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INTRODUCTION TO KOTLIN
COROUTINE
SHIBUYA.APK #19
2017.10.27
HIROSHI KUROKAWA
FABLIC, INC.
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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
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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?
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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!
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SUSPEND TO THE RESCUE
val request = composeRequest()
val post = makeRequest(request)
parsePost(post)
suspend fun composeRequest(): Request = …
suspend fun makeRequest(request: Request): Result = …
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‣ The code suspends at the suspension points
‣ The code resumes when the return value is ready
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SUSPEND TO THE RESCUE
val request = composeRequest()
val post = makeRequest(request)
parsePost(post)
suspend fun composeRequest(): Request = …
suspend fun makeRequest(request: Request): Result = …
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main
composeRequest makeRequest
main main
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SUSPEND TO THE RESCUE
val request = composeRequest()
val post = makeRequest(request)
parsePost(post)
suspend fun composeRequest(): Request = …
suspend fun makeRequest(request: Request): Result = …
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main
composeRequest makeRequest
main main
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The thread is not blocked!
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CODE CAN BE WRITTEN IN A REGULAR STYLE
try {
for (req in list) makeRequest(req)
} catch (e: Exception) {
…
}
loop
exception handling
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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
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Applications
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val first = loadImageAsync("green")
val second = loadImageAsync("red")
overlay(first.await(), second.await())
fun loadImageAsync(name: String) = async { … }
ASYNC/AWAIT
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val first = loadImageAsync("green")
val second = loadImageAsync("red")
overlay(first.await(), second.await())
fun loadImageAsync(name: String) = async { … }
ASYNC/AWAIT
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main
loadImageAsync
main main
2
loadImageAsync
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Coroutine under the hood "
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SUSPEND COROUTINE
suspend fun suspendCoroutine(
block: (Continuation) -> 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!
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SUSPEND COROUTINE
suspendCoroutine { cont: Continuation ->
doLongTask { result, exception ->
if (exception == null)
cont.resume(result)
else
cont.resumeWithException(exception)
}
}
fun doLongTask(cb: (T, Throwable) -> Unit) {
}
suspend fun suspendCoroutine(
block: (Continuation) -> Unit
): T
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SUSPEND COROUTINE
suspend fun doLongTaskSuspend() =
suspendCoroutine { cont: Continuation ->
doLongTask { result, exception ->
if (exception == null)
cont.resume(result)
else
cont.resumeWithException(exception)
䠼}
䠼}
result = doLongTaskSuspend()
…
// do something with the result
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What is continuation?
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CONTINUATION
val request = composeRequest()
val post = makeRequest(request)
parsePost(post)
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val request = composeRequest()
val post = makeRequest(request)
parsePost(post)
CONTINUATION
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Continuation
passed to (1)
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val request = composeRequest()
val post = makeRequest(request)
parsePost(post)
CONTINUATION
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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
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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
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COROUTINE BUILDER
fun postItem(item: Item) {
launch(CommonPool) {
val token = preparePost()
val post = submitPost(token, item)
processPost(post)
}
}
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WRAP UP
‣ Coroutine is like a light-weight thread
‣ Can be suspended/resumed
‣ async/await, generator, channel is just library built on that
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REFERENCE
‣ https://kotlinlang.org/docs/reference/
coroutines.html
‣ https://github.com/Kotlin/kotlin-coroutines