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Márton Braun @zsmb13 Developer Relations Engineer Google Untangling Coroutine Testing KotlinConf’23 Amsterdam

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Agenda ● Testing suspending functions ● The coroutine test APIs ● Best practices ● Handling the Main dispatcher ● Flows & StateFlows

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Testing suspending functions runTest. run.

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Testing suspending functions suspend fun fetchData(): String { delay(1000L) return "Hello world" }

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Testing suspending functions suspend fun fetchData(): String { delay(1000L) return "Hello world" } @Test fun dataIsHelloWorld() { val data = fetchData() assertEquals("Hello world", data) }

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Testing suspending functions suspend fun fetchData(): String { delay(1000L) return "Hello world" } @Test fun dataIsHelloWorld() { val data = fetchData() assertEquals("Hello world", data) }

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Testing suspending functions suspend fun fetchData(): String { delay(1000L) return "Hello world" } @Test fun dataIsHelloWorld() = runTest { val data = fetchData() assertEquals("Hello world", data) } import kotlinx.coroutines.test.runTest

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Testing suspending functions suspend fun fetchData(): String { delay(1000L) return "Hello world" } @Test fun dataIsHelloWorld() = runTest { val data = fetchData() assertEquals("Hello world", data) } import kotlinx.coroutines.test.runTest

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Testing suspending functions suspend fun fetchData(): String { delay(1000L) return "Hello world" } @Test fun dataIsHelloWorld() = runTest { val data = fetchData() assertEquals("Hello world", data) } import kotlinx.coroutines.test.runTest

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Testing suspending functions suspend fun fetchData(): String { delay(1000L) return "Hello world" } @Test fun dataIsHelloWorld() = runTest { val data = fetchData() assertEquals("Hello world", data) } import kotlinx.coroutines.test.runTest

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Testing suspending functions suspend fun fetchData(): String { delay(1000L) return "Hello world" } @Test fun dataIsHelloWorld() = runTest { val data = fetchData() assertEquals("Hello world", data) } import kotlinx.coroutines.test.runTest

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Testing Dispatcher changes suspend fun fetchData(): String = withContext(Dispatchers.IO) { delay(1000L) "Hello world" } @Test fun dataIsHelloWorld() = runTest { val data = fetchData() assertEquals("Hello world", data) } import kotlinx.coroutines.test.runTest

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Testing Dispatcher changes suspend fun fetchData(): String = withContext(Dispatchers.IO) { delay(1000L) "Hello world" } @Test fun dataIsHelloWorld() = runTest { val data = fetchData() assertEquals("Hello world", data) } import kotlinx.coroutines.test.runTest *

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Testing Dispatcher changes suspend fun fetchData(): String = withContext(Dispatchers.IO) { delay(1000L) "Hello world" } @Test fun dataIsHelloWorld() = runTest { val data = fetchData() assertEquals("Hello world", data) } Test thread Dispatchers.IO

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Testing Dispatcher changes suspend fun fetchData(): String = withContext(Dispatchers.IO) { delay(1000L) "Hello world" } @Test fun dataIsHelloWorld() = runTest { val data = fetchData() assertEquals("Hello world", data) } Test thread Dispatchers.IO fetchData()

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Testing Dispatcher changes suspend fun fetchData(): String = withContext(Dispatchers.IO) { delay(1000L) "Hello world" } @Test fun dataIsHelloWorld() = runTest { val data = fetchData() assertEquals("Hello world", data) } Test thread Dispatchers.IO fetchData()

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Testing Dispatcher changes suspend fun fetchData(): String = withContext(Dispatchers.IO) { delay(1000L) "Hello world" } @Test fun dataIsHelloWorld() = runTest { val data = fetchData() assertEquals("Hello world", data) } Test thread Dispatchers.IO fetchData() delay()

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Testing Dispatcher changes suspend fun fetchData(): String = withContext(Dispatchers.IO) { delay(1000L) "Hello world" } @Test fun dataIsHelloWorld() = runTest { val data = fetchData() assertEquals("Hello world", data) } Test thread Dispatchers.IO fetchData() delay()

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Testing Dispatcher changes suspend fun fetchData(): String = withContext(Dispatchers.IO) { delay(1000L) "Hello world" } @Test fun dataIsHelloWorld() = runTest { val data = fetchData() assertEquals("Hello world", data) } Test thread Dispatchers.IO fetchData() delay() assert()

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Testing Dispatcher changes suspend fun fetchData(): String = withContext(Dispatchers.IO) { delay(1000L) "Hello world" } @Test fun dataIsHelloWorld() = runTest { val data = fetchData() assertEquals("Hello world", data) } Test thread Dispatchers.IO fetchData() delay() assert()

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Testing new coroutines

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Testing new coroutines @Test fun directExample() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals( listOf("Alice", "Bob"), repo.getAllUsers() ) }

