Streams with Kotlin Coroutines

Asynchronous operations are part of our challenges that we face daily. Sending, receiving or retrieving data has to be offloaded to another thread so that we don't freeze or crash our app.
Luckily with Kotlin Coroutines, this got a whole lot easier. We can model asynchronous data handling operations with the help of suspending functions.
But what if we are trying to represent a stream of data? What options do we have?
This talk will introduce the different options Kotlin Coroutines give us for representing streams. We'll take a look at Channels and Flows, how and when to use either of them, how you can build on top of them with custom operators and some patterns specific to Android development.

Transcript

1. STREAMS WITH KOTLIN COROUTINES ARTHUR NAGY Android Tech Lead
2. WHY COROUTINES ?
3. val user: User = userRepository.getUser() // Update UI
4. userRepository.getUser(object: Callback<User> { override fun onResult(user: User) { // Update UI } override fun onError(throwable: Throwable) { // Handle error } })
5. object: Callback<User> { override fun onResult(user: User) { // Update UI } override fun onError(throwable: Throwable) { // Handle error } } userRepository.getUser(object: Callback<User> { override fun onResult(user: User) { // Update UI } override fun onError(throwable: Throwable) { // Handle error } })
6. val user: User = userRepository.getUser() // Update UI
7. WHAT ARE COROUTINES ?
8. • simplify async code by replacing callbacks • suspend and resume execution
9. • suspend keyword • CoroutineScope • Coroutine context and dispatchers • Coroutine builder
10. suspend keyword • used to mark a function • built into the Kotlin compiler • use other suspending functions from your function
11. • suspend keyword • CoroutineScope • Coroutine context and dispatchers • Coroutine builder
12. CoroutineScope • life span of a coroutine • holds the CoroutineContext • structured concurrency
13. • suspend keyword • CoroutineScope • Coroutine context and dispatchers • Coroutine builder
14. Coroutine context and dispatchers • contains coroutine Elements • Job: used to cancel a coroutine • Dispatcher: defines the thread in which the coroutine will run
15. • suspend keyword • CoroutineScope • Coroutine context and dispatchers • Coroutine builder
16. Coroutine builder • creates a new coroutine • extensions written on Scope • launch(), async(), (withContext())
17. • suspend keyword • CoroutineScope • Coroutine context and dispatchers • Coroutine builder
18. viewModelScope.launch(Dispatchers.Main) { val user: User = userRepository.getUser() // Update UI }
19. viewModelScope.launch(Dispatchers.Main) { val user: User = userRepository.getUser() // Update UI } viewModelScope
20. viewModelScope.launch(Dispatchers.Main) { val user: User = userRepository.getUser() // Update UI } .launch
21. viewModelScope.launch(Dispatchers.Main) { val user: User = userRepository.getUser() // Update UI } Dispatchers.Main
22. viewModelScope.launch(Dispatchers.Main) { val user: User = userRepository.getUser() // Update UI } .getUser()
23. viewModelScope.launch(Dispatchers.Main) { val user: User = userRepository.getUser() // Update UI }
24. STREAMS
25. WHEN TO USE STREAMS ? 1. API’s which are based on streams: • Bluetooth device scanning/notification API • Socket based services, i.e: chat app, etc. 2. Architecture patterns like: MVVM, MVI, etc, • Unidirectional data flow • Reactive UI’s
26. • Channels • Flows
27. Channels • hot data sources • low-level communication primitives • send/receive, similar to BlockingQueue • synchronisation between coroutines
28. Channels • subscription management • verbose and error prone • used as an implementation detail, never expose a channel, expose a Flow
29. • Channels • Flows
30. Flows • cold asynchronous data sources • runs in one coroutine, no-concurrency • based on suspending functions • simple design: safe, easy to learn and to use
31. Flows interface Flow<out T> { suspend fun collect(collector: FlowCollector<T>) } interface FlowCollector<in T> { suspend fun emit(value: T) } fun <T> flow( block: suspend FlowCollector<T>.() -> Unit ): Flow<T>
32. Flows interface Flow<out T> { suspend fun collect(collector: FlowCollector<T>) } interface FlowCollector<in T> { suspend fun emit(value: T) } fun <T> flow( block: suspend FlowCollector<T>.() -> Unit ): Flow<T>
33. Flows interface Flow<out T> { suspend fun collect(collector: FlowCollector<T>) } interface FlowCollector<in T> { suspend fun emit(value: T) } fun <T> flow( block: suspend FlowCollector<T>.() -> Unit ): Flow<T>
34. Flows interface Flow<out T> { suspend fun collect(collector: FlowCollector<T>) } interface FlowCollector<in T> { suspend fun emit(value: T) } fun <T> flow( block: suspend FlowCollector<T>.() -> Unit ): Flow<T>
