Untangling Coroutine Testing (KotlinConf '23)

Coroutines are embraced on Android as a tool to perform asynchronous operations and manage threading in your apps. Testing them requires some extra work and a solid understanding of scopes and dispatchers.

In this talk, we’ll look at how to test coroutines with the latest available testing APIs introduced in kotlinx.coroutines 1.6, from the simplest cases all the way to Flows.

More info and resources: https://zsmb.co/appearances/kotlinconf-2023-day1/

Transcript

1. Márton Braun @zsmb13 Developer Relations Engineer Google Untangling Coroutine Testing KotlinConf’23 Amsterdam
3. Agenda • Testing suspending functions • The coroutine test APIs • Best practices • Handling the Main dispatcher • Flows & StateFlows
4. Testing suspending functions runTest. run.
5. Testing suspending functions suspend fun fetchData(): String { delay(1000L) return "Hello world" }
6. Testing suspending functions suspend fun fetchData(): String { delay(1000L) return "Hello world" } @Test fun dataIsHelloWorld() { val data = fetchData() assertEquals("Hello world", data) }
7. Testing suspending functions suspend fun fetchData(): String { delay(1000L) return "Hello world" } @Test fun dataIsHelloWorld() { val data = fetchData() assertEquals("Hello world", data) }
8. 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
9. 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
10. 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
11. 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
12. 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
13. 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
14. 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 *
15. 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
16. 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()
17. 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()
18. 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()
19. 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()
20. 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()
21. 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()
22. Testing new coroutines
23. Testing new coroutines @Test fun directExample() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals( listOf("Alice", "Bob"), repo.getAllUsers() ) }
24. Testing new coroutines @Test fun directExample() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals( listOf("Alice", "Bob"), repo.getAllUsers() ) }
25. 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() ) }
26. 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() ) }
27. 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() ) }
28. 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() ) }
29. The coroutine test APIs The latest and greatest in kotlinx.coroutines.test
30. Test APIs runTest
31. Test APIs runTest TestScope
32. Test APIs runTest TestDispatcher TestScope
33. Test APIs runTest TestDispatcher TestCoroutineScheduler TestScope
34. Test APIs runTest TestDispatcher TestCoroutineScheduler TestScope TestDispatcher TestDispatcher
35. Test APIs runTest TestCoroutineScheduler TestScope TestDispatcher TestDispatcher All TestDispatchers must share the same scheduler TestDispatcher
36. Test APIs TestDispatcher TestCoroutineScheduler
37. Test APIs TestCoroutineScheduler StandardTestDispatcher UnconfinedTestDispatcher TestDispatcher
38. StandardTestDispatcher
39. • Queues up coroutines on the scheduler StandardTestDispatcher
40. • Queues up coroutines on the scheduler • You need to advance those coroutines manually StandardTestDispatcher
41. • Queues up coroutines on the scheduler • You need to advance those coroutines manually • runTest uses a StandardTestDispatcher by default StandardTestDispatcher runTest StandardTestDispatcher TestScope
42. StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) }
43. StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } this: TestScope
44. StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) }
45. StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } UserRepo()
46. 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")
47. 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")
48. 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")
49. 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")
50. StandardTestDispatcher • You need to advance those coroutines manually
51. StandardTestDispatcher now now 80 ms 90 ms 100 ms 500 ms 750 ms • You need to advance those coroutines manually
52. StandardTestDispatcher • runCurrent() now now 80 ms 90 ms 100 ms 500 ms 750 ms • You need to advance those coroutines manually
53. StandardTestDispatcher • runCurrent() now now 80 ms 90 ms 100 ms 500 ms 750 ms • You need to advance those coroutines manually
54. StandardTestDispatcher • runCurrent() • advanceTimeBy(delayTimeMillis: Long) now now 80 ms 90 ms 100 ms 500 ms 750 ms • You need to advance those coroutines manually
