Aplicações assíncronas no Android com Coroutines & Jetpack

Aplicações assíncronas no Android com 
Coroutines & Jetpack
Nelson Glauber
@nglauber

• Carregar arquivo de layout
• Desenhar as views
• Tratar os eventos de UI
• …
A main thread do Android

• O processamento desses eventos deve ocorrer em:
• Menos de 16ms para devices com taxas de atualização
de 60Hz
• Menos de 12ms para dispositivos com taxas de 90Hz
• Menos de < 8ms para dispositivos com taxas de 120Hz
A main thread do Android

• AsyncTask 👴
• Thread + Handler 👷
• Loaders (deprecated) 👴
• Volley 🤦🤦
• RxJava 🧐
Async no Android

• Essencialmente, coroutines são light-weight threads.
• Fácil de usar (sem mais “callbacks hell” e/ou centenas de
operadores).
• Úteis para qualquer tarefa computacional mais onerosa
(como operações de I/O).
• Permite a substituição de callbacks por operações
assíncronas.
Coroutines

Dependências
dependencies {
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:1.3.5"
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:1.3.5"
...
}

• Suspending functions são o centro de tudo em Coroutines.

• São funções que podem ser pausadas e retomadas após algum
tempo.

• Podem executar longas tarefas e aguardar o resultado sem
bloquear a thread atual.

• A sintaxe é idêntica a uma função “normal”, exceto pela adição
da palavra reservada suspend.

• Por si só, uma suspending function não é “assíncrona".

• Só pode ser chamada a partir de outra suspending function.
suspend

import kotlinx.coroutines.delay
class Calculator {
suspend fun sum(a: Int, b: Int): Int {
delay(5_000)
return a + b
}
}

import kotlinx.coroutines.delay
class Calculator {
suspend fun sum(a: Int, b: Int): Int {
delay(5_000)
return a + b
}
}
import kotlinx.coroutines.runBlocking
import org.junit.*
class CalculatorUnitTest {
@Test
fun sum_isCorrect() = runBlocking {
val calc = Calculator()
assertEquals(4, calc.sum(2, 2))
}
}

• Job
• Context
• Scope
• Dispatcher
Principais Classes

class MainActivity : AppCompatActivity() {
private val job = Job()
private val coroutineScope =
CoroutineScope(job + Dispatchers.Main)
override fun onDestroy() {
super.onDestroy()
job.cancel()
}
fun callWebService() {
coroutineScope.launch {
txtOutput.text = ""
val books = withContext(Dispatchers.IO) {
BookHttp.loadBooks()
}
// update the UI using books
}
}
...

super.onDestroy()
job.cancel()
}
txtOutput.text = ""
}
}
}
...
Job
• Um Job representa uma tarefa ou
conjunto de tarefas em execução.
• Pode possuir “ﬁlhos”.
• A função launch retorna um Job.
• Pode ser cancelado usando a função
cancel.
• Possui um ciclo de vida (novo, ativo,
completo ou cancelado)

super.onDestroy()
job.cancel()
}
txtOutput.text = ""
}
}
}
...
Context
• A interface CoroutineContext
Representa o conjunto de atributos
que conﬁguram uma coroutine.
• Pode deﬁnir a política de threading;
job raiz; tratamento de exceções;
nome da coroutine (debug).
• Uma coroutine herda o contexto do
pai.

super.onDestroy()
job.cancel()
}
txtOutput.text = ""
}
}
}
...
Scope
• Uma coroutine sempre roda
em um escopo.
• Serve como uma espécie de
ciclo de vida para um
conjunto de coroutines.
• Permite um maior controle
das tarefas em execução.

super.onDestroy()
job.cancel()
}
txtOutput.text = ""
}
}
}
...
Dispatcher
• Deﬁne o pool de threads  
onde a coroutine executará.
• Default: para processos  
que usam a CPU mais
intensamente.
• IO: para tarefas de rede  
ou arquivos. O pool de
threads é compartilhado
com o dispatcher
Default.
• Main - main thread do
Android.

