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Introduction to Coroutines Roman Elizarov elizarov at JetBrains - PowerPoint PPT Presentation

Sep 04, 2022 •361 likes •1.83k views

Introduction to Coroutines Roman Elizarov elizarov at JetBrains Asynchronous programming How do we write code that waits for something most of the time? A toy problem fun requestToken(): Token { 1 Kotlin // makes request for a token &

  1. Higher-order functions val post = retryIO { createPost (token, item) } suspending lambda suspend fun <T> retryIO(block: suspend () -> T): T { var curDelay = 1000L // start with 1 sec while ( true ) { try { return block() } catch (e: IOException) { e.printStackTrace() // log the error } delay (curDelay) curDelay = (curDelay * 2). coerceAtMost (60000L) } }

  2. Higher-order functions val post = retryIO { createPost (token, item) } suspend fun <T> retryIO(block: suspend () -> T): T { var curDelay = 1000L // start with 1 sec while ( true ) { try { return block() } catch (e: IOException) { e.printStackTrace() // log the error } delay (curDelay) curDelay = (curDelay * 2). coerceAtMost (60000L) } } Everything like in blocking code

  3. Higher-order functions val post = retryIO { createPost (token, item) } suspend fun <T> retryIO(block: suspend () -> T): T { var curDelay = 1000L // start with 1 sec while ( true ) { try { return block() } catch (e: IOException) { e.printStackTrace() // log the error } delay (curDelay) curDelay = (curDelay * 2). coerceAtMost (60000L) } }

  4. Coroutine builders

  5. 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) }

  6. 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) }

  7. 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

  8. Coroutines revisited suspend fun requestToken(): Token { … } suspend fun createPost(token: Token, item: Item): Post { … } fun processPost(post: Post) { … } Can suspend execution fun postItem(item: Item) { val token = requestToken () val post = createPost (token, item) processPost (post) }

  9. Coroutines revisited suspend fun requestToken(): Token { … } suspend fun createPost(token: Token, item: Item): Post { … } fun processPost(post: Post) { … } A regular function cannot Can suspend execution fun postItem(item: Item) { val token = requestToken () val post = createPost (token, item) processPost (post) }

  10. Coroutines revisited suspend fun requestToken(): Token { … } suspend fun createPost(token: Token, item: Item): Post { … } fun processPost(post: Post) { … } A regular function cannot Can suspend execution fun postItem(item: Item) { val token = requestToken () val post = createPost (token, item) processPost (post) } One cannot simply invoke a suspending function

  11. Launch coroutine builder fun postItem(item: Item) { launch { val token = requestToken () val post = createPost (token, item) processPost (post) } }

  12. Returns immediately, coroutine works in background thread pool fun postItem(item: Item) { launch { val token = requestToken () val post = createPost (token, item) processPost (post) } Fire and forget! }

  13. fun postItem(item: Item) { launch { val token = requestToken () val post = createPost (token, item) processPost (post) } }

  14. UI Context Just specify the context fun postItem(item: Item) { launch (UI) { val token = requestToken () val post = createPost (token, item) processPost (post) } }

  15. UI Context fun postItem(item: Item) { launch (UI) { val token = requestToken () val post = createPost (token, item) processPost (post) } } And it gets executed on UI thread

  16. Where’s the magic of launch?

  17. A regular function fun launch( context: CoroutineContext = DefaultDispatcher, block: suspend () -> Unit ): Job { … }

  18. fun launch( context: CoroutineContext = DefaultDispatcher, block: suspend () -> Unit ): Job { … } suspending lambda

  19. fun launch( context: CoroutineContext = DefaultDispatcher, block: suspend () -> Unit ): Job { … }

  20. async / await

  21. Kotlin-way suspend fun requestToken(): Token { … } suspend fun createPost(token: Token, item: Item): Post { … } fun processPost(post: Post) { … } suspend fun postItem(item: Item) { Kotlin val token = requestToken () val post = createPost (token, item) processPost (post) }