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Testing new coroutines @Test fun directExample() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals( listOf("Alice", "Bob"), repo.getAllUsers() ) }

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Testing new coroutines class ProfileViewModel : ViewModel() { lateinit var user: User fun initialize() { viewModelScope.launch { user = fetchUser() } } } @Test fun directExample() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals( listOf("Alice", "Bob"), repo.getAllUsers() ) }

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Testing new coroutines class ProfileViewModel : ViewModel() { lateinit var user: User fun initialize() { viewModelScope.launch { user = fetchUser() } } } @Test fun indirectExample() = runTest { val viewModel = ProfileViewModel() viewModel.initialize() assertEquals("Sam", viewModel.user.name) } @Test fun directExample() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals( listOf("Alice", "Bob"), repo.getAllUsers() ) }

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Testing new coroutines class ProfileViewModel : ViewModel() { lateinit var user: User fun initialize() { viewModelScope.launch { user = fetchUser() } } } @Test fun indirectExample() = runTest { val viewModel = ProfileViewModel() viewModel.initialize() assertEquals("Sam", viewModel.user.name) } @Test fun directExample() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals( listOf("Alice", "Bob"), repo.getAllUsers() ) }

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Testing new coroutines class ProfileViewModel : ViewModel() { lateinit var user: User fun initialize() { viewModelScope.launch { user = fetchUser() } } } @Test fun indirectExample() = runTest { val viewModel = ProfileViewModel() viewModel.initialize() assertEquals("Sam", viewModel.user.name) } @Test fun directExample() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals( listOf("Alice", "Bob"), repo.getAllUsers() ) }

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The coroutine test APIs The latest and greatest in kotlinx.coroutines.test

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Test APIs runTest

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Test APIs runTest TestScope

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Test APIs runTest TestDispatcher TestScope

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Test APIs runTest TestDispatcher TestCoroutineScheduler TestScope

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Test APIs runTest TestDispatcher TestCoroutineScheduler TestScope TestDispatcher TestDispatcher

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Test APIs runTest TestCoroutineScheduler TestScope TestDispatcher TestDispatcher All TestDispatchers must share the same scheduler TestDispatcher

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Test APIs TestDispatcher TestCoroutineScheduler

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Test APIs TestCoroutineScheduler StandardTestDispatcher UnconfinedTestDispatcher TestDispatcher

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StandardTestDispatcher

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● Queues up coroutines on the scheduler StandardTestDispatcher

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● Queues up coroutines on the scheduler ● You need to advance those coroutines manually StandardTestDispatcher

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● Queues up coroutines on the scheduler ● You need to advance those coroutines manually ● runTest uses a StandardTestDispatcher by default StandardTestDispatcher runTest StandardTestDispatcher TestScope

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) }

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } this: TestScope

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) }

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } UserRepo()

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } UserRepo() reg("Alice")

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } UserRepo() reg("Bob") reg("Alice")

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } UserRepo() assert() reg("Bob") reg("Alice")

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StandardTestDispatcher UserRepo() assert() @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } reg("Bob") reg("Alice")

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StandardTestDispatcher ● You need to advance those coroutines manually

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StandardTestDispatcher now now 80 ms 90 ms 100 ms 500 ms 750 ms ● You need to advance those coroutines manually

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StandardTestDispatcher ● runCurrent() now now 80 ms 90 ms 100 ms 500 ms 750 ms ● You need to advance those coroutines manually

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StandardTestDispatcher ● runCurrent() now now 80 ms 90 ms 100 ms 500 ms 750 ms ● You need to advance those coroutines manually

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StandardTestDispatcher ● runCurrent() ● advanceTimeBy(delayTimeMillis: Long) now now 80 ms 90 ms 100 ms 500 ms 750 ms ● You need to advance those coroutines manually

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StandardTestDispatcher now now 80 ms 90 ms 100 ms 500 ms 750 ms ● runCurrent() ● advanceTimeBy(delayTimeMillis = 100) ● You need to advance those coroutines manually

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StandardTestDispatcher ● runCurrent() ● advanceTimeBy(delayTimeMillis = 100) now now 80 ms 90 ms 100 ms 500 ms 750 ms ● You need to advance those coroutines manually

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● You need to advance those coroutines manually StandardTestDispatcher ● runCurrent() ● advanceTimeBy(delayTimeMillis: Long) ● advanceUntilIdle() now now 80 ms 90 ms 100 ms 500 ms 750 ms

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● You need to advance those coroutines manually StandardTestDispatcher ● runCurrent() ● advanceTimeBy(delayTimeMillis: Long) ● advanceUntilIdle() now now 80 ms 90 ms 100 ms 500 ms 750 ms

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) }

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } advanceUntilIdle() assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) }

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } advanceUntilIdle() assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) }

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } advanceUntilIdle() assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } UserRepo()

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } advanceUntilIdle() assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } UserRepo() reg("Alice")

