Asynchronous Data Streams with Kotlin Flow

Kotlin Flow is a declarative mechanism for working with asynchronous data streams that builds on top of Kotlin coroutines and structured concurrency. Kotlin Flows are doing the same revolution in simplicity for data streams that suspending functions did for data values.

In this talk we will discuss the challenges of working with asynchronous streams and how Kotlin Flows solve them. We will study the basics of Kotlin Flow design, see their typical usage patterns, peek behind the scenes and into some of the implementation details, checkout flow performance and see how they enable writing safe, reliable, and leak-free systems. We will also discuss how they relate to and incorporate ideas from reactive extensions and reactive streams, how they are similar and different, and how they can be used together.

Transcript

1. Copenhagen
   Denmark
   ASYNCHRONOUS
   DATA STREAMS WITH
   KOTLIN FLOW
   ROMAN ELIZAROV
   @relizarov
   Copenhagen
   Denmark
   

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2. RECAP ON KOTLIN
   COROUTINES
   Kotlin Coroutines FTW
   

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3. Callback hell before
   fun requestTokenAsync(): Promise { … }
   fun createPostAsync(token: Token, item: Item): Promise …
   fun processPost(post: Post) { … }
   fun postItem(item: Item) {
   requestTokenAsync()
   .thenCompose { token -> createPostAsync(token, item) }
   .thenAccept { post -> processPost(post) }
   }
   

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4. Direct style with Kotlin Corou6nes
   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)
   }
   Like regular code
   

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5. Direct style with Kotlin Coroutines
   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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6. Asynchronous yet sequen6al
   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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7. Asynchronous yet sequential
   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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8. Asynchronous yet sequential
   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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9. suspend fun foo(): Response One response
   suspend fun foo(): List Many responses
   

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10. suspend fun foo(): List
    

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11. suspend fun foo(): List = buildList {
    …
    }
    

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12. suspend fun foo(): List = buildList {
    add(compute("A"))
    add(compute("B"))
    add(compute("C"))
    }
    

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13. suspend fun foo(): List = buildList {
    add(compute("A"))
    add(compute("B"))
    add(compute("C"))
    }
    fun main() = runBlocking {
    val list = foo()
    for (x in list) println(x)
    }
    

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14. suspend fun foo(): List = buildList {
    add(compute("A"))
    add(compute("B"))
    add(compute("C"))
    }
    fun main() = runBlocking {
    val list = foo()
    for (x in list) println(x)
    }
    

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15. suspend fun foo(): List = buildList {
    add(compute("A"))
    add(compute("B"))
    add(compute("C"))
    }
    fun main() = runBlocking {
    val list = foo()
    for (x in list) println(x)
    }
    

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16. main()
    foo()
    suspend fun foo(): List = buildList {
    add(compute("A"))
    add(compute("B"))
    add(compute("C"))
    }
    fun main() = runBlocking {
    val list = foo()
    for (x in list) println(x)
    }
    

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17. main()
    foo()
    suspend fun foo(): List = buildList {
    add(compute("A"))
    add(compute("B"))
    add(compute("C"))
    }
    fun main() = runBlocking {
    val list = foo()
    for (x in list) println(x)
    }
    

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18. main()
    foo() A
    suspend fun foo(): List = buildList {
    add(compute("A"))
    add(compute("B"))
    add(compute("C"))
    }
    fun main() = runBlocking {
    val list = foo()
    for (x in list) println(x)
    }
    

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19. main()
    foo()
    suspend fun foo(): List = buildList {
    add(compute("A"))
    add(compute("B"))
    add(compute("C"))
    }
    fun main() = runBlocking {
    val list = foo()
    for (x in list) println(x)
    }
    B
    A
    

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20. main()
    foo()
    suspend fun foo(): List = buildList {
    add(compute("A"))
    add(compute("B"))
    add(compute("C"))
    }
    fun main() = runBlocking {
    val list = foo()
    for (x in list) println(x)
    }
    C
    B
    A
    

