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Structured Concurrency Asynchronous Shift in Kotlin @manuelvicnt Manuel Vicente Vivo

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What problems are coroutines trying to solve?

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Coroutines simplify async programming

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Sync blocking fun loadData() { val data = networkRequest() show(data) } networkRequest show onDraw onDraw onDraw

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Sync blocking fun networkRequest(): Data { // Blocking network request code }

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fun loadData() { networkRequest { data -> show(data) } } Async with callbacks networkRequest show onDraw onDraw onDraw Network thread

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fun networkRequest(onSuccess: (Data) -> Unit) { DefaultScheduler.execute { // Blocking network request code postToMainThread(onSuccess(result)) } } Async with callbacks

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fun networkRequest(onSuccess: (Data) -> Unit) { DefaultScheduler.execute { // Blocking network request code postToMainThread(onSuccess(result)) } } Async with callbacks

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fun networkRequest(onSuccess: (Data) -> Unit) { DefaultScheduler.execute { // Blocking network request code postToMainThread(onSuccess(result)) } } Async with callbacks

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fun loadData() { networkRequest { data -> show(data) } } Async with callbacks

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fun loadData() { networkRequest { data -> anotherRequest(data) { otherData -> // ... } } } Async with callbacks

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fun loadData() { networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> networkRequest { data -> anotherRequest(data) { otherData -> // Hey there! You want more? } } } } } Callback Hell fun loadData() { networkRequest { data -> anotherRequest(data) { otherData ->

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Best of both worlds?

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suspend fun loadData() { val data = networkRequest() show(data) } Async with coroutines onDraw onDraw onDraw Network thread networkRequest show

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suspend fun loadData() { val data = networkRequest() show(data) }

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suspend fun loadData() { val data = networkRequest() show(data) } suspend

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suspend fun loadData() { val data = networkRequest() show(data) } suspend resume

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suspend fun loadData() { val data = networkRequest() show(data) } suspend Callback resume

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Callbacks? The Kotlin compiler writes them under the hood when the computation can suspend

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Continuation Coroutines call those “callbacks” Continuation

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Continuation suspend fun loadData() { val data = networkRequest() show(data) }

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Continuation fun loadData(continuation: Continuation) { val data = networkRequest(continuation) show(data) }

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State Machine Continuations form a state machine

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State Machine LoadData Network Request State 0 - init

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State Machine LoadData Network Request State 1 suspend

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State Machine LoadData Network Request State 2 resume

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State Machine LoadData Network Request State 3 - exit

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Continuation-Passing Style (CPS) Disclaimer Not Covered

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With Coroutines… Computation gets suspended without blocking the thread

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suspend fun loadData() { val data = networkRequest() show(data) }

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suspend fun loadData() { val data = networkRequest() show(data) } suspend fun networkRequest(): Data

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suspend fun loadData() { val data = networkRequest() show(data) } suspend fun networkRequest(): Data = withContext(Dispatchers.IO) { }

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suspend fun loadData() { val data = networkRequest() show(data) } suspend fun networkRequest(): Data = withContext(Dispatchers.IO) { // Blocking network request code } Dispatchers.IO

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.Default .Main Dispatchers .IO

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Dispatchers .IO .Main .Default

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.IO .Main .Default Network & Disk

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.IO .Main .Default Network & Disk CPU

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.IO .Main .Default Network & Disk UI/Non-blocking CPU

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suspend fun loadData() { val data = networkRequest() show(data) } suspend fun networkRequest(): Data = withContext(Dispatchers.IO) { // Blocking network request code }

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suspend fun loadData() { val data = networkRequest() show(data) } suspend fun networkRequest(): Data = withContext(Dispatchers.IO) { // Blocking network request code } Main Safe

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But… what is a coroutine?

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Coroutine Runnable with super powers Takes a block of code to run in a thread

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Asynchronicity expressed as sequential code that is easy to read and reason about Other perks: exception handling and cancellation ❤ Coroutine

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suspend fun loadData() { val data = networkRequest() show(data) }

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suspend fun loadData() { val data = networkRequest() show(data) } fun onButtonClicked() { loadData() } Suspend fun ‘loadData’ must be called from a coroutine

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suspend fun loadData() { val data = networkRequest() show(data) } fun onButtonClicked() { launch { loadData() } }

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suspend fun loadData() { val data = networkRequest() show(data) } fun onButtonClicked() { launch { loadData() } } Who can cancel this execution? Does it follow a particular lifecycle? Who gets exceptions if it fails?