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } advanceUntilIdle() assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } UserRepo() reg("Alice") reg("Bob")

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } advanceUntilIdle() assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } UserRepo() reg("Alice") reg("Bob")

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Idle StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } advanceUntilIdle() assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } UserRepo() reg("Alice") reg("Bob")

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } advanceUntilIdle() assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } UserRepo() reg("Alice") reg("Bob") Idle

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } advanceUntilIdle() assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } UserRepo() reg("Alice") reg("Bob") Idle assert()

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } advanceUntilIdle() assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } UserRepo() reg("Alice") reg("Bob") assert()

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StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } advanceUntilIdle() assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } UserRepo() reg("Alice") reg("Bob") assert()

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UnconfinedTestDispatcher

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UnconfinedTestDispatcher ● Starts new coroutines eagerly

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UnconfinedTestDispatcher ● Starts new coroutines eagerly ● Can be a good choice for simple tests

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UnconfinedTestDispatcher ● Starts new coroutines eagerly ● Can be a good choice for simple tests ● Does not emulate real concurrency

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UnconfinedTestDispatcher @Test fun unconfinedTest() = runTest(UnconfinedTestDispatcher()) { val userRepo = UserRepository() launch { userRepo.register("Alice") } launch { userRepo.register("Bob") } assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) }

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UnconfinedTestDispatcher @Test fun unconfinedTest() = runTest(UnconfinedTestDispatcher()) { val userRepo = UserRepository() launch { userRepo.register("Alice") } launch { userRepo.register("Bob") } assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) }

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UnconfinedTestDispatcher @Test fun unconfinedTest() = runTest(UnconfinedTestDispatcher()) { val userRepo = UserRepository() launch { userRepo.register("Alice") } launch { userRepo.register("Bob") } assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) } this: TestScope

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UnconfinedTestDispatcher @Test fun unconfinedTest() = runTest(UnconfinedTestDispatcher()) { val userRepo = UserRepository() launch { userRepo.register("Alice") } launch { userRepo.register("Bob") } assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) } UserRepo()

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UnconfinedTestDispatcher @Test fun unconfinedTest() = runTest(UnconfinedTestDispatcher()) { val userRepo = UserRepository() launch { userRepo.register("Alice") } launch { userRepo.register("Bob") } assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) } UserRepo() reg("Alice")

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UnconfinedTestDispatcher @Test fun unconfinedTest() = runTest(UnconfinedTestDispatcher()) { val userRepo = UserRepository() launch { userRepo.register("Alice") } launch { userRepo.register("Bob") } assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) } UserRepo() reg("Alice") reg("Bob")

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UnconfinedTestDispatcher @Test fun unconfinedTest() = runTest(UnconfinedTestDispatcher()) { val userRepo = UserRepository() launch { userRepo.register("Alice") } launch { userRepo.register("Bob") } assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) } UserRepo() reg("Alice") reg("Bob") assert()

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UnconfinedTestDispatcher @Test fun unconfinedTest() = runTest(UnconfinedTestDispatcher()) { val userRepo = UserRepository() launch { userRepo.register("Alice") } launch { userRepo.register("Bob") } assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) } UserRepo() reg("Alice") reg("Bob") assert()

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TestDispatchers StandardTestDispatcher Queues up new coroutines Use by default UnconfinedTestDispatcher Starts new coroutines eagerly Use selectively

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TestDispatchers Queues up new coroutines Use by default Starts new coroutines eagerly Use selectively • As the Main dispatcher • For coroutines that collect Flows StandardTestDispatcher UnconfinedTestDispatcher

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Injecting dispatchers Something something heat exchangers

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Injecting dispatchers class Repository(private val database: Database) { private val scope = CoroutineScope(Dispatchers.IO) fun initialize() { scope.launch { database.populate() } } suspend fun fetchData(): String = withContext(Dispatchers.IO) { database.read() } }

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Injecting dispatchers class Repository(private val database: Database) { private val scope = CoroutineScope(Dispatchers.IO) fun initialize() { scope.launch { database.populate() } } suspend fun fetchData(): String = withContext(Dispatchers.IO) { database.read() } }

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Injecting dispatchers class Repository(private val database: Database) { private val scope = CoroutineScope(Dispatchers.IO) fun initialize() { scope.launch { database.populate() } } suspend fun fetchData(): String = withContext(Dispatchers.IO) { database.read() } }

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository(FakeDatabase()) repository.initialize() val data = repository.fetchData() assertEquals("Hello world", data) }

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository(FakeDatabase()) repository.initialize() val data = repository.fetchData() assertEquals("Hello world", data) } Test thread Dispatchers.IO

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository(FakeDatabase()) repository.initialize() val data = repository.fetchData() assertEquals("Hello world", data) } Repo(FakeDb()) Test thread Dispatchers.IO

