Kotlin Coroutines and Android sitting in a tree - 2018 version

KOTLIN COROUTINES AND ANDROID
SITTING IN A TREE...
KAI KOENIG (@AGENTK)

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HELLO
HELLO
My name is Kai!
Software Architect in the web and mobile (Android) space  
from New Zealand.
Stuff I enjoy: Android, Kotlin, CFML, compilers and parsers, aviation and ﬂying
small aircraft, cats and chickens, Nintendo video games of all ages!
Follow me on Twitter: @AgentK

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AGENDA
AGENDA
▸ Kotlin in 3 minutes
▸ Why coroutines?
▸ Using coroutines in your Kotlin code
▸ Libraries and coroutines on Android

KOTLIN IN 3 MINUTES
https://www.ﬂickr.com/photos/tschiae/8080742303/

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HISTORY OF KOTLIN (I)
▸ Jetbrains wanted a more efﬁcient JVM language when building products
▸ Looked at Scala, Groovy, etc. but came up with their own language spec
▸ First shown at the JVM Language Summit in 2011
▸ Got some traction in Android-land in 2014
▸ Modern language features (lambdas, HOF etc) but Java 6 byte code
▸ Low methods count (~7000)
▸ Strong focus on concision and an efﬁcient, bloat-free language
KOTLIN IN 3 MINUTES

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HISTORY OF KOTLIN (II)
▸ Since 1.0 release in early 2016:
▸ multiple maintenance releases
▸ now at 1.0.7
▸ 1.1 was released in Feb 2017,
▸ currently at 1.1.60
▸ experimental new feature: Kotlin coroutines
KOTLIN IN 3 MINUTES

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HISTORY OF KOTLIN (III)
▸ At Google IO 2017, Kotlin became a ﬁrst-grade language for Android
▸ Release of 1.2 happened in late 2017 (CHECK)
▸ Java 9 compatibility
▸ Multiplatform projects
▸ Huge improvements to StdLib, type inference, smart cast etc.
▸ Strong community, lots of interesting frameworks, awesome support from
Jetbrains
KOTLIN IN 3 MINUTES

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KOTLIN IN 5 MINUTES
PROMINENT LANGUAGE FEATURES
▸ Immutability
▸ String templates
▸ Explicit null handling
▸ Properties and Fields
▸ Data classes
▸ Extension functions
▸ Syntactic sugar
▸ Type inference
▸ Lambdas
▸ Collection API
▸ Type-safe builders
▸ Java-Kotlin-Interop

WHY COROUTINES?
https://www.ﬂickr.com/photos/61299367@N05/9448165205

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WHY COROUTINES?
MOTIVATION
▸ Asynchronous programming is becoming increasingly important
▸ Problem: the need to avoid blocking introduces a lot of complexity
▸ Threads are:
▸ expensive to manage and limited
▸ complex in applications with lots of mutable state
▸ usually even more complex to deal with in UI-driven applications
▸ Callback hell (very common in Javascript)

#droidconVNhttp://slides.com/michaelholroyd/asyncnodejs/#/

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WHY COROUTINES?
HISTORY
▸ Coroutines were ﬁrst mentioned and used in Simula 67
▸ detach & resume keywords to suspend and then later resume execution
▸ Pushed aside by industry trend towards multi-threading
▸ C# has async/await (language-level keywords)
▸ Go was one of the ﬁrst modern languages re-introducing coroutines

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WHY COROUTINES?
COROUTINES IN KOTLIN (I)
▸ Kotlin’s approach: Suspending functions
▸ Function/lambda that can be suspended and resumed
▸ Requires no context-switching on the OS level
▸ Minimal integration into the core language and stdlib, most of functionality
provided by libraries
▸ Design allows for a variety of asynchronous API methodologies to be
implemented on top of Kotlin coroutines
▸ Supposed to feel like writing traditional code

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WHY COROUTINES?
COROUTINES IN KOTLIN (II)
▸ Easiest way to think about a coroutine is a very light-weighted thread
▸ They can run in parallel
▸ Coroutines can wait for each other
▸ They can communicate with each other
▸ Very, very cheap to create (compared to threads)
▸ A thread can run a lot of coroutines

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WHY COROUTINES?
COROUTINES IN KOTLIN (III)
▸ Multiple layers of libraries and integration points:
▸ Language support: suspending functions
▸ Low-level library & generators in kotlin.coroutines
▸ Mid-level library in kotlinx.coroutines
▸ High-level libraries (Anko etc.)

