Unit Testing Kotlin Channels & Flows

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● Use case with ViewModel & Repo ● Testing Repo & ViewModel ● Best practices for unit testing

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ViewModel Repository

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interface ApiService { @GET("/users/{id}") suspend fun userDetails(@Path("id") id: Int): UserDetails }

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class UserRepository(val apiService: ApiService) { fun userDetails(id: Int): Flow { } }

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fun userDetails(id: Int): Flow { } Coroutines Library fun flow( block: suspend FlowCollector.() .> Unit ): Flow

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fun userDetails(id: Int): Flow { } Coroutines Library interface FlowCollector { suspend fun emit(value: T) }

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fun userDetails(id: Int): Flow { return flow { val userDetails = apiService.userDetails(id) emit(Result.success(userDetails)) } }

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fun userDetails(id: Int): Flow { return flow { val userDetails = apiService.userDetails(id) emit(Result.success(userDetails)) } }

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fun userDetails(id: Int): Flow { return flow { val userDetails = apiService.userDetails(id) emit(Result.success(userDetails)) } } Coroutines Library Dispatcher - Control scheduling threads - Default, IO, Main, etc...

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class UserRepository( val apiService: ApiService, val dispatcher: CoroutineDispatcher ) { fun userDetails(id: Int): Flow { return flow { val users = apiService.userDetails(id) emit(Result.success(users)) }.flowOn(dispatcher) }

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ViewModel Repository

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class UserDetailViewModel : ViewModel { }

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class UserDetailViewModel : ViewModel { val scope = CoroutineScope(Dispatchers.Main) } Coroutines Library fun CoroutineScope(context: CoroutineContext)

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class UserDetailViewModel : ViewModel { val scope = CoroutineScope(Dispatchers.Main) scope.launch { } }

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class UserDetailViewModel(val repository: UserRepository) : ViewModel { val scope = CoroutineScope(Dispatchers.Main) scope.launch { } }

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class UserDetailViewModel(val repository: UserRepository) : ViewModel { val scope = CoroutineScope(Dispatchers.Main) scope.launch { val flow = repository.getUserDetails(id = 1) } }

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class UserDetailViewModel(val repository: UserRepository) : ViewModel { val scope = CoroutineScope(Dispatchers.Main) scope.launch { val flow = repository.getUserDetails(id = 1) } } Coroutines Library fun Flow.collect( action: suspend (value: T) .> Unit )

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class UserDetailViewModel( val repository: UserRepository, val stateManager: StateManager ) : ViewModel { val scope = CoroutineScope(Dispatchers.Main) scope.launch { val flow = repository.getUserDetails(id = 1) flow.collect { result: Result -> stateManager.dispatchState(result) } } }

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class UserDetailViewModel( val repository: UserRepository, val stateManager: StateManager ): ViewModel { { val scope = CoroutineScope(Dispatchers.Main) } Architecture Components val ViewModel.viewModelScope: CoroutineScope get() { val scope: CoroutineScope? = this.getTag(JOB_KEY) return scope }

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ViewModel Repository

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1. Flow Collection 2. Error handling 3. Retries with Delays (Advanced)

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testImplementation 'org.jetbrains.kotlinx:kotlinx-coroutines-test:1.3.3' ● Experimental ● runBlockingTest ● Test Coroutine Scope ● Test Coroutine Dispatcher

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1. Flow emits Success 2. Flow emits Error 3. Retries with delay (Advanced)

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class UserRepository( val apiService: ApiService, val dispatcher: CoroutineDispatcher ) { fun userDetails(id: Int): Flow { return flow { val userDetails = apiService.userDetails(id) emit(Result.success(userDetails)) }.flowOn(dispatcher) } }

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@Test fun `should get users details on success`() = runBlocking { } Coroutines Library RunBlocking - Blocks untils coroutine completes

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@Test fun `should get users details on success`() = runBlocking { val userDetails = UserDetails(1, "User 1", "avatar_url") val apiService = mock() }

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@Test fun `should get users details on success`() = runBlocking { val userDetails = UserDetails(1, "User 1", "avatar_url") val apiService = mock() } API Service suspend fun userDetails(id: Int): UserDetails

