Unit Testing Kotlin Flows (Chicago Roboto 2021)

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Mohit Sarveiya Unit Testing Kotlin Flows @heyitsmohit

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Unit Testing Kotlin Flows ● Foundations ● Useful Libraries ● State Flows ● Shared Flows

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Foundations How run blocking test works

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Flow of data

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flowOfData: Flow .filter { it.locationType == LocationType.RESTAURANT } .map { it.name }

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flowOfData: Flow .filter { ... } .map { ... } .flatmap { ... }

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flowOfData: Flow .filter { ... } .map { ... } .flatmap { ... } How do we test this flow?

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Test Cases ● Successfully received data

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

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

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

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

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

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

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

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flowOfData: Flow .filter { ... } .map { ... } .flatmap { ... }

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@Test fun `should get data`() = runBlockingTest { val flowOfData = mockFlow() val locations: List = flowOfLocations.toList() } Create Mock Flow

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@Test fun `should get data`() = runBlockingTest { val flowOfData = mockFlow() val items: List = flowOfData.toList() }

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@Test fun `should get locations`() = runBlockingTest { val items: List = flowOfData.toList() }

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@Test fun `should get locations`() = runBlockingTest { val items: List = flowOfData.toList() }

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@Test fun `should get locations`() = runBlockingTest { val items: List = flowOfData.toList() }

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@Test fun `should get locations`() = runBlockingTest { val items: List = flowOfData.toList() }

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@Test fun `should get locations`() = runBlockingTest { val items: List = flowOfData.toList() items shouldContainAll listOf(item1, item2, item3) }

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@Test fun `should get data`() = runBlockingTest { val flowOfData = mockFlow() val items: List = flowOfData.toList() items shouldContainAll listOf(item1, item2, item3) }

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@Test fun `should get data`() = runBlockingTest { val locations: List = flowOfLocations.toList() locations shouldContainAll listOf(location1, location2, location3) } How does this work?

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

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

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@Test fun `should get data`() = runBlockingTest { val flowOfData = mockFlow() val items: List = flowOfData.toList() items shouldContainAll listOf(items1, items2, items3) }

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Run Blocking Test Different Approaches 1. Use run blocking test for all tests 2. Testing Delays -> Use run blocking test 3. Non Delay Uses Cases -> Use run blocking

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Useful Libraries Turbine

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@Test fun `should get data`() = runBlockingTest { val flowOfLocations = mockFlow() val items: List = flowOfData.toList() locations shouldContainAll listOf(location1, location2, location3) } Better way to collect from Flows?

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

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Test Cases ● Verify order of items ● Completion ● Error ● Timeouts

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What Turbine Provides? suspend fun Flow.test( ... ) coroutineScope { val events = Channel> (UNLIMITED)   launch { try { collect { ... } } catch { events.send(Event.Error) }

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interface FlowTurbine { suspend fun expectItem(): T fun expectNoEvents() fun expectError(): Throwable suspend fun expectComplete() } What Turbine Provides?

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interface FlowTurbine { suspend fun expectItem(): T fun expectNoEvents() fun expectError(): Throwable suspend fun expectComplete() } What Turbine Provides?

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interface FlowTurbine { suspend fun expectItem(): T fun expectNoEvents() fun expectError(): Throwable suspend fun expectComplete() } What Turbine Provides?

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interface FlowTurbine { suspend fun expectItem(): T fun expectNoEvents() fun expectError(): Throwable suspend fun expectComplete() } What Turbine Provides?

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Test Cases ● Verify order of items ● Completion ● Error ● Timeouts

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@Test fun `should get data`() = runBlockingTest { flowOfData.test { expectItem() shouldBeEqualTo location1 expectItem() shouldBeEqualTo location2 expectItem() shouldBeEqualTo location3 expectComplete() }   }

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@Test fun `should get data`() = runBlockingTest { flowOfData.test { expectItem() shouldBeEqualTo item1 expectItem() shouldBeEqualTo location2 expectItem() shouldBeEqualTo location3 expectComplete() }   } Assertion

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@Test fun `should get data`() = runBlockingTest { flowOfData.test { expectItem() shouldBeEqualTo item1 expectItem() shouldBeEqualTo item2 expectItem() shouldBeEqualTo item3 expectComplete() }   } Get 1st, 2nd, 3rd item

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@Test fun `should get data`() = runBlockingTest { flowOfData.test { expectItem() shouldBeEqualTo item1 expectItem() shouldBeEqualTo item2 }   } Missed collecting item

