModel(repo) Inject mock into view model

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@Test fun `should get data`() = runBlockingTest { whenever(repo.getUsers()) doReturn flowOf(Result.success("User")) viewModel.getData() viewModel.states.test { expectItem().data shouldBe "User" } }

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@Test fun `should get data`() = runBlockingTest { whenever(repo.getUsers()) doReturn flowOf(Result.success("User")) viewModel.getData() viewModel.states.test { expectItem().data shouldBe "User" } } Mock flow

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@Test fun `should get data`() = runBlockingTest { whenever(repo.getUsers()) doReturn flowOf("User") viewModel.getData() viewModel.states.test { expectItem().data shouldBe "User" } } Emit Successful response

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@Test fun `should get data`() = runBlockingTest { whenever(repo.getUsers()) doReturn flowOf("User") viewModel.getData() viewModel.states.test { expectItem().data shouldBe "User" } } Run test

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@Test fun `should get data`() = runBlockingTest { whenever(repo.getUsers()) doReturn flowOf("User") viewModel.getData() viewModel.states.test { expectItem().data shouldBe "User" } } Verify emission

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@Test fun `should get data`() = runBlockingTest { whenever(repo.getUsers()) doReturn flowOf(Result.success("User")) viewModel.getData() viewModel.states.test { expectItem().data shouldBe "User" } } ViewModelTest.kt Run: ViewModelTest should get data Module with the Main dispatcher had failed to initialize. For tests Dispatchers.setMain from kotlinx-coroutines-test module can be used

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val dispatcher = TestCoroutineDispatcher() @Before fun setUp() { Dispatchers.setMain(dispatcher) } @After fun tearDown() { Dispatchers.resetMain() } Set Main Dispatcher

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val dispatcher = TestCoroutineDispatcher() @Before fun setUp() { Dispatchers.setMain(dispatcher) } @After fun tearDown() { Dispatchers.resetMain() } Reset Main Dispatcher

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class CoroutineTestRule(testDispatcher) : TestWatcher() { override fun starting(description) { Dispatchers.setMain(testDispatcher) } override fun finished(description) { Dispatchers.resetMain() testDispatcher.cleanupTestCoroutines() } }

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class CoroutineTestRule(testDispatcher) : TestWatcher() { override fun starting(description) { Dispatchers.setMain(testDispatcher) } override fun finished(description) { Dispatchers.resetMain() testDispatcher.cleanupTestCoroutines() } }

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class CoroutineTestRule(testDispatcher) : TestWatcher() { override fun starting(description) { Dispatchers.setMain(testDispatcher) } override fun finished(description) { Dispatchers.resetMain() testDispatcher.cleanupTestCoroutines() } }

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@get:Rule var testRule = CoroutineTestRule() @Test fun `should get data`() = runBlockingTest { whenever(repo.getUsers()) doReturn flowOf("User") viewModel.getData() viewModel.states.test { expectItem().data shouldBe "User" } }

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@get:Rule var testRule = CoroutineTestRule() @Test fun `should get data`() = runBlockingTest { whenever(repo.getUsers()) doReturn flowOf(Result.success("User")) viewModel.getData() viewModel.states.test { expectItem().data shouldBe "User" } } ViewModelTest.kt Run: ViewModelTest should get data

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@Test fun `should get data`() = runBlockingTest { whenever(repo.getUsers()) doReturn flowOf( "User 1", "User 2" ) viewModel.getData() viewModel.states.test { expectItem().data shouldBe "User 1" expectItem().data shouldBe "User 2" expectComplete() } Return 2 results

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@Test fun `should get data`() = runBlockingTest { whenever(repo.getUsers()) doReturn flowOf( "User 1", "User 2" ) viewModel.getData() viewModel.states.test { expectItem().data shouldBe "User 1" expectItem().data shouldBe "User 2" expectComplete() } Return 2 results

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@Test fun `should get data`() = runBlockingTest { viewModel.getData() viewModel.states.test { expectItem().data shouldBe "User 1" expectItem().data shouldBe "User 2" expectComplete() } Verification

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@Test fun `should get data`() = runBlockingTest { viewModel.getData() viewModel.states.test { expectItem().data shouldBe "User 1" expectItem().data shouldBe "User 2" expectComplete() } ViewModelTest.kt Run: ViewModelTest should get data java.lang.AssertionError: Expected , actual are not the same instance.

