Coroutines Made Easy

I reference the content of Kotlinconf 2019 and share some of my thoughts about Coroutines.

Transcript

1. Coroutines Made Easy Ivan Wu 1
2. Coroutines Basics 2
3. Why Coroutines? • Coroutines ◦ Light-weight threads • Threads are expensive ◦ Occupy 1-2 mb ◦ Require Thread pools to manage 3
4. Coroutine Builders • runBlocking ◦ Bridge between regular and suspend functions • launch ◦ Fire and forget • async ◦ Expect result 4
5. CoroutineScope • Keep track of coroutines • Ability to cancel ongoing work • Notified when a failure happens val scope = CoroutineScope(Job()) val job = scope.launch { ... } 5
6. finish complete start Job Lifecycle New Active Completing Completed Cancelling cancel/fail cancel/fail Cancelled finish 6
7. CoroutineContext • CoroutineDispatcher -> Threading • Job -> Lifecycle • CoroutineExceptionHandler • CoroutineName 7
8. How to Debug Coroutines? • Intellij ◦ jvmArgs ‘-ea’ • Android ◦ • Debug Agent 8 System.setProperty("kotlinx.coroutines.debug", "on") https://proandroiddev.com/android-coroutine-recipes-33467a4302e9 D/main @coroutine#587 [stopCapturing] D/DefaultDispatcher-worker-1 @coroutine#589 [saveImageToGallery]
9. Coroutines Cancellation 9
10. Cancellation by CoroutineScope val scope = CoroutineScope(Job()) val job = scope.launch { ... } val job2 = scope.launch { ... } scope.cancel() 10
11. Scopes in Android Active Cancelling Cancelled viewModelScope lifecycleScope .cancel() X https://android.jlelse.eu/coroutine-in-android-working-with-lifecycle-fc9c1a31e5f3 11
12. val startTime = System.currentTimeMillis() val job = launch (Dispatchers.Default) { var nextPrintTime = startTime var i = 0 while (i < 5) { if (...) { println("Hello ${i++}") nextPrintTime += 500L } } } delay(1000L) println("Cancel:") job.cancel() println("Done!") Hello 0 Hello 1 Hello 2 Cancel! Done! Hello 3 Hello 4 12
13. Cooperative Cancellation • job.isActive • ensureActive() • yield() 13
14. val startTime = System.currentTimeMillis() val job = launch (Dispatchers.Default) { var nextPrintTime = startTime var i = 0 while (i < 5 && isActive) { if (...) { println("Hello ${i++}") nextPrintTime += 500L } } } delay(1000L) println("Cancel:") job.cancel() println("Done!") Hello 0 Hello 1 Hello 2 Cancel! Done! 14
15. val startTime = System.currentTimeMillis() val job = launch (Dispatchers.Default) { var nextPrintTime = startTime var i = 0 while (i < 5) { ensureActive() if (...) { println("Hello ${i++}") nextPrintTime += 500L } } } delay(1000L) println("Cancel:") job.cancel() println("Done!") Hello 0 Hello 1 Hello 2 Cancel! Done! 15
16. val startTime = System.currentTimeMillis() val job = launch (Dispatchers.Default) { var nextPrintTime = startTime var i = 0 while (i < 5) { yeild() if (...) { println("Hello ${i++}") nextPrintTime += 500L } } } delay(1000L) println("Cancel:") job.cancel() println("Done!") Hello 0 Hello 1 Hello 2 Cancel! Done! 16
17. Cooperative Cancellation • isActive ◦ Do an action before finishing the coroutine • ensureActive() ◦ Instantaneously stop work • yield() ◦ CPU heavy computation that may exhaust thread-pool 17
18. Cancellable Suspend Functions val job = launch { workOne() yield() // or ensureActive() workTwo() } suspend fun workOne() = withContext(Dispatcher.IO) { otherWork() yield() // or ensureOtherWork() someOtherWork() } 18
19. Suspend Jobs Until the Job Is Completed • join() • await() 19
20. val job = launch(Dispatchers.Default) { repeat(5) { println("Hello $it") delay(500) } } delay(1000) job.join() println("Cancel!") job.cancel() println("Done!") Hello 0 Hello 1 Hello 2 Hello 3 Hello 4 Cancel! Done! 20
21. val job = launch(Dispatchers.Default) { repeat(5) { println("Hello $it") delay(500) } } delay(1000) println("Cancel!") job.cancel() job.join() println("Done!") Hello 0 Hello 1 Cancel! Done! 21
22. val job = async(Dispatchers.Default) { repeat(5) { println("Hello $it") delay(500) } } delay(1000) val result = job.await() println("Cancel!") job.cancel() println("Done!") Hello 0 Hello 1 Hello 2 Hello 3 Hello 4 Cancel! Done! 22
23. val job = async(Dispatchers.Default) { repeat(5) { println("Hello $it") delay(500) } } delay(1000) println("Cancel!") job.cancel() val result = job.await() println("Done!") Hello 0 Hello 1 Cancel! Done! JobCancellationException: Job was cancelled! 23