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21. main()
    foo()
    suspend fun foo(): List = buildList {
    add(compute("A"))
    add(compute("B"))
    add(compute("C"))
    }
    fun main() = runBlocking {
    val list = foo()
    for (x in list) println(x)
    }
    List
    C
    B
    A
    

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22. main()
    foo()
    suspend fun foo(): List = buildList {
    add(compute("A"))
    add(compute("B"))
    add(compute("C"))
    }
    fun main() = runBlocking {
    val list = foo()
    for (x in list) println(x)
    }
    List
    C
    B
    A
    

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23. main()
    foo()
    List
    A B C
    B C
    A
    suspend fun foo(): List = buildList {
    add(compute("A"))
    add(compute("B"))
    add(compute("C"))
    }
    fun main() = runBlocking {
    val list = foo()
    for (x in list) println(x)
    }
    

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24. main()
    foo()
    List
    A B C
    B C
    A
    suspend fun foo(): List = buildList {
    add(compute("A"))
    add(compute("B"))
    add(compute("C"))
    }
    fun main() = runBlocking {
    val list = foo()
    for (x in list) println(x)
    }
    

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25. Channel
    receive()
    send()
    

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26. fun CoroutineScope.foo(): ReceiveChannel = produce {
    …
    }
    

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27. fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    

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28. fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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29. fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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30. fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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31. main()
    foo()
    fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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32. main()
    foo()
    fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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33. main()
    foo()
    Channel
    fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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34. main()
    foo()
    Channel
    fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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35. main()
    foo()
    Channel
    fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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36. main()
    foo()
    send
    Channel
    A
    fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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37. main()
    foo()
    send
    Channel
    A
    A
    fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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38. main()
    foo()
    send send
    Channel
    A
    A B
    fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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39. main()
    foo()
    send send
    Channel
    A B
    A B
    fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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40. main()
    foo()
    send send send
    Channel
    A B
    A B C
    fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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41. main()
    foo()
    send send send
    Channel
    A B C
    A B C
    fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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42. main()
    foo()
    send send send
    Channel
    A B C
    A B C
    fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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43. main()
    foo()
    send send send
    Channel
    A B C
    A B C
    fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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44. main()
    foo()
    send send send
    Channel
    A B C
    A B C ❌
    fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    

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45. Channel is hot
    

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46. fun main() = runBlocking {
    val channel = foo()
    for (x in channel) println(x)
    }
    Channel is hot
    

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47. Channel is hot
    fun main() = runBlocking {
    val channel = foo()
    // for (x in channel) println(x)
    }
    

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48. main()
    foo()
    Channel
    fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    // for (x in channel) println(x)
    }
    

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49. main()
    foo()
    send
    Channel
    
    A
    fun CoroutineScope.foo(): ReceiveChannel = produce {
    send(compute("A"))
    send(compute("B"))
    send(compute("C"))
    }
    fun main() = runBlocking {
    val channel = foo()
    // for (x in channel) println(x)
    }
    

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50. fun CoroutineScope.foo(): ReceiveChannel
    
    

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51. KOTLIN FLOW
    Image: Markus Trienke, Sunset over dri6 ice
    

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52. fun foo(): Flow = flow {
    …
    }
    

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53. fun foo(): Flow = flow {
    …
    }
    

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54. fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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55. fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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56. fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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57. fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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58. main()
    foo()
    fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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59. main()
    foo()
    fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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60. main()
    foo()
    Flow
    fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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61. main()
    foo()
    Flow
    fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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62. main()
    foo()
    collect
    Flow
    fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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63. main()
    foo()
    emit
    collect
    A
    Flow
    fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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64. main()
    foo()
    emit
    A
    collect
    A
    Flow
    fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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65. main()
    foo()
    emit
    A
    collect
    A
    Flow
    fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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66. main()
    foo()
    emit emit
    A
    collect
    A B
    Flow
    fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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67. main()
    foo()
    emit emit
    A B
    collect
    A B
    Flow
    fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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68. main()
    foo()
    emit emit
    A B
    collect
    A B
    Flow
    fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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69. main()
    foo()
    emit emit emit
    A B
    collect
    A B C
    Flow
    fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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70. main()
    foo()
    emit emit emit
    A B C
    collect
    A B C
    Flow
    fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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71. main()
    foo()
    emit emit emit
    A B C
    collect
    A B C
    Flow
    fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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72. main()
    foo()
    emit emit emit
    A B C
    collect
    A B C ❌
    Flow
    fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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73. main()
    foo()
    emit emit emit
    A B C
    collect
    A B C ❌
    Flow
    fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    