  22. Classic-way async Task<Token> requestToken() { … } async Task<Post> createPost(Token token, Item item) { … } void processPost(Post post) { … } C# approach to the same problem (also Python, TS, Dart, coming to JS) async Task postItem(Item item) { C# var token = await requestToken (); var post = await createPost (token, item); processPost (post); }

  23. Classic-way async Task<Token> requestToken() { … } async Task<Post> createPost(Token token, Item item) { … } void processPost(Post post) { … } mark with async async Task postItem(Item item) { C# var token = await requestToken (); var post = await createPost (token, item); processPost (post); }

  24. Classic-way async Task<Token> requestToken() { … } async Task<Post> createPost(Token token, Item item) { … } void processPost(Post post) { … } async Task postItem(Item item) { C# var token = await requestToken (); var post = await createPost (token, item); processPost (post); } use await to suspend

  25. Classic-way async Task<Token> requestToken() { … } async Task<Post> createPost(Token token, Item item) { … } void processPost(Post post) { … } returns a future async Task postItem(Item item) { C# var token = await requestToken (); var post = await createPost (token, item); processPost (post); }

  26. Why no await keyword in Kotlin? The problem with async requestToken () VALID –> produces Task<Token> C# concurrent behavior default VALID –> produces Token await requestToken () C# sequential behavior

  27. Kotlin suspending functions are designed to imitate sequential behavior by default Concurrency is hard Concurrency has to be explicit

  28. Kotlin approach to async Concurrency where you need it

  29. Use-case for async async Task<Image> loadImageAsync(String name) { … } C#

  30. Use-case for async async Task<Image> loadImageAsync(String name) { … } C# var promise1 = loadImageAsync(name1); Start multiple operations var promise2 = loadImageAsync(name2); concurrently

  31. Use-case for async async Task<Image> loadImageAsync(String name) { … } C# var promise1 = loadImageAsync(name1); var promise2 = loadImageAsync(name2); var image1 = await promise1; and then wait for them var image2 = await promise2;

  32. Use-case for async async Task<Image> loadImageAsync(String name) { … } C# var promise1 = loadImageAsync(name1); var promise2 = loadImageAsync(name2); var image1 = await promise1; var image2 = await promise2; var result = combineImages(image1, image2);

  33. Kotlin async function fun loadImageAsync(name: String): Deferred<Image> = Kotlin async { … }

  34. Kotlin async function A regular function fun loadImageAsync(name: String): Deferred<Image> = Kotlin async { … }

  35. Kotlin async function Kotlin’s future type fun loadImageAsync(name: String): Deferred<Image> = Kotlin async { … }

  36. Kotlin async function fun loadImageAsync(name: String): Deferred<Image> = Kotlin async { … } async coroutine builder

  37. Kotlin async function fun loadImageAsync(name: String): Deferred<Image> = Kotlin async { … } val deferred1 = loadImageAsync(name1) Start multiple operations val deferred2 = loadImageAsync(name2) concurrently

  38. Kotlin async function fun loadImageAsync(name: String): Deferred<Image> = Kotlin async { … } val deferred1 = loadImageAsync(name1) val deferred2 = loadImageAsync(name2) val image1 = deferred1.await() and then wait for them val image2 = deferred2.await() await function Suspends until deferred is complete

  39. Kotlin async function fun loadImageAsync(name: String): Deferred<Image> = Kotlin async { … } val deferred1 = loadImageAsync(name1) val deferred2 = loadImageAsync(name2) val image1 = deferred1.await() val image2 = deferred2.await() val result = combineImages(image1, image2)

  40. Using async function when needed Is defined as suspending function, not async suspend fun loadImage(name: String): Image { … }

  41. 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()) }

  42. 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()) }

  43. 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()) }

  44. 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()) }

  45. Kotlin approach to async requestToken () VALID –> produces Token Kotlin sequential behavior default VALID –> produces Deferred<Token> async { requestToken () } Kotlin concurrent behavior

  46. Coroutines

  47. What are coroutines conceptually?

  48. What are coroutines conceptually? Coroutines are like very light-weight threads

  49. Example fun main(args: Array<String>) = runBlocking <Unit> { val jobs = List (100_000) { launch { delay (1000L) print ( "." ) } } jobs. forEach { it .join() } }

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