Kotlin Coroutines - How to use it right

Hello! I am Yoyo/Thuy I am here because I am a woman and love sharing knowledge. 2

Problem 1 I want to execute long-running tasks in background thread, then update Views.

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6 Job Deferred join(): Unit await(): T Start a new coroutine launch async Return type Suspend till finish

7 fun main() = runBlocking { val job1 = launch { println("Start Job 1") val job2 = launch { println("Start Job 2") delay(100) launch { println("Start Job 3") delay(200) } println("Job 2 - Done!!") } job2.join() println("Job 1 - Done!!") } println("I am not blocking :)") job1.join() println("Done!!") } I am not blocking :) Start Job 1 Start Job 2 Job 2 - Done!! Start Job 3 Job 1 - Done!! Done!!

8 ThreadPool Thread 1 Thread 2 Dispatcher Coroutine 1 Coroutine 3 Coroutine 2 waits for Coroutine 2 Coroutine 1 starts Coroutine 2 starts Coroutine 3

Problem 2 I have closed Activity/Fragment that launched a coroutine and now its result is no longer needed and its operation should be cancelled.

10 ViewModelScope LifecycleScope class MyViewModel: ViewModel() { init { viewModelScope.launch { // Coroutine that will be canceled when the ViewModel is cleared. } } } class MyFragment: Fragment() { override fun onViewCreated(view: View, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) viewLifecycleOwner.lifecycleScope.launch { // Coroutine that will be canceled when when the Lifecycle is destroyed. } } }

Problem 3 Waiting for a result from a single-shot callback API and to return the result to the caller

12 suspend fun fetchDataFromAPI(): SomeAPIResponse? = suspendCoroutine { continuation -> val callback = object : Callback { override fun onFailure(call: Call,t: Throwable) { continuation.resume(null) } override fun onResponse( call: Call, response: Response ) { continuation.resume(response.body()) } } retrofit.create(ApiService:: class.java).fetchAPIData().enqueue(callback) }

Problem 4 Multi-shot callback APIs

14 Flows fun simple(): Flow = flow { for (i in 1..3) { delay(100) // pretend we are asynchronously waiting 100 ms emit(i) // emit next value } } fun main() = runBlocking { simple().collect { value -> delay(300) // pretend we are processing it for 300 ms println(value) } }

Problem 5 Exception Handling

CancellationException 16 fun main() = runBlocking { val job = launch(handler) { throw CancellationException() } job.join() println("Done!!") } fun main() = runBlocking { val job = launch { throw Exception() } job.join() println("Done!!") }

17 fun main() = runBlocking { val job = launch { try { println("Performing network request in Coroutine") delay(1000) } catch (e: Exception) { println("Handled exception in Coroutine") } println("Coroutine still running ... ") } delay(500) job.cancel() }

18 fun main() = runBlocking { val job = launch { try { println("Performing network request in Coroutine") delay(1000) } catch (e: HttpException) { println("Handled exception in Coroutine") } println("Coroutine still running ... ") } delay(500) job.cancel() }

19 fun main() = runBlocking { val job = launch { try { println("Performing network request in Coroutine") delay(1000) } catch (e: Exception) { if (e is CancellationException) { throw e } println("Handled exception in Coroutine") } println("Coroutine still running ... ") } delay(500) job.cancel() }

“ 20 Don’t catch CancellationExceptions

21 fun main() { val topLevelScope = CoroutineScope(Job()) topLevelScope.launch { try { launch { throw RuntimeException("RuntimeException in nested coroutine") } } catch (exception: RuntimeException) { println("Handle $exception") } } Thread.sleep(100) }

22 fun main() { val exceptionHandler = CoroutineExceptionHandler { _, exception -> println("CoroutineExceptionHandler got $exception") } val topLevelScope = CoroutineScope(Job()) topLevelScope.launch(exceptionHandler) { launch { throw RuntimeException("RuntimeException in nested coroutine") } } Thread.sleep(100) }

23 ▷ SupervisorJob ▷ supervisorScope

Problem 6 Shared mutable state

25 Send logs to server Clear logs Add logs log 1 log 2 log 1 log 2 log 3 log 1 log 2 log 3 Add logs Add logs log 1 log 2 log 3 log 4 log 4 is never sent to server

26 class LogSynchroniser( ... ) : CoroutineScope { private val mutex = Mutex() ... suspend fun addLogs(logs: List) { mutex.withLock { itemsQueue.addAll(logs) } } fun startSync() { launch { for (event in timer) { mutex.withLock { // Sends logs to server itemsQueue.clear() } } } } }

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