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74. fun main() = runBlocking {
    val flow = foo()
    flow.collect { x -> println(x) }
    }
    Flow is cold ❄
    

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75. fun main() = runBlocking {
    val flow = foo()
    // flow.collect { x -> println(x) }
    }
    Flow is cold ❄
    
    

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76. Flow is declara6ve
    

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77. Flow is declarative
    fun foo(): Flow = flow {
    emit(compute("A"))
    emit(compute("B"))
    emit(compute("C"))
    }
    Declaration
    

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78. fun strings(): Flow = flow {
    emit("A")
    emit("B")
    emit(”C")
    }
    

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79. fun strings(): Flow = flow {
    …
    }
    fun foo(): Flow =
    strings().map { name ->
    compute(name)
    }
    

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80. fun strings(): Flow = flow {
    …
    }
    fun foo(): Flow =
    strings().map { name ->
    compute(name)
    }
    

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81. fun strings(): Flow = flow {
    …
    }
    fun foo(): Flow =
    strings().map { name ->
    compute(name)
    }
    

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82. fun strings(): Flow = flow {
    …
    }
    fun foo(): Flow =
    strings().map { name ->
    compute(name)
    }
    Operators
    

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83. fun foo(): Flow =
    flowOf("A", "B", "C").map { name ->
    compute(name)
    }
    Operators
    

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84. Flow vs List
    fun foo(): Flow =
    flowOf("A", "B", "C").map { name ->
    compute(name)
    }
    suspend fun foo(): List =
    listOf("A", "B", "C").map { name ->
    compute(name)
    }
    

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85. fun foo(): Flow =
    flowOf("A", "B", "C").map { name ->
    compute(name)
    }
    suspend fun foo(): List =
    listOf("A", "B", "C").map { name ->
    compute(name)
    }
    Defined – Declarative
    Runs – Impera:ve
    suspend fun Flow.collect(…) Runs the flow
    Flow vs List
    

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86. fun foo(): Flow =
    flowOf("A", "B", "C").map { name ->
    compute(name)
    }
    suspend fun foo(): List =
    listOf("A", "B", "C").map { name ->
    compute(name)
    }
    Defined – Declara:ve
    Runs – Imperative
    suspend fun Flow.toList(): List Runs the flow
    Flow vs List
    

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87. fun foo(): Flow =
    flowOf("A", "B", "C").map { name ->
    compute(name)
    }
    suspend fun foo(): List =
    listOf("A", "B", "C").map { name ->
    compute(name)
    }
    Flow vs List
    Execu6on order
    

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88. fun foo(): Flow =
    flowOf("A", "B", "C").map { name ->
    compute(name)
    }
    suspend fun foo(): List =
    listOf("A", "B", "C").map { name ->
    compute(name)
    } B C
    A
    Flow vs List
    

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89. fun foo(): Flow =
    flowOf("A", "B", "C").map { name ->
    compute(name)
    }
    suspend fun foo(): List =
    listOf("A", "B", "C").map { name ->
    compute(name)
    } B C
    A map
    Flow vs List
    

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90. fun foo(): Flow =
    flowOf("A", "B", "C").map { name ->
    compute(name)
    }
    suspend fun foo(): List =
    listOf("A", "B", "C").map { name ->
    compute(name)
    } B C
    A
    B’ C’
    A’
    map
    Flow vs List
    

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91. fun foo(): Flow =
    flowOf("A", "B", "C").map { name ->
    compute(name)
    }
    suspend fun foo(): List =
    listOf("A", "B", "C").map { name ->
    compute(name)
    } B C
    A
    B’ C’
    A’
    map
    A
    A’
    Flow vs List
    