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Structured Concurrency

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Scopes Keep track of coroutines Ability to cancel them Gets exceptions

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fun onButtonClicked() { launch { loadData() } } Launch must be called in a scope // MyViewModel.kt

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fun onButtonClicked() { launch { loadData() } } // MyViewModel.kt

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val scope = CoroutineScope(Dispatchers.Main) fun onButtonClicked() { launch { loadData() } } // MyViewModel.kt

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val scope = CoroutineScope(Dispatchers.Main) fun onButtonClicked() { scope.launch { loadData() } } // MyViewModel.kt

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val scope = CoroutineScope(Dispatchers.Main) fun onButtonClicked() { scope.launch { loadData() } } When loadData throws, the scope gets the exception

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val scope = CoroutineScope(Dispatchers.Main) Parent fun onButtonClicked() { scope.launch { loadData() } }

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val scope = CoroutineScope(Dispatchers.Main) Parent Child fun onButtonClicked() { scope.launch { loadData() } }

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fun onButtonClicked() { scope.launch { loadData() } } Child fun onCleared() { scope.cancel() }

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Cancelling a Scope Cancels all children coroutines Useless, cannot start more coroutines

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suspend fun loadData() { val data = networkRequest() show(data) }

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suspend fun loadData() { val data = networkRequest() show(data) } Runs in a scope

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When a function returns, it has completed all work

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When a function returns, it has completed all work suspend

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suspend fun loadData() { val data = networkRequest() show(data) }

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Scopes Exception Handling

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fun onButtonClicked() { scope.launch { loadData() } } val scope = CoroutineScope(Dispatchers.Main) // MyViewModel.kt

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fun onButtonClicked() { scope.launch { loadData() } } val scope = CoroutineScope( Dispatchers.Main + Job() ) // MyViewModel.kt

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Scope with a Job When a child fails, it propagates cancellation to other (scope) children When a failure is notified, the scope propagates the exception up

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fun onButtonClicked() { scope.launch { loadData() } } val scope = CoroutineScope( Dispatchers.Main + Job() ) // MyViewModel.kt

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fun onButtonClicked() { scope.launch { loadData() } } val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) // MyViewModel.kt

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Scope with a SupervisorJob The failure of a child doesn’t affect other (scope) children When a failure is notified, the scope doesn’t do anything

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fun onButtonClicked() { scope.launch { loadData() } } val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) // MyViewModel.kt

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TL;DR; work is completed suspend fun returns

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TL;DR; work is completed suspend fun returns scope cancelled children cancel

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TL;DR; work is completed suspend fun returns scope cancelled children cancel coroutine errors scope notified

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Create Coroutines

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Launch Async v s

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Launch Async

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Creates a new Coroutine Creates a new Coroutine Launch Async

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Launch Creates a new Coroutine Fire and Forget scope.launch(Dispatchers.IO) { loggingService.upload(logs) }

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Async Creates a new Coroutine Returns a value suspend fun getUser(userId: String): User = coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } deferred.await() }

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Async Creates a new Coroutine Returns a value suspend fun getUser(userId: String): User = coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } deferred.await() }

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Async Creates a new Coroutine Returns a value suspend fun getUser(userId: String): User = coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } deferred.await() }

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Async Creates a new Coroutine Returns a value suspend fun getUser(userId: String): User = coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } deferred.await() }

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Async Creates a new Coroutine Returns a value suspend fun getUser(userId: String): User = coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } deferred.await() }

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Async Creates a new Coroutine Returns a value suspend fun getUser(userId: String): User = coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } deferred.await() }

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Creates a new Coroutine Creates a new Coroutine Fire and Forget Returns a value Launch Async

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Creates a new Coroutine Creates a new Coroutine Takes a Dispatcher Takes a Dispatcher Fire and Forget Returns a value Launch Async

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Creates a new Coroutine Creates a new Coroutine Takes a Dispatcher Takes a Dispatcher Executed in a Scope Executed in a Scope Fire and Forget Returns a value Launch Async

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Creates a new Coroutine Creates a new Coroutine Takes a Dispatcher Takes a Dispatcher Executed in a Scope Executed in a Scope Not a suspend function Not a suspend function Fire and Forget Returns a value Launch Async

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Creates a new Coroutine Creates a new Coroutine Takes a Dispatcher Takes a Dispatcher Executed in a Scope Executed in a Scope Re-throws exceptions Not a suspend function Not a suspend function Fire and Forget Returns a value Launch Async

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Creates a new Coroutine Creates a new Coroutine Takes a Dispatcher Takes a Dispatcher Executed in a Scope Executed in a Scope Not a suspend function Not a suspend function Holds on exceptions until await is called Fire and Forget Returns a value Launch Async Re-throws exceptions

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Exception handling

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Wrapping in a try-catch block scope.launch(Dispatchers.Default) { try { loggingService.upload(logs) } catch(e: Exception) { // Handle Exception } }

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suspend fun getUser(userId: String): User { coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } try { deferred.await() } catch(e: Exception) { // Handle exception } } }

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suspend fun getUser(userId: String): User { coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } try { deferred.await() } catch(e: Exception) { // Handle exception } } }