txtOutput.text = ""
}
}
}

txtOutput.text = ""
val books = BookHttp.loadBooks()
}
}
android.os.NetworkOnMainThreadException
at android.os.StrictMode$AndroidBlockGuardPolicy.onNetwork(StrictMode.java:1513)
at java.net.Inet6AddressImpl.lookupHostByName(Inet6AddressImpl.java:117)
at java.net.Inet6AddressImpl.lookupAllHostAddr(Inet6AddressImpl.java:105)
at java.net.InetAddress.getAllByName(InetAddress.java:1154)
at com.android.okhttp.Dns$1.lookup(Dns.java:39)

coroutineScope.launch(Dispatchers.IO){
txtOutput.text = ""
val books = BookHttp.loadBooks()
}
}
FATAL EXCEPTION: DefaultDispatcher-worker-1
Process: br.com.nglauber.coroutinesdemo, PID: 26507
android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created
a view hierarchy can touch its views.
at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:7753)
at android.view.ViewRootImpl.requestLayout(ViewRootImpl.java:1225)

txtOutput.text = ""
}
}
}
👍

• Suspending functions
• Job
• Context
• Scope
• Dispatcher
Coroutines

• É possível iniciar coroutines atrelada aos ciclos de vida de
Activity, Fragment e View do Fragment.
• Além da função launch, podemos usar o
launchWhenCreated, launchWhenStarted e
launchWhenResumed.
Lifecycle Scope
dependencies {
...
implementation "androidx.lifecycle:lifecycle-runtime-ktx:2.2.0"
}

class MyActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
lifecycleScope.launch {
...
}
lifecycleScope.launchWhenCreated {
...
}
lifecycleScope.launchWhenStarted {
...
}
lifecycleScope.launchWhenResumed {
...
}
}
}

class MyFragment : Fragment() {
...
}
lifecycleScope.launchWhenCreated {
...
}
lifecycleScope.launchWhenStarted {
...
}
lifecycleScope.launchWhenResumed {
...
}
}
}

class MyFragment: Fragment() {
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
viewLifecycleOwner.lifecycleScope.launch {
...
}
}
}

class MyFragment: Fragment() {
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
viewLifecycleOwner.lifecycleScope.launch {
...
}
}
}
Ambos os escopos são
cancelados automaticamente.

A classe ViewModel possui agora a propriedade viewModelScope.
ViewModel Scope
dependencies {
implementation "androidx.lifecycle:lifecycle-extensions:2.2.0"
implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:2.2.0"
}

class BookListViewModel: ViewModel() {
private val _state = MutableLiveData<State>()
val state: LiveData<State>
get() = _state
fun search(query: String) {
viewModelScope.launch {
_state.value = State.StateLoading
val result = withContext(Dispatchers.IO) {
BookHttp.searchBook(query)
}
_state.value = if (result?.items != null) {
State.StateLoaded(result.items)
} else {
State.StateError(Exception("Error"), false)
}
}
}
}

class BookListViewModel: ViewModel() {
private var query = MutableLiveData<String>()
val state = query.switchMap {
liveData {
emit(State.StateLoading)
val result = withContext(Dispatchers.IO) {
BookHttp.searchBook(it)
}
emit(
if (result?.items != null) State.StateLoaded(result.items)
else State.StateError(Exception("Error"), false)
)
}
}
fun search(query: String) {
this.query.value = query
}
}

class BookListActivity : AppCompatActivity(R.layout.activity_book_list) {
private val viewModel: BookListViewModel …
viewModel.state.observe(this, Observer { state ->
when (state) {
is BookListViewModel.State.StateLoading -> ...
is BookListViewModel.State.StateLoaded -> ...
is BookListViewModel.State.StateError -> ...
}
})
}
...

WorkManager
dependencies {
def work_version = "2.3.4"
implementation "androidx.work:work-runtime-ktx:$work_version"
}

class MyWork(context: Context, params: WorkerParameters) :
CoroutineWorker(context, params) {
override suspend fun doWork(): Result = try {
val output = inputData.run {
val x = getInt("x", 0)
val y = getInt("y", 0)
val result = Calculator().sum(x, y)
workDataOf("result" to result)
}
Result.success(output)
} catch (error: Throwable) {
Result.failure()
}
}

private fun callMyWork() {
val request =
OneTimeWorkRequestBuilder<MyWork>()
.setInputData(workDataOf("x" to 84, "y" to 12))
.build()
WorkManager.getInstance(this).run {
enqueue(request)
getWorkInfoByIdLiveData(request.id)
.observe(this@MainActivity, Observer {
if (it.state == WorkInfo.State.SUCCEEDED) {
val result = it.outputData.getInt("result", 0)
addTextToTextView("Result-> $result")
}
})
}
}

• Lifecycle provê um lifecycleScope para Activity e
Fragment (e a view do Fragment).
• ViewModel possui a propriedade viewModelScope.
• WorkManager disponibiliza a classe CoroutineWorker.
Jetpack + Coroutines