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository(FakeDatabase()) repository.initialize() val data = repository.fetchData() assertEquals("Hello world", data) } Repo(FakeDb()) Test thread Dispatchers.IO db.populate() fun initialize() { scope.launch { database.populate() } }

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository(FakeDatabase()) repository.initialize() val data = repository.fetchData() assertEquals("Hello world", data) } Repo(FakeDb()) Test thread Dispatchers.IO db.read() db.populate() suspend fun fetchData() = withContext(Dispatchers.IO) { database.read() }

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assert() Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository(FakeDatabase()) repository.initialize() val data = repository.fetchData() assertEquals("Hello world", data) } Repo(FakeDb()) Test thread Dispatchers.IO db.read() db.populate()

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assert() Injecting dispatchers Repo(FakeDb()) Test thread Dispatchers.IO db.read() db.populate() @Test fun repoTest() = runTest { val repository = Repository(FakeDatabase()) repository.initialize() val data = repository.fetchData() assertEquals("Hello world", data) }

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Injecting dispatchers Repo(FakeDb()) Test thread Dispatchers.IO db.read() db.populate() assert() @Test fun repoTest() = runTest { val repository = Repository(FakeDatabase()) repository.initialize() val data = repository.fetchData() assertEquals("Hello world", data) }

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Injecting dispatchers Repo(FakeDb()) Test thread Dispatchers.IO db.read() db.populate() assert() @Test fun repoTest() = runTest { val repository = Repository(FakeDatabase()) repository.initialize() val data = repository.fetchData() assertEquals("Hello world", data) }

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, ioDispatcher ioDispatcher Injecting dispatchers class Repository( Dispatchers.IO) fun initialize() { scope.launch { database.populate() } } suspend fun fetchData(): String = withContext( ) { database.read() } } private val database: Database) { private val scope = CoroutineScope( Dispatchers.IO

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Dispatchers.IO Injecting dispatchers class Repository( private val ioDispatcher: CoroutineDispatcher = Dispatchers.IO, ioDispatcher) fun initialize() { scope.launch { database.populate() } } suspend fun fetchData(): String = withContext( ) { database.read() } } private val database: Database ) { private val scope = CoroutineScope( ioDispatcher ,

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Dispatchers.IO Injecting dispatchers class Repository( private val ioDispatcher: CoroutineDispatcher = Dispatchers.IO, ioDispatcher) fun initialize() { scope.launch { database.populate() } } suspend fun fetchData(): String = withContext( ) { database.read() } } private val database: Database ) { private val scope = CoroutineScope( ioDispatcher ,

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Dispatchers.IO Injecting dispatchers class Repository( private val ioDispatcher: CoroutineContext = Dispatchers.IO, ioDispatcher) fun initialize() { scope.launch { database.populate() } } suspend fun fetchData(): String = withContext( ) { database.read() } } private val database: Database ) { private val scope = CoroutineScope( ioDispatcher ,

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Dispatchers.IO Injecting dispatchers class Repository( private val ioDispatcher: CoroutineDispatcher = Dispatchers.IO, ioDispatcher) fun initialize() { scope.launch { database.populate() } } suspend fun fetchData(): String = withContext( ) { database.read() } } private val database: Database ) { private val scope = CoroutineScope( ioDispatcher ,

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FakeDatabase(), @Test fun repoTest() = runTest { val repository = Repository( ) repository.initialize() val data = repository.fetchData() assertEquals("Hello world", data) } Injecting dispatchers

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository( database = , ioDispatcher = StandardTestDispatcher(testScheduler), ) repository.initialize() advanceUntilIdle() val data = repository.fetchData() assertEquals("Hello world", data) } FakeDatabase()

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository( database = FakeDatabase(), ioDispatcher = StandardTestDispatcher(testScheduler), ) repository.initialize() advanceUntilIdle() val data = repository.fetchData() assertEquals("Hello world", data) } this: TestScope

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository( database = FakeDatabase(), ioDispatcher = StandardTestDispatcher(testScheduler), ) repository.initialize() advanceUntilIdle() val data = repository.fetchData() assertEquals("Hello world", data) }

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository( database = FakeDatabase(), ioDispatcher = StandardTestDispatcher(testScheduler), ) repository.initialize() advanceUntilIdle() val data = repository.fetchData() assertEquals("Hello world", data) } Repo(...)

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository( database = FakeDatabase(), ioDispatcher = StandardTestDispatcher(testScheduler), ) repository.initialize() advanceUntilIdle() val data = repository.fetchData() assertEquals("Hello world", data) } fun initialize() { scope.launch { database.populate() } } Repo(...)

