Slides from my talk at Kotlin TechTalks@Lohika Kyiv, 2017-09-20

1. Coroutines in Kotlin
Dmytro Zaitsev
Team Leader @ Lóhika

2. Blocking VS Non-blocking

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

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

6. Callback hell

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

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

9. Experimental status
Kotlin 1.1
-Xcoroutines=enable
kotlin.coroutines.experimental -> kotlin.coroutines

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

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

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

13. suspend fun
computation that can be suspended

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

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

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

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

18. Generators API in
kotlin.coroutines
kotlin.coroutines.experimental:
• buildSequence()
• buildIterator()

19. buildSequence {
print("Start")
yield(1) // suspension point
var prev = 0
var cur = 1
while (true) {
print(“Next")
val next = prev + cur
yield(next) // suspension point
prev = cur
cur = next
}
print("End") // unreachable code
}.take(8).forEach { print(" $it ") }
// Output: Start 1 Next 1 Next 2 Next 3 Next 5 Next 8 Next 13 Next 21

20. kotlinx.coroutines
• kotlinx-coroutines-core
• integration
• guava, jdk8, nio, quasar
• reactive
• reactive, reactor, rx1, rx2
• ui
• android, javafx, swing

21. runBlocking
for main functions
and tests

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

23. launch
fire and forget
(Job)

24. async
promise returned
(Deferred + await)

25. Channel
transfers values
between coroutines

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

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

28. select
waits for the result of
multiple suspending
functions

29. Job lifecycle
New Active
Cancelling
Completed
Cancelled

30. Job states
State isActive isCompleted isCancelled
New
- - -
Active
+ - -
Completed
- + -
Cancelling
- - +
Cancelled
- + +

31. 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

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

33. Demo

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

35. 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

36. Thank you!
@DmitriyZaitsev