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@Test fun `should get data`() = runBlockingTest { flowOfData.test { expectItem() shouldBeEqualTo item1 expectItem() shouldBeEqualTo item2 }   } RepoTest.kt Run: app.cash.turbine.AssertionError: Expected complete but found Item(Data(…)) DataRepoTests should get data

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Test Cases ● Verify order of items ● Completion ● Error ● Timeouts

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@Test fun `should get locations`() = runBlockingTest { flowOfData.test { expectItem() shouldBeEqualTo item1 expectItem() shouldBeEqualTo item2 expectItem() shouldBeEqualTo item3 }   } RepoTests.kt Run: DataRepoTests should get data app.cash.turbine.AssertionError: Unconsumed events found:   - Complete

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How Turbine Works

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Flow to test How Turbine Works

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Channel (Unlimited) How Turbine Works

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Channel Send Receive

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How Turbine Works Channel (Unlimited) Event

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

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

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

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

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

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

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object Complete: Event() How Turbine Works Channel (Unlimited) How do I query items in channels?

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

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

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

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

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

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

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

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Channel (Unlimited) Testing With Turbine Data

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Channel (Unlimited) Testing With Turbine Data

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Channel (Unlimited) Testing With Turbine Query Channel in test

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Channel (Unlimited) Testing With Turbine Verify Flow completed

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@Test fun `should get locations`() = runBlockingTest { flowOfLocations.test { expectItem() shouldBeEqualTo location1 expectItem() shouldBeEqualTo location2 expectItem() shouldBeEqualTo location3 expectComplete() }   }

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Testing State Flows State Flow Use Cases

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State Flow Consumer 1 Consumer 2 State State

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State Flow View View Model

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State Flow View View Model State

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State Flow sealed class UiState {   data class Error(   val exception: Throwable   ): UiState() }

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State Flow sealed class UiState {   data class Success( val data: Data ): UiState() data class Error(   val exception: Throwable   ): UiState() }

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State Flow sealed class UiState {   data class Success( val data: Data ): UiState() data class Error(   val exception: Throwable   ): UiState() }

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State Flow val stateFlow = MutableStateFlow()

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val stateFlow = MutableStateFlow( UiState.Success(Data()) ) State Flow

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State Flow val stateFlow = MutableStateFlow(UiState.Success(Data())) @Test fun `should emit default value`() = runBlockingTest { stateFlow.test { expectItem() shouldBe UIState.Success } }

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State Flow Behavior ● Flow never completes normally

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State Flow @Test fun `should emit default value`() = runBlockingTest { stateFlow.test { expectItem() shouldBe UIState.Success   expectComplete() } }

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State Flow @Test fun `should emit default value`() = runBlockingTest { stateFlow.test { expectItem() shouldBe UIState.Success   expectComplete() } } ! Timed out waiting for 1000 ms kotlinx.coroutines.TimeoutCancellationException: Timed out waiting for 1000 ms

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State Flow Test Coroutine State Flow

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State Flow suspend fun Flow.test( ... ) { coroutineScope { val events = Channel> (UNLIMITED)   launch { collect { item ->  events.send(Event.Item(item)) } events.send(Event.Complete) } }

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State Flow Behavior ● Flow never completes normally ● Subscribers can complete exceptionally

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val stateFlow = MutableStateFlow(UiState.Success(Data())) State Flow stateFlow .onCompletion { println("ON COMPLETE") } .collect { println(it) }

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val stateFlow = MutableStateFlow(UiState.Success(Data())) State Flow stateFlow .onCompletion { println("ON COMPLETE") } .collect { println(it) } Will not log

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@Test fun `should emit default value`() = runBlockingTest { stateFlow .onCompletion { println("ON COMPLETE") } .test { expectItem() shouldBe UIState.Success } } State Flow

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@Test fun `should emit default value`() = runBlockingTest { stateFlow .onCompletion { println("ON COMPLETE") } .test { expectItem() shouldBe UIState.Success } } State Flow Will log

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How Turbine Works suspend fun Flow.test( ... ) { coroutineScope {   val collectJob = launch { collect { item -> ... } ... } ... collectJob.cancel() }

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@Test fun `should emit default value`() = runBlockingTest { stateFlow .onCompletion { println("ON COMPLETE") } .test { expectItem() shouldBe UIState.Success } } State Flow Will log

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State Flow Features ● Conflate Flow Emissions ● Get most recent emitted item

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val stateFlow = MutableStateFlow(UIState.Success) @Test fun `should emit default value`() = runBlockingTest { stateFlow.emit(UIState.Error) stateFlow.test { expectItem() shouldBe UIState.Success expectItem() shouldBe UIState.Error } } State Flow

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State Flow Features ● Conflate Flow Emissions ● Get most recent emitted item

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Mocking Flows View View Model Repository

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Mocking Flows View Model Repository

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Mocking Flows View Model Repository How to create a fake?