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository( database = FakeDatabase(), ioDispatcher = StandardTestDispatcher(testScheduler), ) repository.initialize() advanceUntilIdle() val data = repository.fetchData() assertEquals("Hello world", data) } Repo(...) db.populate() fun initialize() { scope.launch { database.populate() } }

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository( database = FakeDatabase(), ioDispatcher = StandardTestDispatcher(testScheduler), ) repository.initialize() advanceUntilIdle() val data = repository.fetchData() assertEquals("Hello world", data) } Repo(...) db.populate()

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository( database = FakeDatabase(), ioDispatcher = StandardTestDispatcher(testScheduler), ) repository.initialize() advanceUntilIdle() val data = repository.fetchData() assertEquals("Hello world", data) } Repo(...) db.populate()

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository( database = FakeDatabase(), ioDispatcher = StandardTestDispatcher(testScheduler), ) repository.initialize() advanceUntilIdle() val data = repository.fetchData() assertEquals("Hello world", data) } Repo(...) db.populate() db.read() suspend fun fetchData() = withContext(ioDispatcher) { database.read() }

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository( database = FakeDatabase(), ioDispatcher = StandardTestDispatcher(testScheduler), ) repository.initialize() advanceUntilIdle() val data = repository.fetchData() assertEquals("Hello world", data) } Repo(...) db.populate() db.read() assert()

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository( database = FakeDatabase(), ioDispatcher = StandardTestDispatcher(testScheduler), ) repository.initialize() advanceUntilIdle() val data = repository.fetchData() assertEquals("Hello world", data) } fun initialize() { scope.launch { database.populate() } } This is a bad API, don’t do this

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Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository( database = FakeDatabase(), ioDispatcher = StandardTestDispatcher(testScheduler), ) repository.initialize().await(). val data = repository.fetchData() assertEquals("Hello world", data) } fun initialize(): Deferred { return scope.async { } } database.populate()

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suspend fun initialize() { } Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository( database = FakeDatabase(), ioDispatcher = StandardTestDispatcher(testScheduler), ) repository.initialize() val data = repository.fetchData() assertEquals("Hello world", data) } database.populate()

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Handling the Main dispatcher A UI thread without a UI

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Handling the Main dispatcher class HomeViewModel : ViewModel() { private val _message = MutableStateFlow("") val message: StateFlow get() = _message fun loadMessage() { viewModelScope.launch { _message.value = "Greetings!" } } }

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Handling the Main dispatcher class HomeViewModel : ViewModel() { private val _message = MutableStateFlow("") val message: StateFlow get() = _message fun loadMessage() { viewModelScope.launch { _message.value = "Greetings!" } } }

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Handling the Main dispatcher } val viewModel = HomeViewModel() viewModel.loadMessage() assertEquals("Greetings!", viewModel.message.value) @Test fun testGreeting() = runTest {

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Handling the Main dispatcher } val viewModel = HomeViewModel() viewModel.loadMessage() assertEquals("Greetings!", viewModel.message.value) @Test fun testGreeting() = runTest { java.lang.IllegalStateException: 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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Handling the Main dispatcher } val viewModel = HomeViewModel() viewModel.loadMessage() assertEquals("Greetings!", viewModel.message.value) @Test fun testGreeting() = runTest { java.lang.IllegalStateException: 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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Handling the Main dispatcher val testDispatcher = UnconfinedTestDispatcher(testScheduler) Dispatchers.setMain(testDispatcher) val viewModel = HomeViewModel() viewModel.loadMessage() assertEquals("Greetings!", viewModel.message.value) } @Test fun testGreeting() = runTest { java.lang.IllegalStateException: 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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Handling the Main dispatcher val testDispatcher = UnconfinedTestDispatcher(testScheduler) Dispatchers.setMain(testDispatcher) val viewModel = HomeViewModel() viewModel.loadMessage() assertEquals("Greetings!", viewModel.message.value) } @Test fun testGreeting() = runTest { java.lang.IllegalStateException: Module with the Main dispatcher had failed to initialize. For tests Dispatchers.setMain from kotlinx-coroutines-test module can be used this: TestScope

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java.lang.IllegalStateException: Module with the Main dispatcher had failed to initialize. For tests Dispatchers.setMain from kotlinx-coroutines-test module can be used Handling the Main dispatcher val testDispatcher = UnconfinedTestDispatcher(testScheduler) Dispatchers.setMain(testDispatcher) try { } finally { Dispatchers.resetMain() } val viewModel = HomeViewModel() viewModel.loadMessage() assertEquals("Greetings!", viewModel.message.value) } @Test fun testGreeting() = runTest {

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Handling the Main dispatcher @Test fun testGreeting() = runTest { val testDispatcher = UnconfinedTestDispatcher(testScheduler) Dispatchers.setMain(testDispatcher) try { val viewModel = HomeViewModel() viewModel.loadMessage() assertEquals("Greetings!", viewModel.message.value) } finally { Dispatchers.resetMain() } }

