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Loveleen Kaur
Android Educator
Software Engineer, Astrotalk
Asynchronous Programming
With Kotlin Coroutines
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● Loveleen Kaur
● Software Engineer - Android @ Astrotalk
● Core Team Member @ GDG Chandigarh
● Android Educator @ Android Educators Community India
● Phd Research Scholar
● Develop Mobile Applications
● Technical Speaker
● Happy Android Developer :)
Who am I?
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Asynchronous
programming
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How do we write code that waits
for something most of the time?
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A toy problem
Kotlin fun requestToken(): Token {
// makes request for a token & waits
return token // returns result when received
}
1
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fun requestToken(): Token { … }
fun createPost(token: Token, item: Item): Post {
// sends item to the server & waits
return post // returns resulting post
}
A toy problem
2
Kotlin
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A toy problem
Kotlin fun requestToken(): Token { … }
fun createPost(token: Token, item: Item): Post { … }
3 fun processPost(post: Post) {
// does some local processing of result
}
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fun requestToken(): Token { … }
fun createPost(token: Token, item: Item): Post { … }
fun processPost(post: Post) { … }
A toy problem
fun postItem(item: Item) {
val token = requestToken()
val post = createPost(token, item)
processPost(post)
}
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2
3
Can be done with
threads!
Kotlin
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fun requestToken(): Token {
// makes request for a token
// blocks the thread waiting for result
return token // returns result when received
}
fun createPost(token: Token, item: Item): Post { … }
fun processPost(post: Post) { … }
fun postItem(item: Item) {
val token = requestToken()
val post = createPost(token, item)
processPost(post)
}
Threads Is anything wrong with it?
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How many threads we can have?
100
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How many threads we can have?
1000
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How many threads we can have?
10 000
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How many threads we can have?
100 000
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Callbacks to the rescue
Sort of
…
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Callbacks: before
fun requestToken(): Token {
// makes request for a token & waits
return token // returns result when received
}
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Callbacks: after
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callback
fun requestTokenAsync(cb: (Token) -> Unit) {
// makes request for a token, invokes callback when done
// returns immediately
}
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Callbacks: before
fun requestTokenAsync(cb: (Token) -> Unit) { … }
fun createPost(token: Token, item: Item): Post {
// sends item to the server & waits
return post // returns resulting post
}
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Callbacks: after
fun requestTokenAsync(cb: (Token) -> Unit) { … }
fun createPostAsync(token: Token, item: Item,
cb: (Post) -> Unit) {
// sends item to the server, invokes callback when done
// returns immediately
}
2
callback
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Callbacks: before
fun
fun
requestTokenAsync(cb: (Token)
createPostAsync(token: Token,
-> Unit) { … }
item: Item,
cb: (Post) -> Unit) { … }
fun processPost(post: Post) { … }
fun postItem(item: Item) {
val token = requestToken()
val post = createPost(token, item)
processPost(post)
}
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Callbacks: after
fun
fun
requestTokenAsync(cb: (Token)
createPostAsync(token: Token,
-> Unit) { … }
item: Item,
cb: (Post) -> Unit) { … }
fun processPost(post: Post) { … }
fun postItem(item: Item) {
requestTokenAsync { token ->
createPostAsync(token, item) { post ->
processPost(post)
}
}
} aka “callback hell”
This is simplified. Handling
exceptions makes it a real mess
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Kotlin
Coroutines
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● The Kotlin team defines coroutines as “lightweight threads”.
● They are sort of tasks that the actual threads can execute.
● Coroutines were added to Kotlin in version 1.3 and are based on
established concepts from other languages.
● Kotlin coroutines introduce a new style of concurrency that can be used on
Android to simplify async code.