35. Flows • back pressure • operators • intermediate • terminal • collect + flow builder
36. USAGE
37. John Doe me@john.doe
38. ViewModel John Doe me@john.doe
39. ViewModel John Doe me@john.doe
40. ViewModel Repository John Doe me@john.doe
41. ViewModel Repository John Doe me@john.doe
42. ViewModel Repository John Doe me@john.doe
43. ViewModel Repository John Doe me@john.doe
44. ViewModel Repository John Doe me@john.doe
45. ViewModel Repository John Doe me@john.doe
46. ViewModel Repository John Doe me@john.doe
47. class Repository(val userService: UserService) { suspend fun getUser() { val user: User = userService.getUser() channel.send(user) } suspend fun saveUser(user: User) { val updatedUser = userService.updateUser(user) channel.send(updatedUser) } } private val channel = BroadcastChannel<User>() fun getUserFlow(): Flow<User> = channel.asFlow()
48. class Repository(val userService: UserService) { suspend fun getUser() { val user: User = userService.getUser() channel.send(user) } suspend fun saveUser(user: User) { val updatedUser = userService.updateUser(user) channel.send(updatedUser) } } private val channel = BroadcastChannel<User>() fun getUserFlow(): Flow<User> = channel.asFlow()
49. private val channel = BroadcastChannel<User>() fun getUserFlow(): Flow<User> = channel.asFlow()
50. class Repository(val userService: UserService) { suspend fun getUser() { channel.send(user) } suspend fun saveUser(user: User) { channel.send(updatedUser) } } private val channel = BroadcastChannel<User>() fun getUserFlow(): Flow<User> = channel.asFlow() val user: User = userService.getUser() val updatedUser = userService.updateUser(user)
51. class Repository(val userService: UserService) { suspend fun getUser() { channel.send(user) } suspend fun saveUser(user: User) { channel.send(updatedUser) } } private val channel = BroadcastChannel<User>() fun getUserFlow(): Flow<User> = channel.asFlow() val user: User = userService.getUser() val updatedUser = userService.updateUser(user)
52. val user: User = userService.getUser() val updatedUser = userService.updateUser(user)
53. class Repository(val userService: UserService) { suspend fun getUser() { } suspend fun saveUser(user: User) { } } private val channel = BroadcastChannel<User>() fun getUserFlow(): Flow<User> = channel.asFlow() val user: User = userService.getUser() val updatedUser = userService.updateUser(user) channel.send(user) channel.send(updatedUser)
54. class Repository(val userService: UserService) { suspend fun getUser() { } suspend fun saveUser(user: User) { } } private val channel = BroadcastChannel<User>() fun getUserFlow(): Flow<User> = channel.asFlow() val user: User = userService.getUser() val updatedUser = userService.updateUser(user) channel.send(user) channel.send(updatedUser)
55. channel.send(user) channel.send(updatedUser)
56. class ViewModel(private val repository: UserRepository) { val user = LiveData<UserUiModel>() ... viewModelScope.launch { repository } .getUserFlow() .map { user -> UserUiModel.from(user) } .collect { userUiModel -> // Use the data received user.value = userUiModel }
57. class ViewModel(private val repository: UserRepository) { val user = LiveData<UserUiModel>() ... viewModelScope.launch { repository } .getUserFlow() .map { user -> UserUiModel.from(user) } .collect { userUiModel -> // Use the data received user.value = userUiModel }
58. .getUserFlow() .map { user -> UserUiModel.from(user) } .collect { userUiModel -> // Use the data received user.value = userUiModel }
59. fun updateUser( firstName: String, lastName: String, displayName: String ) { viewModelScope.launch { val updatedUser = currentUser.copy( firstName = firstName, lastName = lastName, displayName = displayName ) userRepository.saveUser(updatedUser) } } val updatedUser = currentUser.copy( firstName = firstName, lastName = lastName, displayName = displayName ) userRepository.saveUser(updatedUser)
60. fun updateUser( firstName: String, lastName: String, displayName: String ) { viewModelScope.launch { val updatedUser = currentUser.copy( firstName = firstName, lastName = lastName, displayName = displayName ) userRepository.saveUser(updatedUser) } } val updatedUser = currentUser.copy( firstName = firstName, lastName = lastName, displayName = displayName ) userRepository.saveUser(updatedUser)
61. ViewModel Repository John Doe me@john.doe
62. RESOURCES medium.com/@elizarov github.com/manuelvicnt/MathCoroutinesFlow youtu.be/BOHK_w09pVA youtu.be/B8ppnjGPAGE kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines- core/kotlinx.coroutines.flow/-flow/index.html kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines- core/kotlinx.coroutines.channels/-channel/index.html
63. THANK YOU, QUESTIONS ? arthurnagy.com halcyonmobile.com