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Handling the Main dispatcher @Test fun testGreeting() = runTest { val testDispatcher = UnconfinedTestDispatcher(testScheduler) Dispatchers.setMain(testDispatcher) try { val viewModel = HomeViewModel() viewModel.loadMessage() assertEquals("Greetings!", viewModel.message.value) } finally { Dispatchers.resetMain() } } fun loadMessage() { viewModelScope.launch { _message.value = "Greetings!" } }

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Handling the Main dispatcher @Test fun testGreeting() = runTest { val testDispatcher = UnconfinedTestDispatcher(testScheduler) try { val viewModel = HomeViewModel() viewModel.loadMessage() assertEquals("Greetings!", viewModel.message.value) } finally { } } Dispatchers.setMain(testDispatcher) Dispatchers.resetMain()

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Handling the Main dispatcher class MainDispatcherRule( val testDispatcher: TestDispatcher = UnconfinedTestDispatcher(), ) : TestWatcher() { override fun starting(description: Description) { } override fun finished(description: Description) { } } Dispatchers.setMain(testDispatcher) Dispatchers.resetMain()

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Handling the Main dispatcher class HomeViewModelTestUsingRule { @get:Rule val mainDispatcherRule = MainDispatcherRule() @Test fun testGreeting() = runTest { val viewModel = HomeViewModel() viewModel.loadMessage() assertEquals("Greetings!", viewModel.message.value) } }

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class HomeViewModelTestUsingRule { @get:Rule val mainDispatcherRule = MainDispatcherRule() @Test fun testGreeting() = runTest { Handling the Main dispatcher val viewModel = HomeViewModel() viewModel.loadMessage() assertEquals("Greetings!", viewModel.message.value) } }

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Handling the Main dispatcher val viewModel = HomeViewModel() viewModel.loadMessage() assertEquals("Greetings!", viewModel.message.value) If you replace the Main dispatcher with a TestDispatcher, all new TestDispatchers will automatically share its scheduler class HomeViewModelTestUsingRule { @get:Rule val mainDispatcherRule = MainDispatcherRule() @Test fun testGreeting() = runTest { } }

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Handling the Main dispatcher val unconfinedDispatcher = UnconfinedTestDispatcher() val standardDispatcher = StandardTestDispatcher() class HomeViewModelTestUsingRule { @get:Rule val mainDispatcherRule = MainDispatcherRule() @Test fun testGreeting() = runTest { } } If you replace the Main dispatcher with a TestDispatcher, all new TestDispatchers will automatically share its scheduler

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Flows Suspenseful streams

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Flows interface DataSource { fun counts(): Flow } class Repository(private val dataSource: DataSource) { fun scores(): Flow { return dataSource.counts().map { it * 10 } } }

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Flows Test Repository Fake Data Source

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Flows interface DataSource { fun counts(): Flow } class Repository(private val dataSource: DataSource) { fun scores(): Flow { return dataSource.counts().map { it * 10 } } }

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Flows interface DataSource { fun counts(): Flow } class Repository(private val dataSource: DataSource) { fun scores(): Flow { return dataSource.counts().map { it * 10 } } } class ColdFakeDataSource : DataSource { override fun counts(): Flow { return flowOf(1, 2, 3, 4) } }

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Flows @Test fun useTerminalOperators() = runTest { val repository = Repository(ColdFakeDataSource()) }

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Flows @Test fun useTerminalOperators() = runTest { val repository = Repository(ColdFakeDataSource()) val first = repository.scores().first() assertEquals(10, first) }

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Flows @Test fun useTerminalOperators() = runTest { val repository = Repository(ColdFakeDataSource()) val first = repository.scores().first() assertEquals(10, first) val values = repository.scores().toList() assertEquals(10, values[0]) assertEquals(20, values[1]) assertEquals(4, values.size) }

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Flows @Test fun useTerminalOperators() = runTest { val repository = Repository(ColdFakeDataSource()) val first = repository.scores().first() assertEquals(10, first) val values = repository.scores().toList() assertEquals(10, values[0]) assertEquals(20, values[1]) assertEquals(4, values.size) val someValues = repository.scores().take(2).toList() assertEquals(10, someValues[0]) assertEquals(20, someValues[1]) }

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Flows class HotFakeDataSource : DataSource { private val flow = MutableSharedFlow() suspend fun emit(value: Int) = flow.emit(value) override fun counts(): Flow = flow }

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Flows class HotFakeDataSource : DataSource { private val flow = MutableSharedFlow() suspend fun emit(value: Int) = flow.emit(value) override fun counts(): Flow = flow }

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Flows class HotFakeDataSource : DataSource { private val flow = MutableSharedFlow() suspend fun emit(value: Int) = flow.emit(value) override fun counts(): Flow = flow }

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Flows @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) }

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Flows @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) val values = mutableListOf() }

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Flows launch(UnconfinedTestDispatcher()) { collect { values += it } } @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) val values = mutableListOf() } repository.scores().