55. StandardTestDispatcher now now 80 ms 90 ms 100 ms 500 ms 750 ms • runCurrent() • advanceTimeBy(delayTimeMillis = 100) • You need to advance those coroutines manually
56. StandardTestDispatcher • runCurrent() • advanceTimeBy(delayTimeMillis = 100) now now 80 ms 90 ms 100 ms 500 ms 750 ms • You need to advance those coroutines manually
57. • 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
58. • 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
59. StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) }
60. StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } advanceUntilIdle() assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) }
61. StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } advanceUntilIdle() assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) }
62. StandardTestDispatcher @Test fun standardTest() = runTest { val repo = UserRepository() launch { repo.register("Alice") } launch { repo.register("Bob") } advanceUntilIdle() assertEquals(listOf("Alice", "Bob"), repo.getAllUsers()) } UserRepo()
63. 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")
64. 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")
65. 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")
66. 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")
67. 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
68. 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()
69. 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()
70. 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()
71. UnconfinedTestDispatcher
72. UnconfinedTestDispatcher • Starts new coroutines eagerly
73. UnconfinedTestDispatcher • Starts new coroutines eagerly • Can be a good choice for simple tests
74. UnconfinedTestDispatcher • Starts new coroutines eagerly • Can be a good choice for simple tests • Does not emulate real concurrency
75. UnconfinedTestDispatcher @Test fun unconfinedTest() = runTest(UnconfinedTestDispatcher()) { val userRepo = UserRepository() launch { userRepo.register("Alice") } launch { userRepo.register("Bob") } assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) }
76. UnconfinedTestDispatcher @Test fun unconfinedTest() = runTest(UnconfinedTestDispatcher()) { val userRepo = UserRepository() launch { userRepo.register("Alice") } launch { userRepo.register("Bob") } assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) }
77. 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
78. UnconfinedTestDispatcher @Test fun unconfinedTest() = runTest(UnconfinedTestDispatcher()) { val userRepo = UserRepository() launch { userRepo.register("Alice") } launch { userRepo.register("Bob") } assertEquals(listOf("Alice", "Bob"), userRepo.getAllUsers()) } UserRepo()
79. 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")
80. 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")
81. 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()
82. 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()
83. TestDispatchers StandardTestDispatcher Queues up new coroutines Use by default UnconfinedTestDispatcher Starts new coroutines eagerly Use selectively
84. 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
85. Injecting dispatchers Something something heat exchangers
86. 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() } }
87. 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() } }
88. 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() } }
89. Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository(FakeDatabase()) repository.initialize() val data = repository.fetchData() assertEquals("Hello world", data) }
90. Injecting dispatchers @Test fun repoTest() = runTest { val repository = Repository(FakeDatabase()) repository.initialize() val data = repository.fetchData() assertEquals("Hello world", data) } Test thread Dispatchers.IO
91. 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
92. 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() } }
93. 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() }
94. 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()
95. 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) }
96. 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) }
97. 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) }
98. , 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
99. 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 ,
100. 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 ,
101. 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 ,
102. 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 ,
103. FakeDatabase(), @Test fun repoTest() = runTest { val repository = Repository( ) repository.initialize() val data = repository.fetchData() assertEquals("Hello world", data) } Injecting dispatchers
104. 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()
105. 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
106. 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) }
107. 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(...)
108. 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(...)
