Kotlin/Native Concurrency for Mobile Multiplatform

If you're going to build shared Kotlin code with Kotlin/Native, you'll need to understand the concurrency and state model. In this talk, I give you the basics (and a bit more), plus next steps.

Transcript

1. Kotlin Native Concurrency
   Kevin Galligan
   

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4. Copenhagen
   Denmark
   KOTLIN NATIVE
   CONCURRENCY
   EXPLAINED
   KEVIN GALLIGAN
   @kpgalligan
   

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6. Intro
   

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7. What is Kotlin Multiplatform?
   

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8. kot·lin mul·ti·plat·form
   /ˌkätˈlin məltiˈplatfôrm,ˌkätˈlin məltīˈplatfôrm/
   noun
   noun: kotlin multiplatform
   1.optional, natively-integrated, open-source, code sharing platform, based on the popular,
   modern language kotlin. facilitates non-ui logic availability on many platforms.
   

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9. modern
   

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10. Saner Concurrency?
    concurrency is hard
    

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12. Kotlin Native model is controversial
    lots of disagreement
    

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13. JVM model is controversial
    many other platforms don’t let you do that
    

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14. JVM model is controversial
    many other platforms don’t let you do that
    

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15. JVM model is controversial
    many other platforms don’t let you do that
    

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16. You need to understand it
    libraries won’t hide all details
    

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17. important to relax
    

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18. Kotlin Native Rules
    

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19. 1) Mutable State = one thread
    thread confined
    

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20. 2) Immutable state = many threads
    no changes mean no problems
    

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21. kevin@touchlab.co
    @kpgalligan
    
    

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22. Internalize The Rules
    mutable = 1/immutable = many
    

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23. object BigGlobalState {
    var statusString = ""
    }
    

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24. Don’t manage concurrent state
    no “synchronized”, “volatile”, etc
    

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25. JVM Trusts You
    native does not
    

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26. Runtime Verification
    

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27. Do Things Differently
    not “losing” anything
    

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28. (jvm strict mode would be nice)
    

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29. You can break rules too
    just not a great idea
    

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30. Stages of grief
    skip ahead to acceptance
    

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31. Core Concepts
    

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32. Worker
    kn concurrency queue
    

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33. Similar to ExecutorService
    Handler/MessageQueue/Looper on Android
    

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34. Worker
    kn concurrency queue
    

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35. Mutability & State
    

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36. 1) Mutable State = one thread
    simple state is simple
    

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37. class SimpleState {
    private var i = 0
    val currVal: Int
    get() = i
    fun doStuff(arg: Int) {
    i += arg
    }
    }
    fun main(){
    val s = SimpleState()
    s.doStuff(22)
    s.doStuff(33)
    println("My val is ${s.currVal}")
    }
    

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38. 1) Mutable State = one thread
    you can transfer
    

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39. 1) Mutable State = one thread
    you can transfer
    

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40. 2) Immutable state = many threads
    not source immutable
    

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41. data class SomeData(val s:String, val i:Int)
    Mutable state?
    

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42. freeze()
    runtime immutability designation
    

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43. public fun T.freeze(): T {...}
    

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44. Freeze
    • Recursively freezes everything
    • One-way (no “unfreeze”)
    • Data may be shared between threads
    Int
    String
    Float
    MoreData
    val strData
    var width
    SomeData
    val moreData
    var count
    

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45. Int
    String Float
    Float
    String
    Int
    MoreData
    val strData
    var width
    SomeData
    val moreData
    var count
    

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46. Crossing Threads
    

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47. fun background(block:()->Unit)
    

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48. fun background(block:()->Unit)
    block.freeze()
    

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49. fun printStuffBg() {
    val someData = SomeData("Hello ", 2)
    background {
    println(someData)
    }
    }
    

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50. fun printStuffBg() {
    val someData = SomeData("Hello ", 2)
    background {
    println(someData)
    }
    }
    

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51. fun printStuffBg() {
    val someData = SomeData("Hello ", 2)
    background {
    println(someData)
    }
    }
    

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52. fun printStuffBg() {
    val someData = SomeData("Hello ", 2)
    background {
    println(someData)
    }
    }
    

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53. class CounterModel {
    var count = 0
    fun countClicked() {
    count++
    }
    }
    

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54. class CounterModel {
    var count = 0
    fun countClicked() {
    count++
    background {
    saveToDb(count)
    }
    }
    }
    

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55. class CounterModel {
    var count = 0
    fun countClicked() {
    count++
    background {
    saveToDb(count)
    }
    }
    }
    

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56. class CounterModel {
    var count = 0
    fun countClicked() {
    count++
    background {
    saveToDb(count)
    }
    }
    }
    

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57. class CounterModel {
    var count = 0
    fun countClicked() {
    count++
    background {
    saveToDb(count)
    }
    }
    }
    

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58. class CounterModel {
    var count = 0
    fun countClicked() {
    count++
    background {
    saveToDb(count)
    }
    }
    }
    InvalidMutabilityException: mutation attempt of frozen
    sample.CounterModel
    

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59. Functions Have State
    careful what you capture
    

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60. class CounterModel {
    var count = 0
    fun countClicked() {
    count++
    save(count)
    }
    private fun save(count:Int) = background {
    saveToDb(count)
    }
    }
    

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61. Global State
    some special rules
    