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Flows launch(UnconfinedTestDispatcher()) { toList(values) } repository.scores(). @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) val values = mutableListOf() }

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Flows launch(UnconfinedTestDispatcher()) { } } @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) val values = mutableListOf() dataSource.emit(1) assertEquals(10, values[0]) repository.scores().toList(values) val collectJob =

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Flows launch(UnconfinedTestDispatcher()) { } } @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) val values = mutableListOf() dataSource.emit(1) assertEquals(10, values[0]) repository.scores().toList(values) val collectJob =

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Flows val collectJob = collectJob.cancel() @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) val values = mutableListOf() dataSource.emit(1) assertEquals(10, values[0]) repository.scores().toList(values) launch(UnconfinedTestDispatcher()) { } }

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Flows @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) val values = mutableListOf() dataSource.emit(1) assertEquals(10, values[0]) repository.scores().toList(values) this: TestScope backgroundScope.launch(UnconfinedTestDispatcher()) { } }

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backgroundScope.launch(UnconfinedTestDispatcher()) { repository.scores().toList(values) } dataSource.emit(1) assertEquals(10, values[0]) dataSource.emit(2) dataSource.emit(3) assertEquals(30, values.last()) assertEquals(3, values.size) } @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) val values = mutableListOf() Flows

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StateFlows Look! It’s a stream! It’s a state holder!

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Stream StateFlows State holder

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interface MyRepository { fun scores(): Flow } class MyViewModel(private val myRepository: MyRepository) : ViewModel() { private val _data = MutableStateFlow(0) val data: StateFlow = _data.asStateFlow() fun initialize() { viewModelScope.launch { myRepository.scores().collect { score -> _data.value = score } } } } StateFlows

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interface MyRepository { fun scores(): Flow } class MyViewModel(private val myRepository: MyRepository) : ViewModel() { private val _data = MutableStateFlow(0) val data: StateFlow = _data.asStateFlow() fun initialize() { viewModelScope.launch { myRepository.scores().collect { score -> _data.value = score } } } } StateFlows

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interface MyRepository { fun scores(): Flow } class MyViewModel(private val myRepository: MyRepository) : ViewModel() { private val _data = MutableStateFlow(0) val data: StateFlow = _data.asStateFlow() fun initialize() { viewModelScope.launch { myRepository.scores().collect { score -> _data.value = score } } } } StateFlows

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interface MyRepository { fun scores(): Flow } class MyViewModel(private val myRepository: MyRepository) : ViewModel() { private val _data = MutableStateFlow(0) val data: StateFlow = _data.asStateFlow() fun initialize() { viewModelScope.launch { myRepository.scores().collect { score -> _data.value = score } } } } StateFlows

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StateFlows Test ViewModel Fake Repository

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StateFlows class HotFakeRepository : MyRepository { private val flow = MutableSharedFlow() suspend fun emit(value: Int) = flow.emit(value) override fun scores(): Flow = flow }

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@get:Rule val mainDispatcherRule = MainDispatcherRule() StateFlows @Test fun testHotFakeRepository() = runTest { val fakeRepository = HotFakeRepository() val viewModel = MyViewModel(fakeRepository) }

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@get:Rule val mainDispatcherRule = MainDispatcherRule() StateFlows @Test fun testHotFakeRepository() = runTest { val fakeRepository = HotFakeRepository() val viewModel = MyViewModel(fakeRepository) }

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StateFlows @Test fun testHotFakeRepository() = runTest { val fakeRepository = HotFakeRepository() val viewModel = MyViewModel(fakeRepository) assertEquals(0, viewModel.data.value) }

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StateFlows @Test fun testHotFakeRepository() = runTest { val fakeRepository = HotFakeRepository() val viewModel = MyViewModel(fakeRepository) assertEquals(0, viewModel.data.value) viewModel.initialize() } fun initialize() { viewModelScope.launch { myRepository.scores().collect { score -> _data.value = score } } }

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StateFlows @Test fun testHotFakeRepository() = runTest { val fakeRepository = HotFakeRepository() val viewModel = MyViewModel(fakeRepository) assertEquals(0, viewModel.data.value) viewModel.initialize() fakeRepository.emit(1) assertEquals(1, viewModel.data.value) }

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StateFlows @Test fun testHotFakeRepository() = runTest { val fakeRepository = HotFakeRepository() val viewModel = MyViewModel(fakeRepository) assertEquals(0, viewModel.data.value) viewModel.initialize() fakeRepository.emit(1) assertEquals(1, viewModel.data.value) fakeRepository.emit(2) fakeRepository.emit(3) assertEquals(3, viewModel.data.value) }

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StateFlows @Test fun testHotFakeRepository() = runTest { val fakeRepository = HotFakeRepository() val viewModel = MyViewModel(fakeRepository) assertEquals(0, viewModel.data.value) viewModel.initialize() fakeRepository.emit(1) assertEquals(1, viewModel.data.value) fakeRepository.emit(2) fakeRepository.emit(3) assertEquals(3, viewModel.data.value) }