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@Test fun `should get data`() = runBlockingTest { viewModel.getData() viewModel.states.test { expectItem().data shouldBe "User 1" expectItem().data shouldBe "User 2" expectComplete() } ViewModelTest.kt Run: ViewModelTest should get data java.lang.AssertionError: Expected , actual are not the same instance. Assertion failed

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class UserViewModel(val usersRepository: UsersRepository): ViewModel() { val states = MutableStateFlow(State()) fun getData() { usersRepository.getUsers() .onEach { states.value = State(it) } .launchIn(viewModelScope) } } Conflated

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Channels ● Broadcast Channel ● Unlimited Channel ● Buffered Channel ● Mutable Shared Flow

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class UserViewModel(val usersRepository: UsersRepository): ViewModel() { val sharedFlow = MutableSharedFlow() }

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class UserViewModel(val usersRepository: UsersRepository): ViewModel() { val sharedFlow = MutableSharedFlow() fun getData() { usersRepository.getUsers() .onEach { states.emit(it) } .launchIn(viewModelScope) } }

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Channels ● Broadcast Channel ● Unlimited Channel ● Buffered Channel ● Mutable Shared Flow

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Coroutine Lib Testing How tests are written in the coroutines lib?

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class JobBasicCancellationTest : TestBase() { @Test fun testCancelJobImpl() { } } Defines utilities

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class JobBasicCancellationTest : TestBase() { @Test fun testCancelJobImpl() { } } Test parent job is not cancelled

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@Test fun testCancelJobImpl() = runTest { val parent = launch { expect(1) val child = Job(coroutineContext[Job]) expect(2) child.cancel() child.join() expect(3) } parent.join() finish(4) }

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@Test fun testCancelJobImpl() = runTest { val parent = launch { expect(1) val child = Job(coroutineContext[Job]) expect(2) child.cancel() child.join() expect(3) } parent.join() finish(4) } Launch coroutine

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@Test fun testCancelJobImpl() = runTest { val parent = launch { expect(1) val child = Job(coroutineContext[Job]) expect(2) child.cancel() child.join() expect(3) } parent.join() finish(4) } Child coroutine Job

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@Test fun testCancelJobImpl() = runTest { val parent = launch { expect(1) val child = Job(coroutineContext[Job]) expect(2) child.cancel() child.join() expect(3) } parent.join() finish(4) } Cancel the job

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@Test fun testCancelJobImpl() = runTest { val parent = launch { expect(1) val child = Job(coroutineContext[Job]) expect(2) child.cancel() child.join() expect(3) } parent.join() finish(4) } Verify order of execution Verify completion

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@Test fun testCancelJobImpl() = runTest { val parent = launch { expect(1) val child = Job(coroutineContext[Job]) expect(2) child.cancel() child.join() expect(3) } parent.join() finish(4) }

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expect open class TestBase { fun error(message, cause) fun expect(index: Int) fun finish(index: Int) fun runTest( block: suspend CoroutineScope.() "-> Unit ) }

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JVM JS Native Common TestBase.kt TestBase.kt TestBase.kt TestBase.kt

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actual fun runTest( block: suspend CoroutineScope.() "-> Unit ) { runBlocking( block = block, context = CoroutineExceptionHandler { ""... }) }

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expect open class TestBase { fun error(message, cause) fun expect(index: Int) fun finish(index: Int) fun runTest( block: suspend CoroutineScope.() "-> Unit ) } How does this work?

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private var actionIndex = AtomicInteger() actual fun expect(index: Int) { val wasIndex = actionIndex.incrementAndGet() check(index "== wasIndex) { “Expecting action index $index but it is actually $wasIndex" } }

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private var actionIndex = AtomicInteger() actual fun expect(index: Int) { val wasIndex = actionIndex.incrementAndGet() check(index "== wasIndex) { “Expecting action index $index but it is actually $wasIndex" } }

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private var actionIndex = AtomicInteger() actual fun expect(index: Int) { val wasIndex = actionIndex.incrementAndGet() check(index "== wasIndex) { “Expecting action index $index but it is actually $wasIndex" } }

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expect open class TestBase { fun error(message, cause) fun expect(index: Int) fun finish(index: Int) fun runTest( block: suspend CoroutineScope.() "-> Unit ) } How does this work?

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private var finished = AtomicBoolean() actual fun finish(index: Int) { expect(index) check(!finished.getAndSet(true)) { "Should call 'finish(""...)' at most once" } }

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private var finished = AtomicBoolean() actual fun finish(index: Int) { expect(index) check(!finished.getAndSet(true)) { "Should call 'finish(""...)' at most once" } }

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private var finished = AtomicBoolean() actual fun finish(index: Int) { expect(index) check(!finished.getAndSet(true)) { "Should call 'finish(""...)' at most once" } }

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@Test fun testCancelJobImpl() = runTest { val parent = launch { expect(1) val child = Job(coroutineContext[Job]) expect(2) child.cancel() child.join() expect(3) } parent.join() finish(4) }

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https:"//codingwithmohit.com/coroutines/unit-testing-delays-errors-retries-with-kotlin-flows/ Unit Testing

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Resources ● Unit Testing Delays, Errors & Retries with Kotlin Flows https:"//codingwithmohit.com/coroutines/unit-testing-delays- errors-retries-with-kotlin-flows/ ● Unit Testing Channels & Flows in Practice https:"//www.youtube.com/watch?v=h8bLIUi6HWU

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Thank You! www.twitter.com/heyitsmohit www.codingwithmohit.com