Dmytro Zaitsev. Mobile Team Lead at Lohika, Ukraine.

1. Coroutines in Kotlin:
In-depth review
Dmytro Zaitsev
Team Leader @ Lóhika

2. Blocking VS Non-blocking

4. v1: Sequential
(Direct style)
fun postItem(item: Item) {
val token = prepareToken() // 1
val post = submitPost(token, item) // 2
processPost(post) // 3
}

5. v2: Callbacks
(Continuation-Passing style)
fun postItem(item: Item) {
prepareTokenAsync { token -> // 1
submitPostAsync(token, item) { post -> // 2
processPost(post) // 3
}
}
}
Continuation

6. v2: Callbacks
(Continuation-Passing style)
fun postItem(item: Item) {
prepareTokenAsync { token -> // 1
submitPostAsync(token, item) { post -> // 2
processPost(post) // 3
}
}
}
Callback hell

7. v3: Rx/Futures/Promises
fun postItem(item: Item) {
observeToken()
.concatMap { token -> observePost(token, item) }
.subscribe { post -> processPost(post) }
}

8. v4: Coroutines Direct Style
suspend fun postItem(item: Item) {
val token = prepareToken() // 1
val post = submitPost(token, item) // 2
processPost(post) // 3
}

9. v4: Coroutines Direct Style
suspend fun postItem(item: Item) {
val token = prepareToken() // 1
val post = submitPost(token, item) // 2
processPost(post) // 3
}

10. v4: Coroutines Direct Style
suspend fun postItem(item: Item) {
val token = prepareToken() // 1
val post = submitPost(token, item) // 2
processPost(post) // 3
}

11. Profit: try/catch, loops, std-lib
suspend fun postItems(items: List<Item>) {
try {
val token = prepareToken()
items.forEach { item ->
val post = submitPost(token, item)
processPost(post)
}
} catch (e: BadTokenException) { /*…*/ }
}

12. suspend fun
computation that can be suspended

13. Experimental status
Since Kotlin 1.1
-Xcoroutines=enable
kotlin.coroutines.experimental -> kotlin.coroutines (1.3)
Experimental != unstable
Can and should be used in production

14. Experimental status
New style of programming
The design is not final and expected to change
JetBrains still collects information and feedbacks
Backwards compatibility guaranteed

15. Terminology
coroutine
suspending functionsuspending lambda
suspending function type
coroutine builder
suspension point
continuation

16. Terminology
coroutine
suspending function
suspending lambda
suspending function type
coroutine builder
suspension pointcontinuation

17. A coroutine is…
an instance of suspendable computation
similar to a daemon thread, but very light-weight
similar to a future or promise

18. Why coroutines?
threads are expensive to keep and switch
your code is single threaded
you’ve got lots of mutable states

19. Patterns
• generators/yield: C#, Python, Scala
• async/await: C#, ECMAScript, Dart
• channels, select: Go
• actors: Scala

20. Standard API
• Language support (`suspend` keyword)
• low-level basic API (stdlib: kotlin.coroutines)
• high-level APIs that can be used in user code

21. Low-level API
(kotlin.coroutines)
• kotlin.coroutines.experimental
• create/start/suspendCoroutine()
• Continuation interface
• @RestrictSuspension annotation
• kotlin.coroutines.experimental.intrinsics
• suspendCoroutineOrReturn()

22. How does it work under the
hood?

23. Continuation
interface Continuation<in T> {
val context: CoroutineContext
fun resume(value: T)
fun resumeWithException(exception: Throwable)
}

24. Continuation
// Kotlin
suspend fun submitPost(
token: Token,
item: Item): Post {…}
// Java/JVM
Object submitPost(
Token token,
Item item,
Continuation<Post> cont) {…}
compiler magic

25. Continuations
suspend fun postItem(item: Item) {
val token = prepareToken()
val post = submitPost(token, item)
processPost(post)
}
Initial continuation

26. Continuations
suspend fun postItem(item: Item) {
val token = prepareToken()
val post = submitPost(token, item)
processPost(post)
}
Continuation

27. Continuations
suspend fun postItem(item: Item) {
val token = prepareToken()
val post = submitPost(token, item)
processPost(post)
}
Continuation

28. Labels
suspend fun postItem(item: Item) {
// LABEL 0
val token = prepareToken()
// LABEL 1
val post = submitPost(token, item)
// LABEL 2
processPost(post)
}