USING COROUTINES IN
YOUR KOTLIN CODE
https://www.ﬂickr.com/photos/97114911@N05/14720054418

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USING COROUTINES IN YOUR KOTLIN CODE
FUNDAMENTALS
▸ Kotlin 1.1.4 is the absolute minimum, better 1.1.50+ or right away latest 1.2
▸ Enable experimental feature in Gradle
kotlin {
experimental {
coroutines 'enable'
}
}
compile 'org.jetbrains.kotlinx:kotlinx-coroutines-core:0.22.5'

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“HELLO WORLD”
▸ launch {…} starts new coroutine on CommonPool thread pool
▸ delay() is a suspending function (not blocking a thread)
fun main(args: Array<String>) {
println("Start on main thread")
launch(CommonPool) {
delay(5000)
println("Hello from coroutine")
}
println("Hello from main thread")
Thread.sleep(10000)
println("Stop on main thread")
}

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“HELLO WORLD” IN BETTER
▸ Wait for the coroutines to ﬁnish - launch {…} returns a Job object
fun main(args: Array<String>) = runBlocking {
println("Start on main thread")
val job = launch(CommonPool) {
delay(5000)
println("Hello from coroutine")
}
println("Hello from main thread")
job.join()
println("Stop on main thread")
}

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THREADS VS COROUTINES
val jobs = List(10_000) {
thread(start = true) {
Thread.sleep(1000L)
print(".")
}
}
jobs.forEach { it.join() }
}

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THREADS VS COROUTINES
fun main(args: Array<String>) = runBlocking<Unit> {
val jobs = List(10_000) {
launch(CommonPool) {
delay(1000L)
print(".")
}
}
jobs.forEach { it.join() }
}

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ASYNC/AWAIT (I)
▸ async {…} starts new coroutine and returns a Deferred<T> object
▸ Deferred<T>.await() returns result of the coroutine
▸ await() needs to be called from inside a coroutine, because it needs to suspend
in a non-blocking way
▸ Solution: wrap into runBlocking {…} coroutine
▸ Starts coroutine and wait until it’s ﬁnished

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ASYNC/AWAIT (II)
val deferred = (1..60_000).map { n ->
async (CommonPool) {
n
}
}
runBlocking {
val sum = deferred.sumBy { it.await() }
println("Sum: $sum")
}
}

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SUSPEND FUNCTIONS (I)
▸ Coroutines can suspend without blocking a thread
▸ Functions that might suspend need to be marked with the suspend keyword
▸ They can only be called from another suspend function or a coroutine

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SUSPEND FUNCTIONS (II)
val deferred = (1..1_000_000).map { n ->
async (CommonPool) {
doWork(n)
}
}
runBlocking {
...
}
}
suspend fun doWork(n: Int) : Int {
delay(50)
return n
}

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BEHIND THE SCENES
▸ Kotlin coroutines are very light on language features:
▸ suspend the only new keyword added to the language and acts pretty much
like a compiler ﬂag
▸ Continuation and CoroutineContext in stdlib
▸ All the rest is in the kotlinx.coroutines library
This makes the design of Kotlin coroutines very composable.