• As duas formas de iniciar uma coroutine são:
• A função launch é uma “ﬁre and forget” que signiﬁca
que não retornará o resultado para que a chamou (mas
retornará um Job).
• A função async retorna um objeto Deferred que
permite obter o seu resultado.
Iniciando uma coroutine

launch
launch {
txtOutput.text = ""
val time = measureTimeMillis {
val one = withContext(Dispatchers.IO) { loadFirstNumber() }
val two = withContext(Dispatchers.IO) { loadSecondNumber() }
addTextToTextView("The answer is ${one + two}")
}
addTextToTextView("Completed in $time ms")
}
The answer is 42 
Completed in 2030 ms

async
launch {
txtOutput.text = ""
val one = async(Dispatchers.IO) { loadFirstNumber() }
val two = async(Dispatchers.IO) { loadSecondNumber() }
val s = one.await() + two.await()
addTextToTextView("The answer is $s")
}
}

async
The answer is 42 
Completed in 1038 ms
launch {
txtOutput.text = ""
val one = async(Dispatchers.IO) { loadFirstNumber() }
val two = async(Dispatchers.IO) { loadSecondNumber() }
val s = one.await() + two.await()
addTextToTextView("The answer is $s")
}
}

• O tratamento de exceções é simples, basta tratar no lugar
certo!
• A falha de um Job cancelará o seu “pai" e os demais
“ﬁlhos"
• As exceções não tratadas são propagadas para o Job do
escopo.
• Um escopo cancelado não poderá iniciar coroutines.
Exceptions

Exceptions
launch {
txtOutput.text = ""
try {
val result = methodThatThrowsException()
addTextToTextView("Ok $result")
} catch (e: Exception) {
addTextToTextView("Error! ${e.message}")
}
}
👍

Exceptions
launch {
txtOutput.text = ""
try {
launch {
}
}
}

Exceptions
launch {
txtOutput.text = ""
try {
launch {
}
}
}
Main  
Thread
Flow
launch
Job

Exceptions
launch {
txtOutput.text = ""
try {
launch {
}
}
}
Main  
Thread
Flow
launch
Job
launch
Job

Exceptions
launch {
txtOutput.text = ""
launch {
try {
}
}
}

Exceptions
launch {
txtOutput.text = ""
val task = async { methodThatThrowsException() }
try {
val result = task.await()
}
}

Exceptions
launch {
txtOutput.text = ""
val task = async {
try {
methodThatThrowsException()
"Error! ${e.message}"
}
}
val result = task.await()
}

Exceptions
launch {
txtOutput.text = ""
val task = async(SupervisorJob(job)) {
}
try {
addTextToTextView("Ok ${task.await()}")
} catch (e: Throwable) {
}
}

Exceptions
launch {
txtOutput.text = ""
try {
coroutineScope {
val task = async {
}
}
addTextToTextView("Erro! ${e.message}")
}
}

Exceptions
launch {
txtOutput.text = ""
supervisorScope {
val task = async { methodThatThrowsException() }
try {
addTextToTextView(“Error! ${e.message}")
}
}
}

• Para cancelar um job, basta chamar o método cancel.
• Uma vez cancelado o job não pode ser reusado.
• Para cancelar os jobs ﬁlhos, use cancelChildren.
• A propriedade isActive indica que o job está em
execução, isCancelled se a coroutine foi cancelada, e
isCompleted terminou sua execução.
Cancelamento

• Executa uma coroutine levantando uma
TimeoutCancellationException caso sua duração
exceda o tempo especiﬁcado.

• Uma vez que o cancelamento é apenas uma exceção, é
possível trata-la facilmente.

• É possível usar a função withTimeoutOrNull que é
similar a withTimeout, mas retorna null ao invés de
levantar a exceção.
withTimeout

withTimeout
launch {
txtOutput.text = ""
try {
val s = withTimeout(1300L) {
withContext(Dispatchers.Default) {
aLongOperation()
}
}
txtOutput.text = "Result: $s..."
} catch (e: TimeoutCancellationException) {
txtOutput.text = "Exception! ${e.message}"
}
}

withTimeoutOrNull
launch {
txtOutput.text = ""
val task = async(Dispatchers.Default) {
aLongOperation()
}
val result = withTimeoutOrNull(1300L) { task.await() }
txtOutput.text = "Result: $result"
}

