Kotlin/Native Concurrency for Mobile Multiplatform

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Kotlin Native Concurrency Kevin Galligan

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Copenhagen Denmark KOTLIN NATIVE CONCURRENCY EXPLAINED KEVIN GALLIGAN @kpgalligan

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Intro

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What is Kotlin Multiplatform?

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kot·lin mul·ti·plat·form /ˌkätˈlin məltiˈplatfôrm,ˌkätˈlin məltīˈplatfôrm/ noun noun: kotlin multiplatform 1.optional, natively-integrated, open-source, code sharing platform, based on the popular, modern language kotlin. facilitates non-ui logic availability on many platforms.

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modern

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Saner Concurrency? concurrency is hard

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Kotlin Native model is controversial lots of disagreement

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JVM model is controversial many other platforms don’t let you do that

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JVM model is controversial many other platforms don’t let you do that

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JVM model is controversial many other platforms don’t let you do that

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You need to understand it libraries won’t hide all details

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important to relax

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Kotlin Native Rules

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1) Mutable State = one thread thread confined

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2) Immutable state = many threads no changes mean no problems

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kevin@touchlab.co @kpgalligan

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Internalize The Rules mutable = 1/immutable = many

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object BigGlobalState { var statusString = "" }

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Don’t manage concurrent state no “synchronized”, “volatile”, etc

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JVM Trusts You native does not

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Runtime Verification

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Do Things Differently not “losing” anything

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(jvm strict mode would be nice)

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You can break rules too just not a great idea

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Stages of grief skip ahead to acceptance

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Core Concepts

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Worker kn concurrency queue

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Similar to ExecutorService Handler/MessageQueue/Looper on Android

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Worker kn concurrency queue

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Mutability & State

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1) Mutable State = one thread simple state is simple

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class SimpleState { private var i = 0 val currVal: Int get() = i fun doStuff(arg: Int) { i += arg } } fun main(){ val s = SimpleState() s.doStuff(22) s.doStuff(33) println("My val is ${s.currVal}") }

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1) Mutable State = one thread you can transfer

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1) Mutable State = one thread you can transfer

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2) Immutable state = many threads not source immutable

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data class SomeData(val s:String, val i:Int) Mutable state?

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freeze() runtime immutability designation

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public fun T.freeze(): T {...}

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Freeze • Recursively freezes everything • One-way (no “unfreeze”) • Data may be shared between threads Int String Float MoreData val strData var width SomeData val moreData var count

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Int String Float Float String Int MoreData val strData var width SomeData val moreData var count

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Crossing Threads

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fun background(block:()->Unit)

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fun background(block:()->Unit) block.freeze()

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fun printStuffBg() { val someData = SomeData("Hello ", 2) background { println(someData) } }

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fun printStuffBg() { val someData = SomeData("Hello ", 2) background { println(someData) } }

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fun printStuffBg() { val someData = SomeData("Hello ", 2) background { println(someData) } }

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fun printStuffBg() { val someData = SomeData("Hello ", 2) background { println(someData) } }

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class CounterModel { var count = 0 fun countClicked() { count++ } }

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class CounterModel { var count = 0 fun countClicked() { count++ background { saveToDb(count) } } }

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class CounterModel { var count = 0 fun countClicked() { count++ background { saveToDb(count) } } }

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class CounterModel { var count = 0 fun countClicked() { count++ background { saveToDb(count) } } }

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class CounterModel { var count = 0 fun countClicked() { count++ background { saveToDb(count) } } }

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class CounterModel { var count = 0 fun countClicked() { count++ background { saveToDb(count) } } } InvalidMutabilityException: mutation attempt of frozen sample.CounterModel

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Functions Have State careful what you capture

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class CounterModel { var count = 0 fun countClicked() { count++ save(count) } private fun save(count:Int) = background { saveToDb(count) } }

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Global State some special rules

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var someData = SomeData("a", 1)

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var someData = SomeData("a", 1) object GlobalObject { var data = SomeData("b", 2) }

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@ThreadLocal var someData = SomeData("a", 1) @ThreadLocal object GlobalObject { var data = SomeData("b", 2) }

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@SharedImmutable var someData = SomeData("a", 1) @ThreadLocal object GlobalObject { var data = SomeData("b", 2) }

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var someData:SomeData set(v:SomeData)… get():SomeData = …

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Why Special Rules? you can access it from anywhere

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Debugging Issues

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InvalidMutabilityException changing frozen state

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Why is this frozen() common question

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class CounterModel { var count = 0 fun countClicked() { count++ background { saveToDb(count) } } }

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ensureNeverFrozen()

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class CounterModel { init { ensureNeverFrozen() } var count = 0 fun countClicked() { count++ background { saveToDb(count) } } }

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Int String Float Int MoreData val strData var width SomeData val moreData var count

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Rethinking architecture intentional mutability

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Main Thread Other Thread (maybe?) Data Data Data Content Model iOS View Model

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This can get weird subtle things