24. Suspend Jobs Until the Job Is Completed • join() ◦ No action if completed • await() ◦ Throws if completed 24
25. Try/Catch/Finally 25
26. val job = launch { try { work() } catch (e: CancellationException) { println("Work cancelled!") } } delay(1000L) println("Cancel!") job.cancel() println("Done!") Hello 0 Hello 1 Cancel! Work cancelled! Done! 26
27. val job = launch { try { work() } finally { println("Clean up!") delay(1000L) println("Cleanup done!") } } delay(1000L) println("Cancel!") job.cancel() println("Done!") Hello 0 Hello 1 Cancel! Clean up! Done! 27
28. val job = launch { try { work() } finally { println("Clean up!") withContext(NonCancellable) { delay(1000L) println("Cleanup done!") } } } delay(1000L) println("Cancel!") job.cancel() println("Done!") Hello 0 Hello 1 Cancel! Clean up! Cleanup done! Done! 28
29. Exceptions 29
30. Parent Child Child Child failed 30
31. Parent Child Child Child failed Cancel Cancel 31
32. Failures in a Scope • The failure of child cancels the scope & other children val scop = CoroutineScope(Job()) 32
33. SupervisorJob • The failure or cancellation of a child does not affect other children val scop = CoroutineScope(SupervisorJob()) 33
34. Scope Child Child Child failed 34
35. SupervisorJob only works if it is the coroutine’s direct parent. 35
36. val scope = CoroutineScope(Job()) scope.launch(SupervisorJob()) { launch { // Child 1 } launch { // Child 2 } } 36
37. val scope = CoroutineScope(Job()) scope.launch(SupervisorJob()) { launch { // Child 1 } launch { // Child 2 } } 37
38. val scope = CoroutineScope(Job()) scope.launch(SupervisorJob()) { launch { // Child 1 } launch { // Child 2 } } 38 SupervisorJob Job Child Job 1 Child Job 2 Job
39. val scope = CoroutineScope(Job()) scope.launch(SupervisorJob()) { supervisorScope { launch { // Child 1 } launch { // Child 2 } } } 39
40. val scope = CoroutineScope(Job()) val sharedJob = SupervisorJob() scope.launch(sharedJob) { // Child 1 } scope.launch(sharedJob) { // Child 2 } 40
41. Check the implementation of predefined scopes! E.g. viewModelScope or lifecycleScope 41
42. Handling Exceptions 42
43. Handling Exception in Coroutines • try/catch • runCatching • CoroutineExceptionHandler 43
44. Try/Catch scope.launch { try { codeThatThrowsExpception() } catch (e: Exception) { // Handle it! } } 44
45. Try/Catch supervisorScope { val deferred = async { codeThatThrowsException() } try { deferred.await() } catch (e: Exception) { // Handle it! } } 45
46. Try/Catch coroutineScope { val deferred = async { codeThatThrowsException() } try { deferred.await() } catch (e: Exception) { // This won't be called! } } 46
47. Use runCatching() scope.launch { val result = runCatching { codeThatThrowsExceptions() } if (result.isSuccess) { // Yeah! } else { // No! } } 47
48. CoroutineExceptionHandler val handler = CoroutineExceptionHandler { _, exception -> println("Caught $exception") } val scope = CoroutineScope(Job() + handler) scope.launch { launch { throw Exception("Boom!") } } 48
49. CoroutineExceptionHandler val handler = CoroutineExceptionHandler { _, exception -> println("Caught $exception") } val scope = CoroutineScope(Job()) scope.launch { launch(handler) { throw Exception("Boom!") } } 49
50. CoroutineExceptionHandler val handler = CoroutineExceptionHandler { _, exception -> println("Caught $exception") } val scope = CoroutineScope(Job()) scope.launch(handler) { launch { throw Exception("Boom!") } } 50
51. Testing in Coroutines 51
52. Using kotlinx-coroutines-test • Experimental APIs • runBlockingTest() ◦ It executes code in one thread ◦ Virtual-time control ◦ Cleanup assertions 52
53. Using kotlinx-coroutines-test • TestingScopes ◦ TestCoroutineScope ◦ TestCoroutineDispatcher ◦ TestCoroutineExceptionHandler 53
54. Testing Tips • Always inject scope or dispatcher • Inject main dispatcher ◦ Dispatchers.setMain ◦ Dispatchers.resetMain • Codelab 54
55. CREDITS: This presentation template was created by Slidesgo, including icons by Flaticon, and infographics & images by Freepik. THANKS! Do you have any questions? 55
56. References 56 • KotlinConf 2019: Coroutines! Gotta catch 'em all! by Florina Muntenescu & Manuel Vivo • KotlinConf 2019: Asynchronous Data Streams with Kotlin Flow by Roman Elizarov • KotlinConf 2019: Testing with Coroutines by Sean McQuillan • KotlinConf 2019: Coroutines Case Study - Cleaning Up an Async API by Tom Hanley