9. Introduction to Coroutines [kotlin-workshop]

Part of https://github.com/jetBrains/kotlin-workshop.

Covers:
- Comparing threads and coroutines
- async/await
- Programming with suspend functions

Similar talk was presented here: https://vimeo.com/239334237.

Transcript

1. Kotlin Coroutines
2. Kotlin Coroutines • the key new feature in Kotlin 1.1 • simplify asynchronous programming
3. Coroutine a main routine and a subroutine vs coroutines, which call on each other • the term from 1960s • was used in “The Art of Computer Programming” by Donald Knuth
4. Motivation: async/await
5. Motivation time consuming operation val image = loadImage(url) setImage(image)
6. Solution 1: callbacks loadImageAsync().whenComplete { image -> runOnUiThread { setImage(image) } }
7. Solution 2: async/await async(UI) { val image = loadImageAsync(url).await() setImage(image) }
8. val image = loadImageAsync(url).await() setImage(image) val image = loadImage(url) setImage(image) No callbacks!
9. C# way async Task ProcessImage(String url) { var image = await LoadImage(url); SetImage(image); } Kotlin way fun processImage() = async { val image = loadImageAsync().await() setImage(image) }
10. coroutines async/await Language Library async/await - functions defined in the standard library
11. Threads & Coroutines
12. Coroutine is similar to a thread Thread: • a sequence of instructions Multiple threads: • can be executed concurrently • share resources such as memory Coroutine: Multiple coroutines:
13. Coroutine = “lightweight thread” Thread Coroutine
14. Coroutine computation that can be suspended computation thread
15. Coroutine computation that can be suspended
16. Why suspend?
17. Asynchronous computations: how?
18. New thread for every computation simple & straightforward, but too expensive
19. Executor • fixed number of threads • adding tasks • but: difficult to manage dependencies
20. Example: waiting for two asynchronous computations to complete
21. thenCombine / zip CompletableFutures / RxJava: managing dependencies the rest of computation in a callback
22. Stepping back: what’s wrong with blocking thread? idle too expensive
23. Stepping back: what’s wrong with blocking UI thread? blocked user is blocked UI
24. 1 2 3 • computation that can be suspended • thread is not blocked! Coroutines
25. 1 2 3 • computation that can be suspended • thread is not blocked! UI UI UI Coroutines
26. suspend function computation that can be suspended
27. fun loadImageAsync() = async { /* do the work */ } fun processImage() = async { val image = loadImageAsync().await() setImage(image) } Back to image example
28. async starts new computation fun loadImageAsync() = async { /* do the work */ } loadImageAsync
29. suspending call fun processImage() = async { val image = loadImageAsync().await() setImage(image) } await suspends computation
30. fun loadImageAsync(): Deferred<Image> = async { /* do the work */ } interface Deferred<out T> {
 suspend fun await(): T
 } await is a suspend function
31. await suspends computation fun processImage() = async { val image = loadImageAsync().await() setImage(image) }
32. fun processImage() = async { val deferred = loadImageAsync() val image = deferred.await() setImage(image) } await suspends computation
33. await suspends computation processImage loadImageAsync 1 fun processImage() = async { val deferred = loadImageAsync() val image = deferred.await() setImage(image) }
34. await suspends computation processImage loadImageAsync 1 loadImageAsync processImage 2 await fun processImage() = async { val deferred = loadImageAsync() val image = deferred.await() setImage(image) }
35. await suspends computation loadImageAsync processImage 2 loadImageAsync processImage 3 …and continues it when result is ready fun processImage() = async { val deferred = loadImageAsync() val image = deferred.await() setImage(image) }
36. await suspends computation loadImageAsync processImage 2 loadImageAsync processImage 3 …and continues it when result is ready On which thread?
37. Q: On which thread can the coroutine be continued? A: You specify that.
38. Continue in any thread from thread pool async(CommonPool) {
 ...
 } 1 2 3 3
39. CommonPool is used by default async(CommonPool) {
 ...
 } async { ... } is the same as
40. Continue in the UI thread 1 2 3 async(UI) { ... } UI UI UI