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92. fun foo(): Flow =
    flowOf("A", "B", "C").map { name ->
    compute(name)
    }
    suspend fun foo(): List =
    listOf("A", "B", "C").map { name ->
    compute(name)
    } B C
    A
    B’ C’
    A’
    map
    A
    B’
    A’
    B
    Flow vs List
    

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93. fun foo(): Flow =
    flowOf("A", "B", "C").map { name ->
    compute(name)
    }
    suspend fun foo(): List =
    listOf("A", "B", "C").map { name ->
    compute(name)
    } B C
    A
    B’ C’
    A’
    map
    A
    B’ C’
    A’
    B C
    Flow vs List
    React on emi2ed values
    

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94. Flow is reac6ve
    RxJava
    Project
    Reactor
    Kotlin Flow
    Reactive Streams Specification
    

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95. Publisher
    org.reactivestreams
    Publisher
    

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96. fun Publisher.asFlow(): Flow
    org.reactivestreams
    Publisher
    kotlinx.coroutines.flow
    Flow
    

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97. fun Flow.asPublisher(): Publisher
    fun Publisher.asFlow(): Flow
    org.reactivestreams
    Publisher
    kotlinx.coroutines.flow
    Flow
    

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98. WHY FLOW?
    What’s the difference?
    

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99. A
    A’
    mapper
    fun map(mapper: (T) -> R): Flowable
    fun flatMapSingle(mapper: (T) -> SingleSource): Flowable
    Synchronous
    Asynchronous
    Flowable
    

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100. A
     A’
     mapper
     fun map(mapper: (T) -> R): Flowable
     fun flatMapSingle(mapper: (T) -> SingleSource): Flowable
     fun filter(predicate: (T) -> Boolean): Flowable
     A
     A
     predicate
     
     Synchronous
     Asynchronous
     Synchronous
     Asynchronous
     Flowable
     

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101. A
     A’
     Flow
     fun map(transform: suspend (T) -> R): Flow
     transform
     

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102. A
     A’
     transform
     Flow
     fun map(transform: suspend (T) -> R): Flow
     

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103. A
     A’
     transform fun map(transform: suspend (T) -> R): Flow
     Flow
     fun filter(predicate: suspend (T) -> Boolean): Flow
     A
     predicate
     A
     

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104. Operator avoidance
     startWith(value) onStart { emit(value) }
     delaySubscription(time) onStart { delay(time) }
     startWith(flow) onStart { emitAll(flow) }
     delayEach(time) onEach { delay(time) }
     onErrorReturn(value) catch { emit(value) }
     onErrorResume(flow) catch { emitAll(flow) }
     generate(…) flow { … }
     Composable
     

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105. Reactive + = ❤
     

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106. FLOW
     UNDER THE HOOD
     Image: Flow by Grant Tarrant
     

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107. interface Flow {
     suspend fun collect(collector: FlowCollector)
     }
     interface FlowCollector {
     suspend fun emit(value: T)
     }
     

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108. flow.collect { value ->
     println(value)
     }
     collect
     1
     

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109. flow.collect { value ->
     println(value)
     }
     val flow = flow {
     emit("A")
     }
     collect
     1
     

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110. flow.collect { value ->
     println(value)
     }
     val flow = flow {
     emit("A")
     }
     collect
     1
     

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111. flow.collect { value ->
     println(value)
     }
     val flow = flow {
     emit("A")
     }
     collect
     1
     λ
     

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112. flow.collect { value ->
     println(value)
     }
     val flow = flow {
     emit("A")
     }
     collect
     1
     λ
     

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113. flow.collect { value ->
     println(value)
     }
     val flow = flow {
     emit("A")
     }
     collect
     emit
     λ
     1
     2
     λ
     

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114. flow.collect { value ->
     println(value)
     }
     val flow = flow {
     emit("A")
     }
     collect
     emit
     λ
     println
     1
     2
     3
     λ
     