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Mocking Flows Approaches ● Channels ● State/Shared Flows

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Mocking Flows interface Repository { fun getData(): Flow }

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Mocking Flows class FakeRepository: Repository { private val channel = Channel()   }

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Mocking Flows class FakeRepository: Repository { private val channel = Channel()   override fun getData(): Flow { return channel.consumeAsFlow() } }

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Mocking Flows class FakeRepository: Repository { private val channel = Channel()   suspend fun emitResult(result: Result) { channel.send(result) } }

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Mocking Flows class FakeRepository: Repository { private val channel = Channel()   suspend fun emitFailure(result: Result.Failure) { channel.send(result) } }

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Mocking Flows class FakeRepository: Repository { private val channel = Channel()   fun closeChannel() = apply { channel.close() } }

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Mocking Flows Approaches ● Channels ● State/Shared Flows

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Mocking Flows class FakeRepository: Repository { val _stateFlow = MutableStateFlow(Result.Success)   }

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Mocking Flows class FakeRepository: Repository { val _stateFlow = MutableStateFlow(Result.Success)   val stateFlow = _stateFlow.asStateFlow() }

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Mocking Flows class FakeRepository: Repository { val _stateFlow = MutableStateFlow(Result.Success)   fun emitResult(result: Result) { _stateFlow.value = result } }

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State Flow Considerations ● Hard to test back to back emissions ● Conflates emissions

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Better Approach ● Use Shared Flow ● Test the effects of each emission

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State Flow View View Model State

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State Flow State Flow Flow 1 Flow 2 Flow 3 Combined Flow

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Flow Combine 1 A 1A Flow 1 Flow 2 Combined

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Flow Combine 1 2 A 1A 2A Flow 1 Flow 2 Combined

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Flow Combine 1 2 A 1A 2A Flow 1 Flow 2 Combined B 2B

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State Flow View View Model State Holder

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val stateFlow = MutableStateFlow(UIState.Success)   flow1.combine(flow2) { a, b -> } State Flow

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val stateFlow = MutableStateFlow(UIState.Success)   flow1.combine(flow2) { a, b -> combineItems(a, b) } State Flow

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val stateFlow = MutableStateFlow(UIState.Success)   flow1.combine(flow2) { a, b -> combineItems(a, b) }.collect { stateFlow.emit(it) } State Flow

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@Test fun `should combine flows`() = runBlockingTest {   val mockFlow1 = mockFlow()   val mockFlow2 = mockFlow() } State Flow

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@Test fun `should combine flows`() = runBlockingTest {   val mockFlow1 = mockFlow()   val mockFlow2 = mockFlow()   val stateHolder = StateHolder(mockFlow1, mockFlow2) } State Flow

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@Test fun `should combine flows`() = runBlockingTest {     val stateHolder = StateHolder(mockFlow1, mockFlow2)   stateFlow.test {   ...   } } State Flow

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State Flow Challenges ● Why didn’t the flow emit an item? Conflated? ● Too many flow combines makes it harder to follow the     data flow

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Testing Shared Flows Shared Flow Use Cases

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Shared Flow

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Shared Flow Consumer 1 Consumer 2

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Shared Flow Consumer 1 Consumer 2

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Shared Flow Consumer 1 Consumer 2 Event Event

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Shared Flow Consumer 1 Consumer 2 Replay Replay

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  Shared Flow Buffer

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Shared Flow val flow = MutableSharedFlow <> ()

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Shared Flow val flow = MutableSharedFlow()

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Shared Flow val flow = MutableSharedFlow() @Test fun `collect from shared flow`() = runBlockingTest { flow.emit("Event 1") }

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Shared Flow val flow = MutableSharedFlow() @Test fun `collect from shared flow`() = runBlockingTest { flow.emit("Event 1") flow.test { expectItem() shouldBeEqualTo "Event 1" } } Failed

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Shared Flow val flow = MutableSharedFlow() @Test fun `collect from shared flow`() = runBlockingTest { flow.emit("Event 1") flow.test { expectItem() shouldBeEqualTo "Event 1" } } Subscriber count 0

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Shared Flow val flow = MutableSharedFlow() @Test fun `collect from shared flow`() = runBlockingTest { flow.emit("Event 1") flow.test { expectItem() shouldBeEqualTo "Event 1" } } Flow doesn’t replay