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MORE BEHIND THE SCENES
▸ At compilation:
▸ Suspending functions compile to functions with a general callback interface
of type Continuation
▸ Code with suspension points compiles to a state machine
▸ launch, runBlocking, async etc. are often called coroutine builders

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LAUNCH VS ASYNC
▸ Conceptually very similar
▸ launch {…} returns a Job, no resulting value
▸ async {…} returns a Deferred - a future that can be used to obtain a value
▸ async generally suited better in situations with independent concurrent ﬂows

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WAITING AND CANCELLING
▸ cancel() and then join() can be used to terminate a coroutine execution early
▸ Job has an extension function cancelAndJoin() doing both at once
val job = launch {
repeat(1000) { i ->
println("job: I'm sitting here and delaying $i ...")
delay(500L)
}
}
delay(1300L)
println("main: I'm really over waiting!")
job.cancel()
job.join() // or use: job.cancelAndJoin()
println("main: Let's go.")

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DEALING WITH TIMEOUTS
▸ Quite often the motivation for cancelling is a timeout:
▸ Track yourself via the Job instance
▸ Use withTimeout() {…}
withTimeout(1300L) {
repeat(1000) { i ->
println("I'm waiting $i ...")
delay(500L)
}
}

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THAT’S NOT ALL OF IT YET
▸ Coroutine contexts
▸ Coroutines and threads
▸ Channels
▸ Pipelines
▸ Dealing with state
▸ Shared (mutable) state & Actors
Compare Coroutines with Java Futures API

LIBRARIES AND
COROUTINES ON
ANDROID
https://www.ﬂickr.com/photos/qiaomeng/4665885561/

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MOTIVATION
▸ In general, UI-driven apps need to be aware of long-running processes
▸ Android speciﬁcally:
▸ we can’t do any networking on the UI thread
▸ we need to avoid long-running operations due to ANRs
▸ The kotlinx.coroutines library by Jetbrains provides a starting point for
Android, too.
LIBRARIES AND COROUTINES ON ANDROID
compile 'org.jetbrains.kotlinx:kotlinx-coroutines-android:0.22.5'

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UI CONTEXT
▸ The Android library provides access to a coroutine context for the UI thread
▸ Coroutine launches in UI thread, UI updates and suspending functions are
possible
▸ Non-blocking, UI is not frozen
launch(UI) {
for (i in 10 downTo 1) {
hello.text = "Countdown $i ..."
delay(500)
}
hello.text = "Done!"
}

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OTHER UI THREAD CONCERNS
▸ For Jobs and cancelling coroutines, the same general principles still apply
▸ using launch {…} provides a reference to a Job
▸ can be cancelled with cancel() - for instance via UI control
▸ In UI scenarios useful to write own coroutines builders as extension functions
button.onClick {
...
}
fun View.onClick(action: suspend () -> Unit) {
setOnClickListener {
launch(UI) {
action()
}
}
}

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OTHER UI THREAD CONCERNS
▸ Interesting topics for further study:
▸ Limit and manage coroutines via actors
▸ Dealing with event conﬂation
▸ Channel.CONFLATED
▸ Channel.UNLIMITED

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THREAD BLOCKING OPERATIONS AND THE UI
▸ CPU-intensive computations and/or API calls
▸ Can’t be done from UI thread or UI thread-conﬁned coroutine
▸ Solution: suspending functions with execution context CommonPool
suspend fun fib(x: Int): Int = run(CommonPool) {
fibBlocking(x)
}
fun fibBlocking(x: Int): Int =
if (x <= 1) 1 else fibBlocking(x - 1) + fibBlocking(x - 2)

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NETWORK CALLS (I)
▸ Callback-free API call, handle ofﬂine-exceptions
fun getUsers() : Deferred<List<Users>> {
return async(CommonPool) {
val request = Request.Builder().url(<SOMEURL>).build()
val response = OkHttpClient().newCall(request).execute()
// parse response...
}
}

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NETWORK CALLS (II)
▸ Handle exceptions in calling code
▸ Use result object’s await() to obtain the data (suspending the coroutine)
▸ Use launch {…} builder to trigger execution of getUsers
launch(UI) {
try {
val result = getUsers()
adapter.setElements(result.await())
...
} catch (exception: IOException){
// we’re offline
}