• Nos bastidores, uma suspending function é convertida pelo
compilador para uma função (de mesmo nome) que recebe um
objeto do tipo Continuation. 
fun sum(a: Int, b: Int, Continuation<Int>)
• Continuation é uma interface que contém duas funções que
são invocadas para continuar com a execução da coroutine
(normalmente retornando um valor) ou levantar uma exceção
caso algum erro ocorra. 
interface Continuation<in T> {
val context: CoroutineContext
fun resume(value: T)
fun resumeWithException(exception: Throwable)
}
Convertendo Callbacks

object LocationManager {
fun getCurrentLocation(callback: (LatLng?) -> Unit) {
// get the location...
callback(LatLng(-8.187,-36.156))
}
}
LocationManager.getCurrentLocation { latLng ->
if (latLng != null) {
// Exibir localização
} else {
// Tratar o erro
}
}

suspend fun getMyLocation(): LatLng {
return suspendCoroutine { continuation ->
continuation.resume(latLng)
} else {
continuation.resumeWithException(
Exception("Fail to get user location")
)
}
}
}
}

suspend fun getMyLocation(): LatLng {
return suspendCancellableCoroutine { continuation ->
continuation.resume(latLng)
} else {
continuation.resumeWithException(
Exception("Fail to get user location")
)
}
}
}
}

launch{
try {
val latLng = getMyLocation()
// do something
} catch(e: Exception){
// handle error
}
}

• launch (ﬁre-and-forget) e async (para obter um resultado).
• Trate as exceções no launch ou no async. Ou use
SupervisorJob, SupervisorScope ou
supervisorScope.
• cancel ou cancelChildren para cancelar o Job ou os jobs
ﬁlhos.
• withTimeout ou withTimeoutOrNull.
• Toda suspend function é convertida em um callback usando a
interface Continuation.
Coroutines - Parte 2

• Flow é uma abstração de um cold stream.
• Nada é executado/emitido até que algum consumidor se
registre no ﬂuxo.
• Possui diversos operadores como no RxJava.
Flow

@FlowPreview
public interface Flow<out T> {
public suspend fun collect(collector: FlowCollector<T>)
}
@FlowPreview
public interface FlowCollector<in T> {
public suspend fun emit(value: T)
}

val intFlow = flow {
for (i in 0 until 10) {
emit(i) //calls emit directly from the body of a FlowCollector
}
}
launch {
intFlow.collect { number ->
addTextToTextView("$numbern")
}
addTextToTextView("DONE!")
}

launch {
(0..100).asFlow()
.map { it * it }
.filter { it % 4 == 0 } // here and above is on IO thread pool
.flowOn(Dispatchers.IO) // 👆change the upstream Dispatcher
.map { it * 2 }
.flowOn(Dispatchers.Main)
.onStart { }
.onEach { }
.onCompletion { }
.collect { number ->
addTextToTextView("$numbern")
}
}

class NumberFlow {
private var currentValue = 0
private val numberChannel = BroadcastChannel<Int>(10)
fun getFlow(): Flow<Int> = numberChannel.asFlow()
suspend fun sendNext() {
numberChannel.send(currentValue++)
}
fun close() = numberChannel.close()
}

class FlowActivity : AppCompatActivity(R.layout.activity_flow) {
private val sender = NumberFlow()
btnProduce.setOnClickListener {
sender.sendNext()
}
}
sender.getFlow().collect {
txtOutput.append("Number: $it n")
}
}
}
super.onDestroy()
sender.close()
}
}

Callback para Flow
class YourApi {
fun doSomeCall(callback: YourListener<String>) {
// when new value arrives
callback.onNext("Item 1")
// when some error happens
callback.onApiError(Exception("Error"))
// when we're done
callback.onComplete()
}
interface YourListener<T> {
fun onNext(value: T)
fun onApiError(t: Throwable)
fun onComplete()
}
}

fun myFlow(): Flow<String> = callbackFlow {
val myCallback = object: YourApi.YourListener<String> {
override fun onNext(value: String) { offer(value) }
override fun onApiError(t: Throwable) { close(t) }
override fun onComplete() { close() }
}
val api = YourApi()
api.doSomeCall(myCallback)
awaitClose { /* do something when the stream is closed */ }
}

Room
dependencies {
def room_version = "2.2.5"
implementation "androidx.room:room-runtime:$room_version"
kapt "androidx.room:room-compiler:$room_version"
// 👇 Kotlin Extensions and Coroutines support for Room
implementation "androidx.room:room-ktx:$room_version"
...
}

@Dao
interface BookDao {
@Query("SELECT * FROM book")
suspend fun getAll(): List<Book>
@Query("SELECT * FROM book WHERE id = :id")
suspend fun getBook(id: Long): Book
@Insert
suspend fun insert(book: Book): Long
@Delete
suspend fun delete(book: Book)
}