Coroutines
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Kotlin coroutines to the rescue
Let’s get real
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Coroutines: before
fun requestTokenAsync(): Promise {
// makes request for a token
// returns promise for a future result immediately
}
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Coroutines: after
suspend fun requestToken(): Token {
// makes request for a token & suspends
return token // returns result when received
}
1
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Coroutines: before
suspend fun requestToken(): Token { … }
fun createPostAsync(token: Token, item: Item): Promise {
// sends item to the server
// returns promise for a future result immediately
}
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Coroutines: after
suspend fun requestToken(): Token { … }
suspend fun createPost(token: Token, item: Item): Post {
// sends item to the server & suspends
return post // returns result when received
}
2
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Coroutines: before
fun postItem(item: Item) {
requestTokenAsync()
.thenCompose { token -> createPostAsync(token, item) }
.thenAccept { post -> processPost(post) }
}
Coroutines: after
suspend fun postItem(item: Item) {
val token = requestToken()
val post = createPost(token, item)
processPost(post)
}
suspension
points
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• Regular loops
for ((token, item) in list) {
createPost(token, item)
}
Bonus features
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• Regular exception handing
try {
createPost(token, item)
} catch (e: BadTokenException) {
…
}
Bonus features
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Bonus features
• Regular higher-order functions
file.readLines().forEach { line ->
createPost(token, line.toItem())
}
• forEach, let, apply, repeat, filter, map, use, etc
Everything like in blocking code
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Suspending functions
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Retrofit async
interface Service {
fun createPost(token: Token, item: Item): Call
}
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Retrofit async
interface Service {
fun createPost(token: Token, item: Item):
}
Call
future
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Retrofit async
interface Service {
fun createPost(token: Token, item: Item): Call
}
suspend fun createPost(token: Token, item: Item): Post =
serviceInstance.createPost(token, item).await()
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Retrofit async
suspend fun createPost(token: Token, item: Item): Post =
serviceInstance.createPost(token, item).await()
interface Service {
fun createPost(token: Token, item: Item): Call
}
natural signature
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Retrofit async
interface Service {
fun createPost(token: Token, item: Item): Call
}
suspend fun createPost(token: Token, item: Item): Post =
serviceInstance.createPost(token, item).await()
Suspending extension function
from integration library
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Coroutine builders
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Coroutines revisited
suspend fun requestToken(): Token { … }
suspend fun createPost(token: Token, item: Item): Post { … }
fun processPost(post: Post) { … }
suspend fun postItem(item: Item) {
val token = requestToken()
val post = createPost(token, item)
processPost(post)
}
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Coroutines revisited
suspend fun requestToken(): Token { … }
suspend fun createPost(token: Token, item: Item): Post { … }
fun processPost(post: Post) { … }
fun postItem(item: Item) {
val token = requestToken()
val post = createPost(token, item)
processPost(post)
}
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Coroutines revisited
suspend fun requestToken(): Token { … }
suspend fun createPost(token: Token, item: Item): Post { … }
fun processPost(post: Post) { … }
fun postItem(item: Item) {
val token = requestToken()
val post = createPost(token, item)
processPost(post)
}
Error: Suspend function 'requestToken' should be called only from
a coroutine or another suspend function
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Coroutines revisited
suspend fun requestToken(): Token { … }
suspend fun createPost(token: Token, item: Item): Post { … }
fun processPost(post: Post) { … }
fun postItem(item: Item) {
val token = requestToken()
val post = createPost(token, item)
processPost(post)
}
Can suspend execution
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Coroutines revisited
suspend fun requestToken(): Token { … }
suspend fun createPost(token: Token, item: Item): Post { … }
fun processPost(post: Post) { … }
fun postItem(item: Item) {
val token = requestToken()
val post = createPost(token, item)
processPost(post)
}
Can suspend execution
A regular function cannot
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Coroutines revisited
suspend fun requestToken(): Token { … }
suspend fun createPost(token: Token, item: Item): Post { … }
fun processPost(post: Post) { … }
Can suspend execution
A regular function cannot
fun postItem(item: Item) {
val token = requestToken()
val post = createPost(token, item)
processPost(post)
}
One cannot simply invoke
a suspending function
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}
Launch
coroutine builder
fun postItem(item: Item) {
launch {
val token = requestToken()
val post = createPost(token, item)
processPost(post)
}
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fun postItem(item: Item) {
launch {
val token = requestToken()
val post = createPost(token, item)
processPost(post)
} Fire and forget!
Returns immediately, coroutine
works in background thread pool
}
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fun postItem(item: Item) {
launch {
val token = requestToken()
val post = createPost(token, item)
processPost(post)
}
}
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fun postItem(item: Item) {
launch(UI) {
val token = requestToken()
val post = createPost(token, item)
processPost(post)
}
}
UI Context
Just specify the context
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fun postItem(item: Item) {
launch(UI) {
val token = requestToken()
val post = createPost(token, item)
processPost(post)
}
}
UI Context
And it gets executed on UI thread
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Where’s the magic of launch?
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fun launch(
context: CoroutineContext = DefaultDispatcher,
block: suspend () -> Unit
): Job { … }
A regular function
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fun launch(
context: CoroutineContext = DefaultDispatcher,
block: suspend () -> Unit
): Job { … } suspending lambda
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async / await
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Kotlin-way
Kotlin suspend fun postItem(item: Item) {
val token = requestToken()
val post = createPost(token, item)
processPost(post)
}
suspend fun requestToken(): Token { … }
suspend fun createPost(token: Token, item: Item): Post { … }
fun processPost(post: Post) { … }
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Classic-way
C# approach to the same problem
(also Python, TS, Dart, coming to
JS)
C# async Task postItem(Item item) {
var token = await requestToken();
var post = await createPost(token, item);
processPost(post);
}
async Task requestToken() { … }
async Task createPost(Token token, Item item) { … }
void processPost(Post post) { … }
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Why no await keyword in
Kotlin?
