Kotlin Coroutines in Android Untangle your async code @charlesjuliank Atlanta Android Club 2018

About Me Charles Julian Knight he/him (or they/them) @charlesjuliank [email protected]

About Us www.fixdapp.com

Outline - Why - Intro to Coroutines - Android-Specific Patterns - Gotchas

I/O is Hard Everything is I/O & Problem

We’ve gotten okay at this... - Multithreading - AsyncTask - Callbacks - Promises / Futures - Observables / Reactive Streams

Simple Flow Example error Load Friends from Network Ask for Permissions Contact permissions? Loading Show Retry Dialog retry? yes no Network Error success Canceled Granted? Read Contacts Show Results yes no yes no

Some Examples

We’ve gotten okay at this... - Chaining Steps - Hard to read and maintain - Callbacks - Callback hell, hard to refactor, no cancellation - Promises / Futures - CompletableFuture minSdk 24, no cancellation, concurrent by default - Observables / Reactive Streams - Steep learning curve, different paradigm, too many operators

Introducing Coroutines

Coroutines Simplify Async

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“Experimental” - Stable (as in not-buggy) - API is not finalized - Guaranteed backwards compatibility - Calls covered here are unlikely to change - IDE tools to ease migration Can I use it in production? “Yes! You should.” - Roman Elizarov (seconded by me)

Setup dependencies { compile 'org.jetbrains.kotlinx:kotlinx-coroutines-core:0.22.5' compile 'org.jetbrains.kotlinx:kotlinx-coroutines-android:0.22.5' // RxJava 1 compile 'org.jetbrains.kotlinx:kotlinx-coroutines-rx1:0.22.5' // RxJava 2 compile 'org.jetbrains.kotlinx:kotlinx-coroutines-rx2:0.22.5' // Java 8 CompletableFuture (minSdk 24) compile 'org.jetbrains.kotlinx:kotlinx-coroutines-jdk8:0.22.5' // Java NIO (minSdk 26) compile 'org.jetbrains.kotlinx:kotlinx-coroutines-nio:0.22.5' } kotlin { experimental { coroutines 'enable' } }

github.com/KeepSafe/dexcount-gradle-plugin

Creating Coroutines suspend fun delayedParseInt(i: String): Int { return suspendCoroutine { cont -> // this code gets executed immediately to set up the coroutine Thread { Thread.sleep(1000) // some time later, use the continuation to resume from suspension try { val result = i.toInt() cont.resume(result) }catch (e: NumberFormatException) { cont.resumeWithException(e) } }.start() // after leaving this method, the caller will suspend until resume is called } }

Combining Coroutines suspend fun doFirst(): String suspend fun doSecond(): Boolean suspend fun doThird(data: String) suspend fun example() { val data = doFirst() if (doSecond()) { doThird(data) } }

Calling Coroutines

Coroutine Builders val job = launch(UI) { example() } job.isActive // true launch(CommonPool) { delay(100) print("ok") }

Coroutine Context Where the non-suspending code is run. Or another way: What thread picks back up after a suspension resumes. - UI - CommonPool - newSingleThreadContext(name = "single") - newFixedThreadPoolContext(nThreads = 3, name = "pool")

Concurrency In Kotlin, coroutines are sequential by default. - Concurrency is hard - Most common use case is sequential

Concurrency - Async/Await val one = async(CommonPool) { delay(100) return@async 1 } // one is a Promise (Deferred). It has already started val two = async(CommonPool) { delay(100) return@async 2 } // now both promises are running // await() suspends until the promise resolves val sum = one.await() + two.await()

“Lightweight Threads” val jobs = List(100_000) { launch(CommonPool) { delay(1000) print(".") } } jobs.forEach { it.join() }

Cancellation val job = launch(UI) { delay(1000) print("this never gets called") } Thread.sleep(500) job.cancel()

Under the Hood void example(Continuation cont) { switch (cont.label) { case 0: cont.label = 1; showConfirmDialog(cont); break; case 1: if (cont.lastError != null) throw cont.lastError; boolean isConfirmed = (boolean) cont.lastValue; if (!isConfirmed) { textView.setText("cancelled"); return; } textView.setText("loading"); makeNetworkRequest(cont); break; case 2: if (cont.lastError != null) throw cont.lastError;