29. Labels
suspend fun postItem(item: Item) {
switch(label) {
case 0:
val token = prepareToken()
case 1:
val post = submitPost(token, item)
case 2:
processPost(post)
}
}

30. State
suspend fun postItem(item: Item) {
val stateMachine = object: CoroutineImpl {…}
switch(stateMachine.label) {
case 0:
val token = prepareToken()
case 1:
val post = submitPost(token, item)
case 2:
processPost(post)
}
}

31. CPS Transform
fun postItem(item: Item, cont: Continuation) {
val stateMachine = object: CoroutineImpl {…}
switch(stateMachine.label) {
case 0:
val token = prepareToken(stateMachine)
case 1:
val post = submitPost(token, item, stateMachine)
case 2:
processPost(post)
}
}

32. Save state
fun postItem(item: Item, cont: Continuation) {
val stateMachine = object: CoroutineImpl {…}
switch(stateMachine.label) {
case 0:
stateMachine.item = item
stateMachine.label = 1
prepareToken(stateMachine)
case 1:
…
}
}

33. Callback
fun postItem(item: Item, cont: Continuation) {
val stateMachine = cont as? ThisSM ?: object: ThisSM {
fun resume(…) {
postItem(null, this)
}
}
switch(stateMachine.label) {
case 0:
….
}
}

34. Restore state and continue
fun postItem(item: Item, cont: Continuation) {
…
case 0:
stateMachine.item = item
stateMachine.label = 1
prepareToken(stateMachine)
case 1:
val item = stateMachine.item
val token = stateMachine.result
stateMachine.label = 2
submitPost(token, item, stateMachine)
case 2:
…

35. How to create custom
coroutines?

36. Await for a Single
suspend fun <T> Single<T>.await(): T = TODO()

37. Suspend a coroutine
suspend fun <T> Single<T>.await(): T = suspendCancellableCoroutine { cont ->
// …
}

38. Subscribe
suspend fun <T> Single<T>.await(): T = suspendCancellableCoroutine { cont ->
subscribe(object : SingleObserver<T> {
override fun onSuccess(t: T) {
TODO()
}
override fun onError(error: Throwable) {
TODO()
}
override fun onSubscribe(d: Disposable) {
TODO()
}
})
}

39. Return a result, if successful
suspend fun <T> Single<T>.await(): T = suspendCancellableCoroutine { cont ->
subscribe(object : SingleObserver<T> {
override fun onSuccess(t: T) {
cont.resume(t)
}
override fun onError(error: Throwable) {
TODO()
}
override fun onSubscribe(d: Disposable) {
TODO()
}
})
}

40. Or resume with exception
suspend fun <T> Single<T>.await(): T = suspendCancellableCoroutine { cont ->
subscribe(object : SingleObserver<T> {
override fun onSuccess(t: T) {
cont.resume(t)
}
override fun onError(error: Throwable) {
cont.resumeWithException(error)
}
override fun onSubscribe(d: Disposable) {
TODO()
}
})
}

41. Don’t forget to dispose
suspend fun <T> Single<T>.await(): T = suspendCancellableCoroutine { cont ->
subscribe(object : SingleObserver<T> {
override fun onSuccess(t: T) {
cont.resume(t)
}
override fun onError(error: Throwable) {
cont.resumeWithException(error)
}
override fun onSubscribe(d: Disposable) {
cont.invokeOnCompletion { d.dispose() }
}
})
}

42. And this is it
suspend fun <T> Single<T>.await(): T = suspendCancellableCoroutine { cont ->
subscribe(object : SingleObserver<T> {
override fun onSuccess(t: T) {
cont.resume(t)
}
override fun onError(error: Throwable) {
cont.resumeWithException(error)
}
override fun onSubscribe(d: Disposable) {
cont.invokeOnCompletion { d.dispose() }
}
})
}

43. kotlinx.coroutines
Core
integration
Guava
JDK 8
NIO
Quasar
reactive
Reactor
RxJava 1.x
Reactive Streams
RxJava 2.x
UI
Android
JavaFX
Swing

44. async
promise returned
(Deferred + await)

45. async/await
// C# way
async Task ProcessImage(String url)
{
var image = await LoadImage(url);
imageCache.Add(image);
}
// Kotlin way
suspend fun processImage(url: String) {
val image = loadImageAsync(url).await()
imageCache.add(image)
}