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ANKO
▸ More often than not identiﬁed with declarative UI for Android/Kotlin
▸ But it also has APIs for:
▸ Async
▸ SQLite
▸ Anko 0.9 introduced naming changes around the Async API
▸ Since Anko 0.10, Anko has support for coroutines

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COROUTINES IN ANKO
▸ Add anko-coroutines dependency
▸ Earlier versions of Anko already had support for async handling
▸ New:
▸ Coroutines in listeners
▸ bg()
▸ asReference()
fun getData(): Data { ... }
fun showData(data: Data) { ... }
async(UI) {
val data: Deferred<Data> = bg {
// Runs on the background
getData()
}
// This code is executed on the UI thread
showData(data.await())
}

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COROUTINES WITH ASYNCAWAIT
▸ Async/await approach
▸ Very rich library on top of Kotlin’s
coroutine core
▸ Plugins for Retroﬁt and rxJava
async {
val result = await {
//Long running code
}
// Use result
}
async {
val repos = await { github.getRepos() }
showList(repos)
repos.forEach { repo ->
val stats = await { github.getStats
(repo.name) }
showStats(repo, stats)
}
}

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COROUTINES AND RXJAVA (I)
▸ There is an interop library for RX2 with coroutines: org.jetbrains.kotlinx:kotlinx-
coroutines-rx2
▸ rxJava -> coroutines: openSubscription(), awaitFirstOrDefault()
▸ coroutines -> rxJava:
▸ Job.asCompletable, Deferred.asSingle()
▸ Multiple coroutine builders: rxMaybe, rxCompletable etc.

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COROUTINES AND RXJAVA (II)
▸ Method count:
▸ Coroutines ~10-20% less than rxJava
▸ APK size:
▸ Similar, ~10-20% smaller than with rxJava
▸ Interop libraries add quite a lot to method count and APK size.

OTHER THINGS AND
FINAL THOUGHTS
https://www.ﬂickr.com/photos/chrispiascik/4054331891

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OTHER THINGS & FINAL THOUGHTS
UNIT TESTING SUSPENDING FUNCTIONS
▸ They need a coroutine to run, easiest way seems to be with runBlocking {…}
import kotlinx.coroutines.experimental.runBlocking
import org.testng.annotations.Test
class MyTest {
@Test
fun testMySuspendingFunction() = runBlocking<Unit> {
// your test code here
}
}

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OTHER THINGS & FINAL THOUGHTS
EXPERIMENTAL?
▸ Coroutines are currently experimental in Kotlin 1.x, however:
▸ Very sound approach of dealing with concurrency
▸ Jetbrains guarantees backwards compatibility
▸ Potentially no forward compatibility
▸ Coroutines can and should be used in production
▸ Quite low learning curve compared to rxJava and other libraries

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OTHER THINGS
RESOURCES
▸ Introduction to background processing in Android: 
http://www.vogella.com/tutorials/AndroidBackgroundProcessing/article.html
▸ Coroutines design document: 
https://github.com/Kotlin/kotlin-coroutines
▸ Coroutines guide: 
https://github.com/Kotlin/kotlinx.coroutines/blob/master/coroutines-guide.md
▸ Java Futures & Coroutines: 
https://blog.frankel.ch/concurrency-java-futures-kotlin-coroutines/#gsc.tab=0
▸ Anko:  
https://github.com/Kotlin/anko
▸ AsyncAwait: 
https://github.com/metalabdesign/AsyncAwait
▸ Coroutines and reactive streams: 
https://github.com/Kotlin/kotlinx.coroutines/blob/master/reactive/README.md

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OTHER THINGS
GET IN TOUCH
Kai Koenig
Email: kai@ventego-creative.co.nz
Work: http://www.ventego-creative.co.nz
Twitter: @AgentK
Telegram: @kaikoenig
Slides: http://www.slideshare.com/agentk

Kotlin Coroutines and Android sitting in a tree - 2018 version

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