Introduction to Kotlin Coroutine

This talk introduces Kotlin Coroutine, which was a big feature of Kotlin 1.1.

Kotlin coroutine is a suspendable computation. It empowers programmers in some use cases such as async/await, generator.

The talk describes what Kotlin coroutine is and explains suspendCoroutine method behind it.

Reference:
- https://kotlinlang.org/docs/reference/coroutines.html
- https://github.com/Kotlin/kotlin-coroutines/blob/master/kotlin-coroutines-informal.md#use-cases
- https://github.com/Kotlin/kotlin-coroutines

- Introduction to Kotlin Coroutine
https://vimeo.com/222499934
slides: https://www.slideshare.net/elizarov/introduction-to-kotlin-coroutines
- Guide to kotlinx.coroutines by example

https://github.com/Kotlin/kotlinx.coroutines/blob/master/coroutines-guide.md

Transcript

1. INTRODUCTION TO KOTLIN COROUTINE SHIBUYA.APK #19 2017.10.27 HIROSHI KUROKAWA
 FABLIC, INC.
2. WHAT’S COROUTINE? A coroutine can be thought of as an instance of suspendable computation i.e. the one that can suspend at some points and later resume execution possibly on another thread. https://github.com/Kotlin/kotlin-coroutines/blob/master/kotlin-coroutines-informal.md#use-cases
3. THREAD-BLOCKING CODE SAMPLE val request = composeRequest() val post = makeRequest(request) parsePost(post) fun composeRequest(): Request = … fun makeRequest(request: Request): Result = … main composeRequest makeRequest If it takes a long time? If it takes a long time?
4. THREAD-BLOCKING CODE SAMPLE val request = composeRequest() val post = makeRequest(request) parsePost(post) fun composeRequest(): Request = … fun makeRequest(request: Request): Result = … main composeRequest makeRequest If it takes a long time? If it takes a long time? The thread is blocked!
5. SUSPEND TO THE RESCUE val request = composeRequest() val post = makeRequest(request) parsePost(post) suspend fun composeRequest(): Request = … suspend fun makeRequest(request: Request): Result = … 1 2 ‣ The code suspends at the suspension points ‣ The code resumes when the return value is ready
6. SUSPEND TO THE RESCUE val request = composeRequest() val post = makeRequest(request) parsePost(post) suspend fun composeRequest(): Request = … suspend fun makeRequest(request: Request): Result = … 1 2 main composeRequest makeRequest main main 1 2
7. SUSPEND TO THE RESCUE val request = composeRequest() val post = makeRequest(request) parsePost(post) suspend fun composeRequest(): Request = … suspend fun makeRequest(request: Request): Result = … 1 2 main composeRequest makeRequest main main 1 2 The thread is not blocked!
8. CODE CAN BE WRITTEN IN A REGULAR STYLE try { for (req in list) makeRequest(req) } catch (e: Exception) { … } loop exception handling
9. CAVEAT ‣ “Kotlin suspending functions are designed to imitate sequential behaviour by default”,
 Roman Elizarov (https://www.slideshare.net/elizarov/introduction-to-kotlin-coroutines) ‣ Concurrency has to be explicit
10. Applications
11. val first = loadImageAsync("green") val second = loadImageAsync("red") overlay(first.await(), second.await()) fun loadImageAsync(name: String) = async { … } ASYNC/AWAIT 1 2
12. val first = loadImageAsync("green") val second = loadImageAsync("red") overlay(first.await(), second.await()) fun loadImageAsync(name: String) = async { … } ASYNC/AWAIT 1 2 main loadImageAsync main main 2 loadImageAsync 1
13. Coroutine under the hood "
14. SUSPEND COROUTINE suspend fun <T> suspendCoroutine( block: (Continuation<T>) -> Unit ): T ‣ When the method is called, it suspends the current coroutine ‣ The coroutine is resumed when resume() is called on the passed continuation Scheme call/cc style!
15. SUSPEND COROUTINE suspendCoroutine { cont: Continuation<T> -> doLongTask { result, exception -> if (exception == null) cont.resume(result) else cont.resumeWithException(exception) } } fun <T> doLongTask(cb: (T, Throwable) -> Unit) { } suspend fun <T> suspendCoroutine( block: (Continuation<T>) -> Unit ): T
16. SUSPEND COROUTINE suspend fun <T> doLongTaskSuspend() = suspendCoroutine { cont: Continuation<T> -> doLongTask { result, exception -> if (exception == null) cont.resume(result) else cont.resumeWithException(exception) 䠼} 䠼} result = doLongTaskSuspend() … // do something with the result
17. What is continuation?
18. CONTINUATION val request = composeRequest() val post = makeRequest(request) parsePost(post) 1 2
19. val request = composeRequest() val post = makeRequest(request) parsePost(post) CONTINUATION 1 2 Continuation
 passed to (1)
20. val request = composeRequest() val post = makeRequest(request) parsePost(post) CONTINUATION 1 2 Continuation
 passed to (2) No deep stacks and light-weight $ ‣ Kotlin builds a state machine for each coroutine ‣ A suspension point just adds a state to the state machine
21. COROUTINE BUILDER ‣ Suspending function only can be called from a suspending function or a coroutine ‣ Use coroutine builders to call suspending functions from a regular function ‣ launch ‣ runBlocking ‣ async
22. COROUTINE BUILDER fun postItem(item: Item) { launch(CommonPool) { val token = preparePost() val post = submitPost(token, item) processPost(post) } }
23. WRAP UP ‣ Coroutine is like a light-weight thread ‣ Can be suspended/resumed ‣ async/await, generator, channel is just library built on that
24. REFERENCE ‣ https://kotlinlang.org/docs/reference/ coroutines.html ‣ https://github.com/Kotlin/kotlin-coroutines