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Shared Flow @Test fun `collect from shared flow`() = runBlockingTest {   val job = launch(start = CoroutineStart.LAZY) { flow.emit("Event 1") } }

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Shared Flow @Test fun `collect from shared flow`() = runBlockingTest {   val job = launch(start = CoroutineStart.LAZY) { flow.emit("Event 1") } }

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Shared Flow @Test fun `collect from shared flow`() = runBlockingTest {   val job = launch(start = CoroutineStart.LAZY) { flow.emit("Event 1") } flow.test { job.start() expectItem() shouldBeEqualTo "Event 1" } } Subscriber count 1

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Shared Flow Features ● Replay

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Shared Flow val flow = MutableSharedFlow(replay = 1)

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@Test fun `collect from shared flow`() = runBlockingTest { flow.emit("Event 1") flow.test { expectItem() shouldBeEqualTo "Event 1" } } Shared Flow val flow = MutableSharedFlow(replay = 1) Replay

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Mocking Flows View Model Repository How to create a fake?

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Mocking Flows Approaches ● Channels ● State/Shared Flows

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Mocking Flows class FakeRepository: Repository { val flow = MutableSharedFlow(   replay = 1,   BufferOverFlow.DROP_OLDEST   )   }

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Mocking Flows class FakeRepository: Repository { val flow = MutableSharedFlow(   replay = 1,   BufferOverFlow.DROP_OLDEST   )   suspend fun emitResult(result: Result) { flow.emit(result) } }

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Shared Flow Approach ● No conflation ● Handle testing each emission

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Converting to Shared Flows ● While Subscribed ● Eagerly ● Lazily

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While Subscribed val flow = upstreamFlow()

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flow.shareIn( sharingScope, SharingStarted.WhileSubscribed(), replay = 1 ) While Subscribed

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flow.shareIn( sharingScope, SharingStarted.WhileSubscribed(), replay = 1 ) While Subscribed

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flow.shareIn( sharingScope, SharingStarted.WhileSubscribed(), replay = 1 ) While Subscribed

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flow.shareIn( sharingScope, SharingStarted.WhileSubscribed(), replay = 1 ) While Subscribed

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While Subscribed @Test fun `collect with while subscribed strategy`() = runBlockingTest {   val sharingScope = TestCoroutineScope() val sharedFlow = flow .onStart { println("ON START") } .shareIn( sharingScope, SharingStarted.WhileSubscribed(), 1 ) }

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While Subscribed @Test fun `collect with while subscribed strategy`() = runBlockingTest {   val sharingScope = TestCoroutineScope() val sharedFlow = flow .shareIn( sharingScope, SharingStarted.WhileSubscribed(), 1 ) }

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While Subscribed ● Start upstream flow upon subscriber ● Remain active while external scope is active

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While Subscribed @Test fun `collect with while subscribed strategy`() = runBlockingTest {   val sharedFlow = flow .onStart { println("ON START") } .shareIn( ... ) sharedFlow.test { ... } }

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While Subscribed @Test fun `collect with while subscribed strategy`() = runBlockingTest {   val sharedFlow = flow .onStart { println("ON START") } .shareIn( ... ) sharedFlow.test { ... } }

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While Subscribed ● Start upstream flow upon subscriber ● Remain active while there are subscribers

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Eagerly ● Start upstream flow upon creation

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Eagerly @Test fun `collect with eagerly strategy`() = runBlockingTest {   val sharedFlow = flow .onStart { println("ON START") } .shareIn( sharingScope, SharingStarted.Eagerly, 1 ) }

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Eagerly @Test fun `collect with eagerly strategy`() = runBlockingTest {   val sharedFlow = flow .onStart { println("ON START") } .shareIn( sharingScope, SharingStarted.Eagerly, 1 ) } Will start

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Lazily ● Start upstream flow upon subscription ● Never stops upstream

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Lazily @Test fun `collect with lazily strategy`() = runBlockingTest {   val sharedFlow = flow .onStart { println("ON START") } .shareIn( sharingScope, SharingStarted.Lazily, 1 ) } Will start

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Lazily @Test fun `collect with lazily strategy`() = runBlockingTest {   val sharedFlow = flow .onComplete { println("ON Complete”) } .shareIn( sharingScope, SharingStarted.Lazily, 1 ) }

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Lazily ● Start upstream flow upon subscription ● Never stops upstream

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Converting to Shared Flows ● While Subscribed ● Eagerly ● Lazily

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https: // codingwithmohit.com/coroutines/learning-shared-and-state-flows-with-tests/ Coding with Mohit

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Thank You! www.codingwithmohit.com @heyitsmohit