46. Not idiomatic way
fun loadImageAsync(url: String): Deferred<Image> = async { TODO() }
suspend fun processImage(url: String) {
val image = loadImageAsync(url).await()
imageCache.add(image)
}
Don’t define async functions
in the first place

47. Idiomatic way
suspend fun loadImage(url: String): Image = TODO()
suspend fun processImage(url: String) {
val image = async { loadImage(url) }.await()
imageCache.add(image)
}
Keep concurrency explicit

48. Generators API in
kotlin.coroutines
kotlin.coroutines.experimental
buildIterator()
buildSequence()

49. buildSequence {
print(“Start: ")
var prev = 1; var cur = 1
while (true) {
print(“Next")
yield(prev) // suspension point
val next = prev + cur
prev = cur; cur = next
}
print("End") // unreachable code
}.take(6).forEach { print(" $it ") }
// Output: Start 1 Next 1 Next 2 Next 3 Next 5 Next 8

50. buildSequence {
print(“Start: ")
var prev = 1; var cur = 1
while (true) {
print(“Next")
yield(prev) // suspension point
val next = prev + cur
prev = cur; cur = next
}
print("End") // unreachable code
}.take(6).forEach { print(" $it ") }
// Output: Start 1 Next 1 Next 2 Next 3 Next 5 Next 8

51. buildSequence {
print(“Start: ")
var prev = 1; var cur = 1
while (true) {
print(“Next")
yield(prev) // suspension point
val next = prev + cur
prev = cur; cur = next
}
print("End") // unreachable code
}.take(6).forEach { print(" $it ") }
// Output: Start 1 Next 1 Next 2 Next 3 Next 5 Next 8

52. buildSequence {
print(“Start: ")
var prev = 1; var cur = 1
while (true) {
print(“Next")
yield(prev) // suspension point
val next = prev + cur
prev = cur; cur = next
}
print("End") // unreachable code
}.take(6).forEach { print(" $it ") }
// Output: Start 1 Next 1 Next 2 Next 3 Next 5 Next 8

53. buildSequence {
print(“Start: ")
var prev = 1; var cur = 1
while (true) {
print(“Next")
yield(prev) // suspension point
val next = prev + cur
prev = cur; cur = next
}
print("End") // unreachable code
}.take(6).forEach { print(" $it ") }
// Output: Start 1 Next 1 Next 2 Next 3 Next 5 Next 8

54. launch
fire and forget
(Job)

55. run,
runBlocking
for main functions
and tests

56. delay
like Thread.sleep(),
but non-blocking

57. Channel
transfers values
between coroutines

58. produce
produces a stream of values
by sending them to a channel
(ProducerJob)

59. actor
deals with it’s mailbox
(ActorJob)

60. select
waits for the result of
multiple suspending
functions

61. Job lifecycle
New Active
Cancelling
Completed
Cancelled

62. Job states
State isActive isCompleted isCancelled
New
- - -
Active ✔ - -
Completed
- ✔ -
Cancelling
- - ✔
Cancelled
- ✔ ✔

63. val deferred = async(CommonPool) {
throw SomeException("I'm thrown inside a coroutine")
}
try {
deferred.await() // re-throws
} catch (e: SomeException) {
log(e.message)
}
Exception handling

64. Memory leaks

65. WeakReference “life hack”
suspend operator fun <T> WeakReference<T>.invoke(): T
= suspendCoroutineOrReturn {
get() ?: throw CancellationException()
}
val activityRef = WeakReference(this)
launch(CommonPool) {
activityRef().expensiveComputation()
}

66. Demo

67. A lot more left…
Coroutine dispatchers
Cancellation
Mutable state and concurrency
UI programming
Reactive streams

68. Links
Andrey Breslav FAQ:
https://discuss.kotlinlang.org/t/experimental-status-of-coroutines-in-1-1-and-
related-compatibility-concerns/2236
Design document (KEEP):
https://github.com/Kotlin/kotlin-coroutines/blob/master/kotlin-coroutines-
informal.md
Full kotlinx.coroutines API:
http://kotlin.github.io/kotlinx.coroutines
Coroutines guide by Roman ELizarov:
https://github.com/Kotlin/kotlinx.coroutines/blob/master/coroutines-guide.md

69. We are hiring!

70. Thank you!
@DmitriyZaitsev