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@Test fun `should get users details on success`() = runBlocking { val userDetails = UserDetails(1, "User 1", "avatar_url") val apiService = mock() { on { userDetails(1) } doReturn userDetails } }

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@Test fun `should get users details on success`() = runBlocking { val userDetails = UserDetails(1, "User 1", "avatar_url") val apiService = mock() { on { userDetails(1) } doReturn userDetails } } Mockito-Kotlin fun on(methodCall: T.() .> R)

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@Test fun `should get users details on success`() = runBlocking { val userDetails = UserDetails(1, "User 1", "avatar_url") val apiService = mock() } Mockito-Kotlin fun onBlocking(m: suspend T.() .> R) { return runBlocking { Mockito.`when`(mock.m()) } }

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@Test fun `should get users details on success`() = runBlocking { val userDetails = UserDetails(1, "User 1", "avatar_url") val apiService = mock() { onBlocking { userDetails(1) } doReturn userDetails } val dispatcher = TestCoroutineDispatcher() val repository = UserRepository(userService, dispatcher) val flow = repository.getUserDetails(id = 1) }

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1. Flow emits Success 2. Flow emits Error 3. Retries with delay (Advanced)

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fun userDetails(id: Int): Flow { return flow { val userDetails = apiService.userDetails(id) emit(Result.success(userDetails)) }.flowOn(dispatcher) }

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fun userDetails(id: Int): Flow { return flow { val userDetails = apiService.userDetails(id) emit(Result.success(userDetails)) }.flowOn(dispatcher) } ViewModel scope.launch { val flow: = userRepository.getUserDetails(id = 1) flow.collect { } }

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fun userDetails(id: Int): Flow { return flow { val userDetails = apiService.userDetails(id) emit(Result.success(userDetails)) }.flowOn(dispatcher) } Coroutines-Library fun Flow.catch( action: suspend FlowCollector.(t: Throwable) .> Unit ): Flow

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fun userDetails(id: Int): Flow { return flow { val userDetails = apiService.userDetails(id) emit(Result.success(userDetails)) } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) }

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@Test fun `should get error for user details`() = runBlocking { val apiService = mock { onBlocking { userDetails(1) } doAnswer { throw IOException() } } }

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@Test fun `should get error for user details`() = runBlocking { val apiService = mock { onBlocking { userDetails(1) } doAnswer { throw IOException() } } val repository = UserRepository(apiService, dispatcher) val flow = repository.getUserDetails(id = 1) val result = flow.single() result.isFailure.assertTrue() }

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1. Flow emits Success 2. Flow emits Error 3. Retries with delay (Advanced)

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fun userDetails(id: Int): Flow { return flow { val userDetails = apiService.userDetails(id) emit(Result.success(userDetails)) }.catch { emit(Result.failure(it)) } } Coroutines-Library fun retry( retries: Long, block: suspend (Throwable) .>Boolean ): Flow

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fun userDetails(id: Int): Flow { return flow { val userDetails = apiService.userDetails(id) emit(Result.success(userDetails)) }.retry(retries = 2) { t -> (t is Exception).also { if (it) delay(DELAY_ONE_SECOND) } } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) }

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fun userDetails(id: Int): Flow { return flow { val userDetails = apiService.userDetails(id) emit(Result.success(users)) }.retry(retries = 2) { t -> (t is Exception).also { if (it) delay(DELAY_ONE_SECOND) } } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) }

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1. All Retries fail with Error 2. Retry succeeds

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1. Launch a coroutine with a test scope and test dispatcher. 2. Advance virtual time forward.

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@Test fun `should retry with error`() { } Coroutine-Test-Library fun runBlockingTest( block: suspend TestCoroutineScope.() .> Unit ) fun TestCoroutineDispatcher.runBlockingTest( block: suspend TestCoroutineScope.() .> Unit )

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@Test fun `should retry with error`() = dispatcher.runBlockingTest { }

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@Test fun `should retry with error`() = dispatcher.runBlockingTest { val apiService = mock { onBlocking { userDetails(1) } doAnswer { throw IOException() } } val flow = repository.getUserDetails(id = 1) flow.collect { result: Result -> result.isFailure.assertTrue() } }

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fun userDetails(id: Int): Flow { return flow { val users = userService.userDetails(id) emit(Result.success(users)) }.retry(retries = 2) { t -> (t is Exception).also { if (it) delay(DELAY_ONE_SECOND) } } .catch { emit(Result.failure(it)) } .flowOn(dispatcher) }

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1. All Retries fail with Error 2. Retry succeeds

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1. Pause Dispatcher 2. Advance virtual time forward by certain milliseconds.