@Dao
interface BookDao {
@Query("SELECT * FROM book")
fun getAll(): Flow<List<Book>>
@Query("SELECT * FROM book WHERE id = :id")
fun getBook(id: Long): Flow<Book?>
@Insert
suspend fun insert(book: Book): Long
@Delete
suspend fun delete(book: Book)
}

launch {
withContext(Dispatchers.IO) {
val id = dao.insert(
Book(0, "Dominando o Android", "Nelson Glauber")
)
}
// Do UI stuff
}

launch {
dao.getAll().collect { bookList ->
lstBooks.adapter = BookAdapter(context, bookList)
}
}

class BookFavoritesViewModel(
repository: BookRepository
): ViewModel() {
val favoriteBooks = repository.allFavorites().asLiveData()
}
Flow

class BookFavoritesViewModel(
repository: BookRepository
): ViewModel() {
val favoriteBooks = liveData {
repository.allFavorites().collect { list ->
emit(list)
}
}
}

• Coroutines vêm se tornando a forma de padrão para
realizar código assíncrono no Android.
• Essa é uma recomendação do Google.
• Além do Jetpack, outras bibliotecas estão migrando (ou já
migraram) pra Coroutines (ex: Retroﬁt, Apollo, MockK, …).
Conclusão

• Android Suspenders (Android Dev Summit 2018) 
https://www.youtube.com/watch?v=EOjq4OIWKqM
• Understand Kotlin Coroutines on Android (Google I/O 2019) 
https://www.youtube.com/watch?v=BOHK_w09pVA
• Coroutines Guide 
https://github.com/Kotlin/kotlinx.coroutines/blob/master/coroutines-
guide.md
• Android Suspenders by Chris Banes (KotlinConf 2018) 
https://www.youtube.com/watch?v=P7ov_r1JZ1g
• Room & Coroutines (Florina Muntenescu) 
https://medium.com/androiddevelopers/room-coroutines-422b786dc4c5
Referências #1

• Using Kotlin Coroutines in your Android App 
https://codelabs.developers.google.com/codelabs/kotlin-coroutines
• Use Kotlin coroutines with Architecture Components 
https://developer.android.com/topic/libraries/architecture/coroutines
• Create a Clean-Code App with Kotlin Coroutines and Android Architecture
Components 
https://blog.elpassion.com/create-a-clean-code-app-with-kotlin-coroutines-and-
android-architecture-components-f533b04b5431
• Android Coroutine Recipes (Dmytro Danylyk) 
https://proandroiddev.com/android-coroutine-recipes-33467a4302e9
• Kotlin Coroutines patterns & anti-patterns 
https://proandroiddev.com/kotlin-coroutines-patterns-anti-patterns-f9d12984c68e
Referências #2

• The reason to avoid GlobalScope (Roman Elizarov) 
https://medium.com/@elizarov/the-reason-to-avoid-globalscope-835337445abc
• WorkManager meets Kotlin (Pietro Maggi) 
https://medium.com/androiddevelopers/workmanager-meets-kotlin-b9ad02f7405e
• Coroutine Context and Scope (Roman Elizarov) 
https://medium.com/@elizarov/coroutine-context-and-scope-c8b255d59055
• Easy Coroutines in Android: viewModelScope (Manuel Vivo) 
https://medium.com/androiddevelopers/easy-coroutines-in-android-
viewmodelscope-25bffb605471
• Exceed the Android Speed Limit 
https://medium.com/androiddevelopers/exceed-the-android-speed-limit-
b73a0692abc1
Referências #3

• An Early look at Kotlin Coroutine’s Flow 
https://proandroiddev.com/an-early-look-at-kotlin-coroutines-
flow-62e46baa6eb0
• Coroutines on Android (Sean McQuillan) 
https://medium.com/androiddevelopers/coroutines-on-android-part-i-getting-the-
background-3e0e54d20bb
• Kotlin Flows and Coroutines (Roman Elizarov) 
https://medium.com/@elizarov/kotlin-flows-and-coroutines-256260fb3bdb
• Simple design of Kotlin Flow (Roman Elizarov) 
https://medium.com/@elizarov/simple-design-of-kotlin-flow-4725e7398c4c
• React Streams and Kotlin Flows (Roman Elizarov) 
https://medium.com/@elizarov/reactive-streams-and-kotlin-flows-bfd12772cda4
Referências #4

• KotlinConf 2019: Coroutines! Gotta catch 'em all! by Florina
Muntenescu & Manuel Vivo 
https://www.youtube.com/watch?v=w0kfnydnFWI
• LiveData with Coroutines and Flow (Android Dev Summit
'19) 
https://www.youtube.com/watch?v=B8ppnjGPAGE
• Testing Coroutines on Android (Android Dev Summit '19) 
https://www.youtube.com/watch?v=KMb0Fs8rCRs
Referências #5

Obrigado!
Nelson Glauber
@nglauber

Aplicações assíncronas no Android com Coroutines & Jetpack

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