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StateFlows @Test fun testHotFakeRepository() = runTest { val fakeRepository = HotFakeRepository() val viewModel = MyViewModel(fakeRepository) assertEquals(0, viewModel.data.value) viewModel.initialize() fakeRepository.emit(1) assertEquals(1, viewModel.data.value) fakeRepository.emit(2) fakeRepository.emit(3) assertEquals(3, viewModel.data.value) }

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StateFlows with stateIn private val _data = MutableStateFlow(0) _data.asStateFlow() fun initialize() { .launch { .collect { count -> _data.value = count } } } myRepository: MyRepository myRepository.scores() class MyViewModel( ) : ViewModel() { private val viewModelScope val data: StateFlow = }

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StateFlows with stateIn myRepository: MyRepository myRepository.scores() .stateIn( , SharingStarted.WhileSubscribed(5000), 0) class MyViewModel( ) : ViewModel() { viewModelScope val data: StateFlow = }

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StateFlows with stateIn @Test fun testLazilySharingViewModel() = runTest { val fakeRepository = HotFakeRepository() val viewModel = MyViewModel(fakeRepository) assertEquals(0, viewModel.data.value) fakeRepository.emit(1) assertEquals(1, viewModel.data.value) }

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StateFlows with stateIn @Test fun testLazilySharingViewModel() = runTest { val fakeRepository = HotFakeRepository() val viewModel = MyViewModel(fakeRepository) assertEquals(0, viewModel.data.value) fakeRepository.emit(1) assertEquals(1, viewModel.data.value) }

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StateFlows with stateIn class MyViewModel( myRepository: MyRepository ) : ViewModel() { val data: StateFlow = myRepository.scores() .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5000), 0) }

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StateFlows with stateIn class MyViewModel( myRepository: MyRepository ) : ViewModel() { val data: StateFlow = myRepository.scores() .stateIn(viewModelScope, , 0) } Test ViewModel Fake Repository SharingStarted.WhileSubscribed(5000)

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StateFlows with stateIn class MyViewModel( myRepository: MyRepository ) : ViewModel() { val data: StateFlow = myRepository.scores() .stateIn(viewModelScope, , 0) } Test ViewModel Fake Repository SharingStarted.WhileSubscribed(5000)

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StateFlows with stateIn class MyViewModel( myRepository: MyRepository ) : ViewModel() { val data: StateFlow = myRepository.scores() .stateIn(viewModelScope, , 0) } Test ViewModel Fake Repository SharingStarted.WhileSubscribed(5000)

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StateFlows with stateIn class MyViewModel( myRepository: MyRepository ) : ViewModel() { val data: StateFlow = myRepository.scores() .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5000), 0) } Test ViewModel Fake Repository

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StateFlows with stateIn class MyViewModel( myRepository: MyRepository ) : ViewModel() { val data: StateFlow = myRepository.scores() .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5000), 0) } Test ViewModel Fake Repository

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StateFlows with stateIn class MyViewModel( myRepository: MyRepository ) : ViewModel() { val data: StateFlow = myRepository.scores() .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5000), 0) } SharingStarted.Lazily Test ViewModel Fake Repository

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StateFlows with stateIn @Test fun testLazilySharingViewModel() = runTest { val fakeRepository = HotFakeRepository() val viewModel = MyViewModel(fakeRepository) } assertEquals(0, viewModel.data.value) fakeRepository.emit(1) assertEquals(1, viewModel.data.value)

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assertEquals(0, viewModel.data.value) fakeRepository.emit(1) assertEquals(1, viewModel.data.value) StateFlows with stateIn @Test fun testLazilySharingViewModel() = runTest { val fakeRepository = HotFakeRepository() val viewModel = MyViewModel(fakeRepository) backgroundScope.launch(UnconfinedTestDispatcher()) { viewModel.data.collect() } }

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assertEquals(0, viewModel.data.value) fakeRepository.emit(1) assertEquals(1, viewModel.data.value) StateFlows with stateIn @Test fun testLazilySharingViewModel() = runTest { val fakeRepository = HotFakeRepository() val viewModel = MyViewModel(fakeRepository) backgroundScope.launch(UnconfinedTestDispatcher()) { viewModel.data.collect() } }

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Summary What did we learn today?

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Summary ● Use runTest for tests with coroutines ● Inject dispatchers into your classes to make them testable ● Create TestDispatchers as needed, always share a single scheduler ● Replace the Main dispatcher in unit tests (also simplifies sharing!) ● Use backgroundScope to create Flow collectors

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Resources ● goo.gle/coroutine-test-guide ● goo.gle/flow-test-guide ● github.com/cashapp/turbine

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Thank you, and don’t forget to vote! KotlinConf’23 Amsterdam Márton Braun @zsmb13 Developer Relations Engineer Google