Usage in Android

Wrapping Callback APIs suspend fun Task.await(): T = suspendCoroutine { cont -> addOnCompleteListener { cont.resume(it.result) } addOnFailureListener { cont.resumeWithException(it) } } launch(UI) { val locationClient = LocationServices.getFusedLocationProviderClient(this) val location = locationClient.lastLocation.await() }

- Single.await() - Observable.awaitFirst() - Observable.awaitFirstOrDefault() - Observable.awaitSingle() - Observable.awaitLast() - Observable.consumeEach { } Errors and cancellation work too. Convert Observables to Coroutines

Coroutine Dialogs suspend fun showConfirmationDialog() = suspendCancellableCoroutine { cont -> val dialog = AlertDialog.Builder(this) .setMessage("Are you sure?") .setPositiveButton("Yes", { _, _ -> cont.resume(true) }) .setNegativeButton("No", { _, _ -> cont.resume(false) }) .setCancelable(true) .setOnCancelListener { cont.cancel() } .create() dialog.show() cont.invokeOnCompletion { if(cont.isCancelled) dialog.dismiss() } }

Coroutine Activity Results interface ActivityResultMixin { data class ActivityResult(val requestCode: Int, val resultCode: Int, val data: Intent?) fun startActivityForResult(intent: Intent, requestCode: Int) val _activityResultStream: PublishSubject suspend fun startActivityForResultAsync(intent: Intent, requestCode: Int): ActivityResult { startActivityForResult(intent, requestCode) return _activityResultStream .filter { it.requestCode == requestCode } .awaitFirstOrDefault((ActivityResult(requestCode, Activity.RESULT_CANCELED, null))) } fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) { _activityResultStream.onNext(ActivityResult(requestCode, resultCode, data)) } fun onDestroy() { _activityResultStream.onCompleted() } }

Blocking Code suspend fun saveUser(db: SQLiteDatabase, user: User) = async(CommonPool) { db.insert("users", null, ContentValues().apply { put("id", user.id) put("name", user.name) put("email", user.email) }) }.await()

Job Lifecycle class JobCancellationActivity : Activity() { var job: Job? = null override fun onStart() { super.onStart() job = launch(UI) { /* ... */ } } override fun onStop() { job?.cancel(CancellationException("onStop")) super.onStop() } }

Testing Coroutines @Test fun asyncAddition_isCorrect() = runBlocking { val result = asyncAdd(2, 2) expect(result).to.equal(4) }

Some Pitfalls

Race Conditions, etc. var count = 500 (0..5).forEach { launch(CommonPool) { while(count > 0) { print(count) count = count - 1 delay(10) } } }

Need to be Cancelled override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) launch(UI) { var timer = 0 while (true) { textView.text = "$timer seconds" delay(1, TimeUnit.SECONDS) timer++ } } } // will throw IllegalStateException: Activity has been destroyed

Need to be Cancelled lateinit var job: Job override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) job = launch(UI) { var timer = 0 while (true) { textView.text = "$timer seconds" delay(1, TimeUnit.SECONDS) timer++ } } } override fun onDestroy() { job.cancel() super.onDestroy() }

Cancellation is an Exception val job = launch(UI) { try { dangerousMethod() // throws CancellationException } catch (e: Exception) { // CancellationException got gobbled up! Log.w("EXAMPLE", "dangerous method exploded") } doTheNextThing() // next thing still runs! } job.cancel()

Cancellation is an Exception val job = launch(UI) { try { dangerousMethod() // throws CancellationException } catch (e: CancellationException){ throw e // rethrow, to be handled by the coroutine context } catch (e: Exception) { Log.w("EXAMPLE", "dangerous method exploded") } doTheNextThing() // next thing won't get called anymore } job.cancel()

Cancellation is Cooperative // Not Cancellable! suspend fun Task.await(): T = suspendCoroutine { cont -> addOnCompleteListener { cont.resume(it.result) } addOnFailureListener { cont.resumeWithException(it) } } // Not Cancellable! suspend fun readFromInputStream(inputStream: InputStream) { InputStreamReader(inputStream).forEachLine { line -> print(line) } }

There’s Much More - Channels - Generators - Actors - Mutexes (suspending locks)

Growing the Team [email protected]

Resources - Kotlin Coroutines Guide: github.com/Kotlin/kotlinx.coroutines - Introduction to Kotlin coroutines - Roman Elizarov (GeekOut 2017) vimeo.com/222499934 - Examples from this talk: github.com/rabidaudio/kotlin-coroutines-android