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suspend fun getUser(userId: String): User { coroutineScope { val deferred = async(Dispatchers.IO) { userService.getUser(userId) } try { deferred.await() } catch(e: Exception) { // Handle exception } } }

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scope.launch(Dispatchers.IO) { for (name in files) { readFile(name) } }

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Cancellation requires co-operation

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scope.launch(Dispatchers.IO) { for (name in files) { readFile(name) } }

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Check if the coroutine is Active scope.launch(Dispatchers.IO) { for (name in files) { if (!isActive) break readFile(name) } }

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When to mark a function as suspend

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suspend fun loadData() { val data = networkRequest() show(data) } When it calls other suspend functions

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suspend fun loadData() { val data = networkRequest() show(data) } When it calls other suspend functions

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When NOT to mark a function as suspend

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When it doesn’t call suspend functions fun onButtonClicked() { scope.launch { loadData() } }

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Tip Don’t mark a function suspend unless you’re forced to

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Unit Testing Coroutines

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Use Case 1 The test doesn’t trigger new coroutines

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suspend fun loadData() { val data = networkRequest() show(data) } // MyViewModel.kt

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@Test fun `Test loadData happy path`() = runBlocking { val viewModel = MyViewModel() viewModel.loadData() // Assert show did something } // MyViewModelTest.kt

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Use Case 2 The test triggers new coroutines

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class MyViewModel { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { scope.launch { loadData() } } }

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class MyViewModel { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { scope.launch { loadData() } } }

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@Test fun `Test loadData happy path`() = runBlocking { val viewModel = MyViewModel() viewModel.onButtonClicked() // Assert show did something } // MyViewModelTest.kt

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@Test fun `Test loadData happy path`() = runBlocking { val viewModel = MyViewModel() viewModel.onButtonClicked() // Assert show did something } // MyViewModelTest.kt

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@Test fun `Test loadData happy path`() { val viewModel = MyViewModel() viewModel.onButtonClicked() // Wait for the result -> using a CountDownLatch // LiveDataTestUtil, Mockito await, etc. } // MyViewModelTest.kt

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@Test fun `Test loadData happy path`() { val viewModel = MyViewModel() viewModel.onButtonClicked() // Wait for the result -> using a CountDownLatch // LiveDataTestUtil, Mockito await, etc. } // MyViewModelTest.kt Code Smell!! Bad practice

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class MyViewModel { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { scope.launch { loadData() } } }

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class MyViewModel( private val dispatcher: CoroutineDispatcher ) { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { scope.launch(dispatcher) { loadData() } } }

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class MyViewModel( private val dispatcher: CoroutineDispatcher ) { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { scope.launch(dispatcher) { loadData() } } }

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val testDispatcher = TestCoroutineDispatcher() @Test fun `Test loadData happy path`() = testDispatcher.runBlockingTest { val viewModel = MyViewModel(testDispatcher) viewModel.onButtonClicked() // Assert show did something } // MyViewModelTest.kt

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val testDispatcher = TestCoroutineDispatcher() @Test fun `Test loadData happy path`() = testDispatcher.runBlockingTest { val viewModel = MyViewModel(testDispatcher) viewModel.onButtonClicked() // Assert show did something } // MyViewModelTest.kt

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val testDispatcher = TestCoroutineDispatcher() @Test fun `Test loadData happy path`() = testDispatcher.runBlockingTest { val viewModel = MyViewModel(testDispatcher) viewModel.onButtonClicked() // Assert show did something } // MyViewModelTest.kt

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class MyViewModel( private val dispatcher: CoroutineDispatcher ) { val scope = CoroutineScope( Dispatchers.Main + SupervisorJob() ) fun onButtonClicked() { // Do something else scope.launch(dispatcher) { loadData() } } }

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val testDispatcher = TestCoroutineDispatcher() @Test fun `Test loadData happy path`() = testDispatcher.runBlockingTest { val viewModel = MyViewModel(testDispatcher) testDispatcher.pauseDispatcher() viewModel.onButtonClicked() // Assert onButtonClicked did something else } // MyViewModelTest.kt

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val testDispatcher = TestCoroutineDispatcher() @Test fun `Test loadData happy path`() = testDispatcher.runBlockingTest { val viewModel = MyViewModel(testDispatcher) testDispatcher.pauseDispatcher() viewModel.onButtonClicked() // Assert onButtonClicked did something else testDispatcher.resumeDispatcher() // Assert show did something } // MyViewModelTest.kt

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Testing Use runBlocking when the test doesn’t create new coroutines

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Testing As a good practice, inject Dispatchers and use TestCoroutineDispatcher in tests

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What we covered What problems Coroutines solve Dispatchers & withContext What a Coroutine is How coroutines work under the hood Principles of Structured Concurrency How to create Coroutines Exception Handling When to mark a function as suspend Testing Coroutines & TestCoroutineDispatcher

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Thank You Questions? @manuelvicnt Manuel Vicente Vivo