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115. flow.collect { value ->
     println(value)
     }
     val flow = flow {
     emit("A")
     }
     collect
     λ
     emit
     λ
     println
     1
     2
     3 ^
     4
     ^
     5
     

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116. flow.collect { value ->
     println(value)
     }
     val flow = flow {
     emit("A")
     emit("B")
     }
     collect
     λ
     emit
     λ
     println
     emit
     println
     

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117. flow.collect { value ->
     println(value)
     }
     val flow = flow {
     emit("A")
     delay(100)
     emit("B")
     }
     collect
     λ
     emit
     λ
     println
     emit
     println
     Asynchronous emi@er
     

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118. flow.collect { value ->
     delay(100)
     println(value)
     }
     val flow = flow {
     emit("A")
     delay(100)
     emit("B")
     }
     collect
     λ
     emit
     λ
     println
     emit
     println
     Backpressure
     

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119. Simple design
     

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120. Simple design ⇒ performance
     

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121. Kotlin Flow Plays Scrabble
     • Benchmark originally developed by José Paumard
     • Implemented for RxJava by David Karnok
     

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122. Kotlin Flow Plays Scrabble
     • Benchmark originally developed by José Paumard
     • Implemented for RxJava by David Karnok
     SequencePlaysScrabble 9.824 ± 0.190 ms/op
     https://github.com/Kotlin/kotlinx.coroutines/tree/develop/benchmarks/src/jmh/kotlin/benchmarks/flow/scrabble/README.md
     

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123. Kotlin Flow Plays Scrabble
     • Benchmark originally developed by José Paumard
     • Implemented for RxJava by David Karnok
     SequencePlaysScrabble 9.824 ± 0.190 ms/op
     RxJava2PlaysScrabbleOpt 23.653 ± 0.379 ms/op
     https://github.com/Kotlin/kotlinx.coroutines/tree/develop/benchmarks/src/jmh/kotlin/benchmarks/flow/scrabble/README.md
     

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124. Kotlin Flow Plays Scrabble
     • Benchmark originally developed by José Paumard
     • Implemented for RxJava by David Karnok
     SequencePlaysScrabble 9.824 ± 0.190 ms/op
     RxJava2PlaysScrabbleOpt 23.653 ± 0.379 ms/op
     FlowPlaysScrabbleOpt 13.958 ± 0.278 ms/op
     https://github.com/Kotlin/kotlinx.coroutines/tree/develop/benchmarks/src/jmh/kotlin/benchmarks/flow/scrabble/README.md
     

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125. FLOW IS
     ASYNCHRONOUS
     

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126. FLOW IS
     ASYNCHRONOUS
     YET SEQUENTIAL
     

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127. main()
     emit emit emit
     A B C
     collect
     A B C ❌
     100 ms 100 ms 100 ms 100 ms
     100 ms 100 ms 100 ms
     700 ms
     Single corouDne
     

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128. GOING
     CONCURRENT
     WITH A FLOW
     Image: Channels by Tom Doel
     

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129. flow.buffer().collect { … }
     

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130. flow.buffer().collect { … }
     main()
     emit emit emit
     A B C
     collect
     A B C ❌
     100 ms 100 ms 100 ms 100 ms
     400 ms
     100 ms 100 ms
     100 ms
     

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131. flow.buffer().collect { … }
     main()
     emit emit emit
     A B C
     collect
     A B C
     Separate corouDne
     

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132. flow.buffer().collect { … }
     main()
     send send send
     A B C
     collect
     A B C ❌
     Channel
     Declarative & safe
     

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133. FLOW
     EXECUTION CONTEXT
     Image: Context by Bart Everson
     

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134. fun foo(): Flow =
     flowOf("A", "B", "C").map { name ->
     compute(name)
     }
     

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135. fun foo(): Flow =
     flowOf("A", "B", "C").map { name ->
     compute(name)
     }
     fun main() = runBlocking {
     val flow = foo()
     flow.collect { x -> println(x) }
     }
     Where does it execute?
     

