Coroutines in Kotlin Dmytro Zaitsev Team Leader @ Lóhika 

Blocking VS Non-blocking 

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

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

Callback hell 

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

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

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

Terminology coroutine suspending function suspending lambda suspending function type coroutine builder suspension point continuation 

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

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

suspend fun computation that can be suspended 

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

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

Continuation interface Continuation {
 val context: CoroutineContext
 fun resume(value: T)
 fun resumeWithException(exception: Throwable)
 } 

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

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

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 

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

runBlocking for main functions and tests 

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

launch fire and forget (Job) 

async promise returned (Deferred + await) 

Channel transfers values between coroutines 

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

actor deals with it’s mailbox (ActorJob) 

select waits for the result of multiple suspending functions 

Job lifecycle New Active Cancelling Completed Cancelled 

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

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 

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

Demo 

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

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 

Thank you! @DmitriyZaitsev