109. 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() } }
110. 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()
111. 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()
112. 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() }
113. 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()
114. 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
115. 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<Unit> { return scope.async { } } database.populate()
116. 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()
117. Handling the Main dispatcher A UI thread without a UI
118. Handling the Main dispatcher class HomeViewModel : ViewModel() { private val _message = MutableStateFlow("") val message: StateFlow<String> get() = _message fun loadMessage() { viewModelScope.launch { _message.value = "Greetings!" } } }
119. Handling the Main dispatcher class HomeViewModel : ViewModel() { private val _message = MutableStateFlow("") val message: StateFlow<String> get() = _message fun loadMessage() { viewModelScope.launch { _message.value = "Greetings!" } } }
120. Handling the Main dispatcher } val viewModel = HomeViewModel() viewModel.loadMessage() assertEquals("Greetings!", viewModel.message.value) @Test fun testGreeting() = runTest {
121. 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
122. 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
123. 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
124. 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
125. 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 {
126. 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() } }
127. 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!" } }
128. 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()
129. 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()
130. 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) } }
131. 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) } }
132. 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 { } }
133. 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
134. Flows Suspenseful streams
135. Flows interface DataSource { fun counts(): Flow<Int> } class Repository(private val dataSource: DataSource) { fun scores(): Flow<Int> { return dataSource.counts().map { it * 10 } } }
136. Flows Test Repository Fake Data Source
137. Flows interface DataSource { fun counts(): Flow<Int> } class Repository(private val dataSource: DataSource) { fun scores(): Flow<Int> { return dataSource.counts().map { it * 10 } } }
138. Flows interface DataSource { fun counts(): Flow<Int> } class Repository(private val dataSource: DataSource) { fun scores(): Flow<Int> { return dataSource.counts().map { it * 10 } } } class ColdFakeDataSource : DataSource { override fun counts(): Flow<Int> { return flowOf(1, 2, 3, 4) } }
139. Flows @Test fun useTerminalOperators() = runTest { val repository = Repository(ColdFakeDataSource()) }
140. Flows @Test fun useTerminalOperators() = runTest { val repository = Repository(ColdFakeDataSource()) val first = repository.scores().first() assertEquals(10, first) }
141. 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) }
142. 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]) }
143. Flows class HotFakeDataSource : DataSource { private val flow = MutableSharedFlow<Int>() suspend fun emit(value: Int) = flow.emit(value) override fun counts(): Flow<Int> = flow }
144. Flows class HotFakeDataSource : DataSource { private val flow = MutableSharedFlow<Int>() suspend fun emit(value: Int) = flow.emit(value) override fun counts(): Flow<Int> = flow }
145. Flows class HotFakeDataSource : DataSource { private val flow = MutableSharedFlow<Int>() suspend fun emit(value: Int) = flow.emit(value) override fun counts(): Flow<Int> = flow }
146. Flows @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) }
147. Flows @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) val values = mutableListOf<Int>() }
148. Flows launch(UnconfinedTestDispatcher()) { collect { values += it } } @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) val values = mutableListOf<Int>() } repository.scores().
149. Flows launch(UnconfinedTestDispatcher()) { toList(values) } repository.scores(). @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) val values = mutableListOf<Int>() }
150. Flows launch(UnconfinedTestDispatcher()) { } } @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) val values = mutableListOf<Int>() dataSource.emit(1) assertEquals(10, values[0]) repository.scores().toList(values) val collectJob =
151. Flows launch(UnconfinedTestDispatcher()) { } } @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) val values = mutableListOf<Int>() dataSource.emit(1) assertEquals(10, values[0]) repository.scores().toList(values) val collectJob =
152. Flows val collectJob = collectJob.cancel() @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) val values = mutableListOf<Int>() dataSource.emit(1) assertEquals(10, values[0]) repository.scores().toList(values) launch(UnconfinedTestDispatcher()) { } }
153. Flows @Test fun continuouslyCollect() = runTest { val dataSource = HotFakeDataSource() val repository = Repository(dataSource) val values = mutableListOf<Int>() dataSource.emit(1) assertEquals(10, values[0]) repository.scores().toList(values) this: TestScope backgroundScope.launch(UnconfinedTestDispatcher()) { } }