41. Custom executor async(CustomContext) {
 ...
 } You can launch coroutine in the custom executor
42. Programming with suspend functions
43. Q: Can I define my custom suspend functions? A: Yes.
44. Example: simple consecutive logic fun login(credentials: Credentials): UserID fun loadUserData(userID: UserID): UserData fun showData(data: UserData) fun showUserInfo(credentials: Credentials) { val userID = login(credentials) val userData = loadUserData(userID) showData(userData) }
45. Rewrite with CompletableFuture fun loginAsync(credentials: Credentials): CompletableFuture<UserID> fun loadUserDataAsync(userID: UserID): CompletableFuture<UserData> fun showData(data: UserData) fun showUserInfo(credentials: Credentials) { loginAsync(credentials) .thenCompose { loadUserDataAsync(it) } .thenAccept { showData(it) } }
46. Rewrite with async/await fun login(credentials: Credentials): Deferred<UserID> fun loadUserData(userID: UserID): Deferred<UserData> fun showData(data: UserData) fun showUserInfo(credentials: Credentials) = async { val userID = login(credentials).await() val userData = loadUserData(userID).await() showData(userData) }
47. Rewrite with suspend functions suspend fun login(credentials: Credentials): UserID suspend fun loadUserData(userID: UserID): UserData fun showData(data: UserData) suspend fun showUserInfo(credentials: Credentials) { val userID = login(credentials) val userData = loadUserData(userID) showData(userData) }
48. Q: Where can I call suspend functions? A: Inside other suspend functions and inside async, launch, etc. (so-called, coroutine builders).
49. val deferred = async { … } deferred.await() val job = launch { … } job.join() runBlocking { … } Coroutine Builders start a coroutine
50. fun <T> async( context: CoroutineContext, block: suspend CoroutineScope.() -> T ) : Deferred<T> fun launch( context: CoroutineContext, block: suspend CoroutineScope.() -> Unit ): Job suspend lambdas
51. fun showUserInfo(credentials: Credentials) = launch(UI) {
 val userID = login(credentials)
 val data = loadUserData(userID)
 val image = async {
 loadImage(data.imageID)
 }
 showData(data)
 showImage(image.await())
 } Nested coroutines
52. Implementation details
53. suspend fun foo(): Int suspend fun foo(continuation: Continuation<Int>): Int Hidden parameter: continuation Continuation is a generic callback interface: public interface Continuation<in T> { public val context: CoroutineContext public fun resume(value: T) public fun resumeWithException(exception: Throwable) }
54. Coroutine body is compiled to a state machine 1 2 3 4 … state 1 state 2
55. Coroutines Regular world
56. Q: Can I call a suspend function from Java? suspend fun foo(continuation: Continuation<Int>): Int A: You don’t want to call it directly (because of Continuation parameter).
57. suspend fun foo(): Int fun fooAsync(): CompletableFuture<Int> = future { foo() } fun fooAsync(): Single<Int> = rxSingle(CommonPool) { foo() } To call suspend foo from Java wrap it into fooAsync if you have Java 8 if you have RxJava
58. fun loadUserData(userId: UserID, callback: (UserData?, Throwable?) -> Unit) suspend fun loadUserData(userId: UserID): UserData { return suspendCoroutine { continuation -> loadUserData(userId) { data, exception -> if (data != null) { continuation.resume(data) } else { continuation.resumeWithException(exception!!) } } } } Wrapping third-party callback-based API
59. fun loadUserDataRx(userId: UserID): Single<UserData> suspend fun loadUserData(userId: UserID): UserData { return suspendCoroutine { continuation -> loadUserDataRx(userId) .doOnSuccess { continuation.resume(it) } .doOnError { continuation.resumeWithException(it) } .subscribe() } } Wrapping a function returning rx.Single or just loadUserDataRx(userId).await()
61. coroutines async/await Language Library channels actors kotlinx.coroutines yield
62. Experimental status of Coroutines • We want the community to try it! • Migration aids will be provided • Old code will continue to work via the support library
63. kotlinx.coroutines • https://github.com/Kotlin/kotlinx.coroutines/ • Guide to kotlinx.coroutines by example • by Roman Elizarov (@relizarov)
65. • “Deep dive into coroutines on JVM” by Roman Elizarov
66. Copyright © 2017 https://github.com/JetBrains/kotlin-workshop