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136. fun foo(): Flow =
     flowOf("A", "B", "C").map { name ->
     compute(name)
     }
     .flowOn(Dispatchers.Default)
     fun main() = runBlocking {
     val flow = foo()
     flow.collect { x -> println(x) }
     }
     Executes in background
     

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137. fun foo(): Flow =
     flowOf("A", "B", "C").map { name ->
     compute(name)
     }
     .flowOn(Dispatchers.Default)
     Executes in collector’s context
     fun main() = runBlocking {
     val flow = foo()
     flow.collect { x -> println(x) }
     }
     Context preservation
     

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138. FLOW IN REACTIVE UI
     

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139. fun events(): Flow
     

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140. fun events(): Flow
     scope.launch {
     …
     }
     

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141. fun events(): Flow
     scope.launch {
     events().collect { event ->
     …
     }
     }
     

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142. fun events(): Flow
     scope.launch {
     events().collect { event ->
     updateUI(event)
     }
     }
     

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143. fun events(): Flow
     scope.launch {
     events().collect { event ->
     updateUI(event)
     }
     }
     “Subscribe” to events
     

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144. fun events(): Flow
     events()
     .onEach { event -> updateUI(event) }
     .launchIn(scope)
     “Subscribe” to events
     

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145. fun events(): Flow
     events()
     .onEach { event -> updateUI(event) }
     .launchIn(scope)
     “Subscribe” to events
     

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146. MANAGING LIFETIME
     

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147. Observable
     • ……….
     • …..
     • ……..
     

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148. Observable
     • ……….
     • …..
     • ……..
     subscribe
     observable.subscribe { event ->
     updateUI(event)
     }
     

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149. Observable
     • ……….
     • …..
     • ……..
     subscribe
     SubscripIon
     observable.subscribe { event ->
     updateUI(event)
     }
     

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150. Observable
     • ……….
     • …..
     • ……..
     subscribe
     SubscripIon
     val composite = CompositeDisposable()
     composite.add(observable.subscribe { event ->
     updateUI(event)
     })
     

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151. Observable
     • ……….
     • …..
     • ……..
     subscribe
     SubscripIon
     val composite = CompositeDisposable()
     composite.add(observable.subscribe { event ->
     updateUI(event)
     })
     composite.clear()
     

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152. Flow
     • ……….
     • …..
     • ……..
     launch
     Job
     CorouIneScope
     Structured Concurrency
     

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153. Flow
     • ……….
     • …..
     • ……..
     launch
     Job
     events()
     .onEach { event -> updateUI(event) }
     .launchIn(scope)
     

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154. Flow
     • ……….
     • …..
     • ……..
     launch
     Job
     val scope = MainScope()
     events()
     .onEach { event -> updateUI(event) }
     .launchIn(scope)
     

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155. Flow
     • ……….
     • …..
     • ……..
     launch
     Job
     val scope = MainScope()
     events()
     .onEach { event -> updateUI(event) }
     .launchIn(scope)
     scope.cancel()
     

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156. Flow
     • ……….
     • …..
     • ……..
     launch
     Job
     val scope = MainScope()
     events()
     .onEach { event -> updateUI(event) }
     .launchIn(scope)
     scope.cancel()
     

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157. LiveData
     • ……….
     • …..
     • ……..
     observe
     data.observe(livecycleOwner) { event ->
     updateUI(event)
     }
     

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158. STATUS & ROADMAP
     What’s next?
     

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159. Status & Roadmap
     Flow is stable in kotlinx.corouInes version 1.3.0
     Future improvements
     ØOut-of-the box support for UI models (StateFlow / EventFlow)
     ØSharing / caching flows
     ØConcurrency / parallelism operators
     ØChunking / windowing operators
     Want more? Give us your feedback
     h2ps://github.com/Kotlin/kotlinx.corouInes/issues
     

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160. Learn more
     vKotlin Flow by example guide
     https://kotlinlang.org/docs/reference/coroutines/flow.html
     vAPI docs
     https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/
     vStories in my blog
     https://medium.com/@elizarov
     

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161. #KotlinConf
     THANK YOU
     AND
     REMEMBER
     TO VOTE
     Roman Elizarov @relizarov
     #KotlinConf
     

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