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class BreedModel( private val viewUpdate: (ItemDataSummary) -> Unit, private val errorUpdate: (String) -> Unit ) : BaseModel() { private val dbHelper: DatabaseHelper by inject() private val settings: Settings by inject() init { ensureNeverFrozen() scope.launch { dbHelper.selectAllItems().asFlow() .map { q -> val itemList = q.executeAsList() ItemDataSummary(/* etc */) } .flowOn(Dispatchers.Default) .collect { summary -> viewUpdate(summary) } } }

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class BreedModel( private val viewUpdate: (ItemDataSummary) -> Unit, private val errorUpdate: (String) -> Unit ) : BaseModel() { private val dbHelper: DatabaseHelper by inject() private val settings: Settings by inject() init { ensureNeverFrozen() scope.launch { dbHelper.selectAllItems().asFlow() .map { q -> val itemList = q.executeAsList() settings.set("Hey", "Bad idea") ItemDataSummary(/* etc */) } .flowOn(Dispatchers.Default) .collect { summary -> viewUpdate(summary) } } }

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class BreedModel( private val viewUpdate: (ItemDataSummary) -> Unit, private val errorUpdate: (String) -> Unit ) : BaseModel() { private val dbHelper: DatabaseHelper by inject() private val settings: Settings by inject() init { ensureNeverFrozen() scope.launch { dbHelper.selectAllItems().asFlow() .map { q -> val itemList = q.executeAsList() settings.set("Hey", "Bad idea") ItemDataSummary(/* etc */) } .flowOn(Dispatchers.Default) .collect { summary -> viewUpdate(summary) } } }

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This may be frustrating but should make you think

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Concurrent Mutability

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Atomics mutating frozen state

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class CounterModel { val count = AtomicInt(0) fun countClicked() { count.value++ background { saveToDb(count.value) } } }

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class CounterModel { val count = AtomicInt(0) fun countClicked() { count.value++ background { saveToDb(count.value) } } }

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val atom = AtomicReference(SomeData(“abc", 123))

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class SomeModel { val atom = AtomicReference(SomeData(“abc", 123)) fun update(sd:SomeData){ atom.value = sd } init { freeze() } }

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Don’t use too many atomics AtomicRef can leak memory

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Worker dispatched state mutable state thread local

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State Thread IsolateState ??? State Container Data

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State Thread IsolateState ??? State Container ??? Data

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open class IsolateState

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open class IsolateState constructor(producer: () -> T)

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open class IsolateState constructor(producer: () -> T) fun access(block: (T) -> R): R

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IsoArrayDeque IsoMutableCollection IsoMutableIterator IsoMutableList IsoMutableMap IsoMutableSet

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fun IsoMutableMap.checkFor2(first: K, second: K, ins: V)= access { map -> if (map.containsKey(first)) map.put(first, ins) else map.put(second, ins) }

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Objective-C/C/C++ just do what you want!

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Multiplatform & State

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Multiplatform State

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freeze() is native only

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expect fun T.freeze(): T expect val T.isFrozen: Boolean expect fun Any.ensureNeverFrozen() expect annotation class Throws

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freeze() on iOS threading rules apply

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Swift/Objc can’t be frozen be aware of concurrency issues

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Concurrency Libraries

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Official Native kotlinx.coroutines single-threaded

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ktor on native called on main thread

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multithreaded coroutines!

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multithreaded coroutines?

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Switching Threads use withContext (or equivalents) data passed in (or captured) is frozen data returned is frozen

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fun printStuffBg() { val someData = SomeData("Hello ", 2) background { println(someData) } }

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suspend fun printStuffBg() { val someData = SomeData("Hello ", 2) withContext(workerDispatcher) { println(someData) } }

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suspend fun printStuffBg() { val someData = SomeData("Hello ", 2) withContext(workerDispatcher) { println(someData) } }

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suspend fun printStuffForeground() { val someData = SomeData("Hello ", 2) val someMoreData = withContext(workerDispatcher) { SomeData("${someData.s}, Dogs!", someData.i + 1) } println(someMoreData) println("I am frozen? ${someMoreData.isFrozen()}") }

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SomeData(s=Hello , Dogs!, i=3) I am frozen? true

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Flow, Channel, etc all freezable

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Main Thread Other Thread (maybe?) Data Data Data Content Model iOS View Model

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iOS Suspend Functions new for 1.4!

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Stately

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Stately (Probably) deprecated

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Stately v1! (ish)

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Modules • stately-common • stately-collections • stately-concurrency

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Modules • stately-isolate • stately-iso-collections • alpha. Feedback please!

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github.com/Autodesk/coroutineworker

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github.com/Badoo/reaktive

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Next Steps

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go.touchlab.co/kotlinlang

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go.touchlab.co/kndevto

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go.touchlab.co/knthreads

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JetBrains Links • https://github.com/JetBrains/kotlin-native/blob/ master/IMMUTABILITY.md • https://github.com/JetBrains/kotlin-native/blob/ master/CONCURRENCY.md • https://www.youtube.com/watch?v=nw6YTfEyfO0