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@Test fun `should retry with success`() = dispatcher.runBlockingTest { pauseDispatcher { } } Coroutine-Test-Library pausedDispatcher - Do not start coroutines eagerly

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@Test fun `should retry with success`() = dispatcher.runBlockingTest { var throwError = true val userDetails = UserDetails(1, "User 1", "avatar_url") val apiService = mock { onBlocking { userDetails(1) } doAnswer { if (throwError) throw IOException() else userDetails } } }

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@Test fun `should retry with success`() = dispatcher.runBlockingTest { var throwError = true val apiService = mock { onBlocking { userDetails(1) } doAnswer { if (throwError) throw IOException() else userDetails } } pauseDispatcher { val flow = repository.getUserDetails(id = 1) launch { flow.collect { it.isSuccess.assertTrue() } } } }

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@Test fun `should retry with success`() = dispatcher.runBlockingTest { val apiService = mock { onBlocking { userDetails(1) } doAnswer { if (throwError) throw IOException() else userDetails } } pauseDispatcher { val flow = repository.getUserDetails(id = 1) launch { flow.collect { it.isSuccess.assertTrue() } } advanceTimeBy(DELAY_ONE_SECOND) throwError = false advanceTimeBy(DELAY_ONE_SECOND) } }

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1. Flow emits Success 2. Flow emits Error 3. Retries with delay

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ViewModel Repository

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class UserDetailViewModel( val repository: UserRepository, val stateManager: StateManager ) : ViewModel { viewModelScope.launch { val flow = repository.getUserDetails(id = 1) flow.collect { result: Result -> stateManager.dispatchState(result) } } }

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1. Dispatchers.setMain() 2. Dispatchers.resetMain()

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class CoroutineTestRule( val dispatcher= TestCoroutineDispatcher() ) : TestWatcher() { override fun starting(description: Description?) { super.starting(description) Dispatchers.setMain(dispatcher) } }

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class CoroutineTestRule( val dispatcher= TestCoroutineDispatcher() ) : TestWatcher() { override fun finished(description: Description?) { super.finished(description) Dispatchers.resetMain() dispatcher.cleanupTestCoroutines() } }

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@get:Rule val rule = CoroutineTestRule() val repository = mock() val stateManager = mock() val viewModel = UserDetailsViewModel(repository, stateManager)

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@Test fun `should dispatch details`() = rule.dispatcher.runBlockingTest { }

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Flow Channel

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@Test fun `should dispatch details`() = rule.dispatcher.runBlockingTest { val userDetails = UserDetails(1, "User 1", "avatar") val result = Result.success(userDetails) }

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@Test fun `should dispatch details`() = rule.dispatcher.runBlockingTest { val userDetails = UserDetails(1, "User 1", "avatar") val result = Result.success(userDetails) val channel = Channel() val flow = channel.consumeAsFlow() whenever(repository.getUserDetails(id = 1)) doReturn flow } Flow Channel Convert to Flow

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ViewModel Repository

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@Test fun `should get users details on success`() = runBlocking { ... flow.collect { } } RxJava Observable.test()

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Flow Channel API fun expectItem(): T fun expectNoMoreEvents() fun expectComplete() fun expectError(): Throwable

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@Test fun `should get users details on success`() = runBlocking { ... flow.test { expectItem() assertEquals userDetails expectComplete() } }

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● Channels & Flow in Practice https://speakerdeck.com/heyitsmohit/channels-and-ﬂows-in-practice ● Dissecting Coroutines https://speakerdeck.com/heyitsmohit/dissecting-coroutines ● Koltin Channels Under the Hood https://medium.com/@heyitsmohit/kotlin-coroutine-channels-under-the-hood-part-1

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