The problem with async
requestToken()
VALID –> produces Task
await requestToken()
VALID –> produces Token
concurrent behavior
sequential behavior
C#
C#
default
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Kotlin suspending functions
are designed to imitate
sequential behavior
by default
Concurrency is hard
Concurrency has to be explicit
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Kotlin approach to async
Concurrency where you need it
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Kotlin async function
fun loadImageAsync(name: String): Deferred![]()
=
async { … }
Kotlin
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Kotlin async function
fun loadImageAsync(name: String): Deferred![]()
=
async { … }
Kotlin
A regular function
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Kotlin async function
fun loadImageAsync(name: String): Deferred![]()
=
async { … }
Kotlin’s future type
Kotlin
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Kotlin async function
fun loadImageAsync(name: String): Deferred![]()
=
async { … }
async coroutine builder
Kotlin
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Kotlin async function
fun loadImageAsync(name: String): Deferred![]()
=
async { … }
val deferred1 = loadImageAsync(name1)
val deferred2 = loadImageAsync(name2)
Start multiple operations
concurrently
Kotlin
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Kotlin async function
fun loadImageAsync(name: String): Deferred![]()
=
async { … }
val deferred1 = loadImageAsync(name1)
val deferred2 = loadImageAsync(name2)
and then wait for them
val image1 = deferred1.await()
val image2 = deferred2.await()
await function
Suspends until deferred is complete
Kotlin
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Kotlin async function
fun loadImageAsync(name: String): Deferred![]()
=
async { … }
val deferred1 = loadImageAsync(name1)
val deferred2 = loadImageAsync(name2)
val image1 = deferred1.await()
val image2 = deferred2.await()
val result = combineImages(image1, image2)
Kotlin
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Using async function when needed
suspend fun loadImage(name: String): Image { … }
Is defined as suspending function, not async
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Using async function when needed
suspend fun loadImage(name: String): Image { … }
suspend fun loadAndCombine(name1: String, name2: String): Image {
val deferred1 = async { loadImage(name1) }
val deferred2 = async { loadImage(name2) }
return combineImages(deferred1.await(), deferred2.await())
}
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Using async function when needed
suspend fun loadImage(name: String): Image { … }
suspend fun loadAndCombine(name1: String, name2: String): Image {
async
val deferred1 = { loadImage(name1) }
val deferred2 = async { loadImage(name2) }
return combineImages(deferred1.await(), deferred2.await())
}
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Using async function when needed
suspend fun loadImage(name: String): Image { … }
suspend fun loadAndCombine(name1: String, name2: String): Image {
val deferred1 = async { loadImage(name1) }
async
val deferred2 = { loadImage(name2) }
return combineImages(deferred1.await(), deferred2.await())
}
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Using async function when needed
suspend fun loadImage(name: String): Image { … }
suspend fun loadAndCombine(name1: String, name2: String): Image {
val deferred1 = async { loadImage(name1) }
val deferred2 = async { loadImage(name2) }
return combineImages(deferred1.await(), deferred2.await())
}
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Kotlin approach to async
requestToken()
VALID –> produces Token
async { requestToken() }
VALID –> produces Deferred
sequential behavior
concurrent behavior
Kotlin
Kotlin
default
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Coroutines
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What are coroutines
conceptually?
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What are coroutines
conceptually?
Coroutines are like very light-weight threads
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fun main(args: Array) = runBlocking {
val jobs = List(100_000) {
launch {
delay(1000L)
print(".")
}
}
jobs.forEach { it.join() }
}
Example
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fun main(args: Array) = runBlocking {
val jobs = List(100_000) {
launch {
delay(1000L)
print(".")
}
}
jobs.forEach { it.join() }
}
Example
This coroutine builder runs coroutine
in the context of invoker thread
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fun main(args: Array) = runBlocking {
val jobs = List(100_000) {
launch {
delay(1000L)
print(".")
}
}
jobs.forEach { it.join() }
}
Example
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fun main(args: Array) = runBlocking {
val jobs = List(100_000) {
launch {
delay(1000L)
print(".")
}
}
jobs.forEach { it.join() }
}
Example
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fun main(args: Array) = runBlocking {
val jobs = List(100_000) {
launch {
delay(1000L)
print(".")
}
}
jobs.forEach { it.join() }
}
Example
Suspends for 1 second
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fun main(args: Array) = runBlocking {
val jobs = List(100_000) {
launch {
delay(1000L)
print(".")
}
}
jobs.forEach { it.join() }
}
Example
We can join a job
just like a thread
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Example
Try that with 100k threads!
fun main(args: Array) = runBlocking {
val jobs = List(100_000) {
launch {
delay(1000L)
print(".")
}
}
jobs.forEach { it.join() }
}
Prints 100k dots after one second delay
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fun main(args: Array) =
val jobs = List(100_000) {
runBlocking {
launch {
delay(1000L)
print(".")
}
}
jobs.forEach { it.join() }
}
Example
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fun main(args: Array) {
val jobs = List(100_000) {
thread {
Thread.sleep(1000L)
print(".")
}
}
jobs.forEach { it.join() }
}
Example
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Library vs Language
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Classic async
async/await
generate/yield
Keywords
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Kotlin coroutines
suspend Modifier
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Kotlin coroutines
Standard
library
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Kotlin coroutines
Standard
library
kotlinx-coroutines
launch, async,
runBlocking, future, delay,
Job, Deferred, etc
http://github.com/kotlin/kotlinx.coroutines
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Thank you
Loveleen Kaur
https://www.linkedin.com/in/loveleen-kaur/
https://twitter.com/loveleen_nancy