Kotlin Coroutines in Android Untangle your async code @charlesjuliank Connect.Tech 2017 

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

About Us fixdapp.com 

Outline - The Problem - Existing Solutions - Intro to Coroutines - Android-Specific Patterns 

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

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

Simple Flow Example Show Confirm Dialog Append Result Network Error error Canceled confirmed? Load Words from Network Count usage of word in file For each yes no Loading 

Some Examples 

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

We’ve gotten pretty good at this... - AsyncTask - Boilerplate, blocking only, hard to chain - Callbacks - Promises / Futures - Observables / Reactive Streams 

We’ve gotten pretty good at this... - AsyncTask - Boilerplate, blocking only, hard to chain - Callbacks - Callback hell, hard to refactor, no cancellation - Promises / Futures - Observables / Reactive Streams 

We’ve gotten pretty good at this... - AsyncTask - Boilerplate, blocking only, hard to chain - Callbacks - Callback hell, hard to refactor, no cancellation - Promises / Futures - CompletableFuture minSdk 24, no cancellation, concurrent by default - Observables / Reactive Streams 

We’ve gotten pretty good at this... - AsyncTask - Boilerplate, blocking only, hard to chain - 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 

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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:0.18' compile 'org.jetbrains.kotlinx:kotlinx-coroutines-android:0.18' // RxJava 1 compile 'org.jetbrains.kotlinx:kotlinx-coroutines-rx1:0.18' // RxJava 2 compile 'org.jetbrains.kotlinx:kotlinx-coroutines-rx2:0.18' // Java 8 CompletableFuture (minSdk 24) compile 'org.jetbrains.kotlinx:kotlinx-coroutines-jdk8:0.18' // Java NIO (minSdk 26) compile 'org.jetbrains.kotlinx:kotlinx-coroutines-nio:0.18' } kotlin { experimental { coroutines 'enable' } } 

github.com/KeepSafe/dexcount-gradle-plugin 

github.com/KeepSafe/dexcount-gradle-plugin 

github.com/KeepSafe/dexcount-gradle-plugin 

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) } 

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