154. 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<Int>() Flows
156. StateFlows Look! It’s a stream! It’s a state holder!
157. Stream StateFlows State holder
158. interface MyRepository { fun scores(): Flow<Int> } class MyViewModel(private val myRepository: MyRepository) : ViewModel() { private val _data = MutableStateFlow(0) val data: StateFlow<Int> = _data.asStateFlow() fun initialize() { viewModelScope.launch { myRepository.scores().collect { score -> _data.value = score } } } } StateFlows
159. interface MyRepository { fun scores(): Flow<Int> } class MyViewModel(private val myRepository: MyRepository) : ViewModel() { private val _data = MutableStateFlow(0) val data: StateFlow<Int> = _data.asStateFlow() fun initialize() { viewModelScope.launch { myRepository.scores().collect { score -> _data.value = score } } } } StateFlows
160. interface MyRepository { fun scores(): Flow<Int> } class MyViewModel(private val myRepository: MyRepository) : ViewModel() { private val _data = MutableStateFlow(0) val data: StateFlow<Int> = _data.asStateFlow() fun initialize() { viewModelScope.launch { myRepository.scores().collect { score -> _data.value = score } } } } StateFlows
161. interface MyRepository { fun scores(): Flow<Int> } class MyViewModel(private val myRepository: MyRepository) : ViewModel() { private val _data = MutableStateFlow(0) val data: StateFlow<Int> = _data.asStateFlow() fun initialize() { viewModelScope.launch { myRepository.scores().collect { score -> _data.value = score } } } } StateFlows
162. StateFlows Test ViewModel Fake Repository
163. StateFlows class HotFakeRepository : MyRepository { private val flow = MutableSharedFlow<Int>() suspend fun emit(value: Int) = flow.emit(value) override fun scores(): Flow<Int> = flow }
164. @get:Rule val mainDispatcherRule = MainDispatcherRule() StateFlows @Test fun testHotFakeRepository() = runTest { val fakeRepository = HotFakeRepository() val viewModel = MyViewModel(fakeRepository) }
165. @get:Rule val mainDispatcherRule = MainDispatcherRule() StateFlows @Test fun testHotFakeRepository() = runTest { val fakeRepository = HotFakeRepository() val viewModel = MyViewModel(fakeRepository) }
166. StateFlows @Test fun testHotFakeRepository() = runTest { val fakeRepository = HotFakeRepository() val viewModel = MyViewModel(fakeRepository) assertEquals(0, viewModel.data.value) }
167. 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 } } }
168. 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) }
169. 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) }
170. 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) }
171. 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) }
172. 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<Int> = }
173. StateFlows with stateIn myRepository: MyRepository myRepository.scores() .stateIn( , SharingStarted.WhileSubscribed(5000), 0) class MyViewModel( ) : ViewModel() { viewModelScope val data: StateFlow<Int> = }
174. 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) }
175. 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) }
176. StateFlows with stateIn class MyViewModel( myRepository: MyRepository ) : ViewModel() { val data: StateFlow<Int> = myRepository.scores() .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5000), 0) }
177. StateFlows with stateIn class MyViewModel( myRepository: MyRepository ) : ViewModel() { val data: StateFlow<Int> = myRepository.scores() .stateIn(viewModelScope, , 0) } Test ViewModel Fake Repository SharingStarted.WhileSubscribed(5000)
178. StateFlows with stateIn class MyViewModel( myRepository: MyRepository ) : ViewModel() { val data: StateFlow<Int> = myRepository.scores() .stateIn(viewModelScope, , 0) } Test ViewModel Fake Repository SharingStarted.WhileSubscribed(5000)
179. StateFlows with stateIn class MyViewModel( myRepository: MyRepository ) : ViewModel() { val data: StateFlow<Int> = myRepository.scores() .stateIn(viewModelScope, , 0) } Test ViewModel Fake Repository SharingStarted.WhileSubscribed(5000)
180. StateFlows with stateIn class MyViewModel( myRepository: MyRepository ) : ViewModel() { val data: StateFlow<Int> = myRepository.scores() .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5000), 0) } Test ViewModel Fake Repository
181. StateFlows with stateIn class MyViewModel( myRepository: MyRepository ) : ViewModel() { val data: StateFlow<Int> = myRepository.scores() .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5000), 0) } Test ViewModel Fake Repository
182. StateFlows with stateIn class MyViewModel( myRepository: MyRepository ) : ViewModel() { val data: StateFlow<Int> = myRepository.scores() .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5000), 0) } SharingStarted.Lazily Test ViewModel Fake Repository
183. 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)
184. 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() } }
185. 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() } }
186. Summary What did we learn today?
187. 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
188. Resources • goo.gle/coroutine-test-guide • goo.gle/flow-test-guide • github.com/cashapp/turbine
189. Thank you, and don’t forget to vote! KotlinConf’23 Amsterdam Márton Braun @zsmb13 Developer Relations Engineer Google