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62. var someData = SomeData("a", 1)
    

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63. var someData = SomeData("a", 1)
    object GlobalObject {
    var data = SomeData("b", 2)
    }
    

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64. @ThreadLocal
    var someData = SomeData("a", 1)
    @ThreadLocal
    object GlobalObject {
    var data = SomeData("b", 2)
    }
    

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65. @SharedImmutable
    var someData = SomeData("a", 1)
    @ThreadLocal
    object GlobalObject {
    var data = SomeData("b", 2)
    }
    

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66. var someData:SomeData
    set(v:SomeData)…
    get():SomeData = …
    

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67. Why Special Rules?
    you can access it from anywhere
    

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68. Debugging Issues
    

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69. InvalidMutabilityException
    changing frozen state
    

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70. Why is this frozen()
    common question
    

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71. class CounterModel {
    var count = 0
    fun countClicked() {
    count++
    background {
    saveToDb(count)
    }
    }
    }
    

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72. ensureNeverFrozen()
    

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73. class CounterModel {
    init {
    ensureNeverFrozen()
    }
    var count = 0
    fun countClicked() {
    count++
    background {
    saveToDb(count)
    }
    }
    }
    

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74. Int
    String Float
    Int
    MoreData
    val strData
    var width
    SomeData
    val moreData
    var count
    

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75. Rethinking architecture
    intentional mutability
    

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76. Main Thread
    Other Thread (maybe?)
    Data
    Data
    Data
    Content
    Model
    iOS View
    Model
    

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77. This can get weird
    subtle things
    

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78. class BreedModel(
    private val viewUpdate: (ItemDataSummary) -> Unit,
    private val errorUpdate: (String) -> Unit
    ) : BaseModel() {
    private val dbHelper: DatabaseHelper by inject()
    private val settings: Settings by inject()
    init {
    ensureNeverFrozen()
    scope.launch {
    dbHelper.selectAllItems().asFlow()
    .map { q ->
    val itemList = q.executeAsList()
    ItemDataSummary(/* etc */)
    }
    .flowOn(Dispatchers.Default)
    .collect { summary ->
    viewUpdate(summary)
    }
    }
    }
    

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79. class BreedModel(
    private val viewUpdate: (ItemDataSummary) -> Unit,
    private val errorUpdate: (String) -> Unit
    ) : BaseModel() {
    private val dbHelper: DatabaseHelper by inject()
    private val settings: Settings by inject()
    init {
    ensureNeverFrozen()
    scope.launch {
    dbHelper.selectAllItems().asFlow()
    .map { q ->
    val itemList = q.executeAsList()
    settings.set("Hey", "Bad idea")
    ItemDataSummary(/* etc */)
    }
    .flowOn(Dispatchers.Default)
    .collect { summary ->
    viewUpdate(summary)
    }
    }
    }
    

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80. class BreedModel(
    private val viewUpdate: (ItemDataSummary) -> Unit,
    private val errorUpdate: (String) -> Unit
    ) : BaseModel() {
    private val dbHelper: DatabaseHelper by inject()
    private val settings: Settings by inject()
    init {
    ensureNeverFrozen()
    scope.launch {
    dbHelper.selectAllItems().asFlow()
    .map { q ->
    val itemList = q.executeAsList()
    settings.set("Hey", "Bad idea")
    ItemDataSummary(/* etc */)
    }
    .flowOn(Dispatchers.Default)
    .collect { summary ->
    viewUpdate(summary)
    }
    }
    }
    

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82. This may be frustrating
    but should make you think
    

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83. class BreedModel(
    private val viewUpdate: (ItemDataSummary) -> Unit,
    private val errorUpdate: (String) -> Unit
    ) : BaseModel() {
    private val dbHelper: DatabaseHelper by inject()
    private val settings: Settings by inject()
    init {
    ensureNeverFrozen()
    scope.launch {
    dbHelper.selectAllItems().asFlow()
    .map { q ->
    val itemList = q.executeAsList()
    ItemDataSummary(/* etc */)
    }
    .flowOn(Dispatchers.Default)
    .collect { summary ->
    viewUpdate(summary)
    }
    }
    }
    

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84. Concurrent Mutability
    

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85. Atomics
    mutating frozen state
    

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86. class CounterModel {
    val count = AtomicInt(0)
    fun countClicked() {
    count.value++
    background {
    saveToDb(count.value)
    }
    }
    }
    

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87. class CounterModel {
    val count = AtomicInt(0)
    fun countClicked() {
    count.value++
    background {
    saveToDb(count.value)
    }
    }
    }
    

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88. val atom = AtomicReference(SomeData(“abc", 123))
    

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89. class SomeModel {
    val atom = AtomicReference(SomeData(“abc", 123))
    fun update(sd:SomeData){
    atom.value = sd
    }
    init {
    freeze()
    }
    }
    

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90. Don’t use too many atomics
    AtomicRef can leak memory
    

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91. Worker dispatched state
    mutable state thread local
    

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92. State Thread
    IsolateState
    ???
    State Container
    Data
    

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93. State Thread
    IsolateState
    ???
    State Container
    ???
    Data
    

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94. open class IsolateState
    

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95. open class IsolateState
    constructor(producer: () -> T)
    

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96. open class IsolateState
    constructor(producer: () -> T)
    fun access(block: (T) -> R): R
    

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97. IsoArrayDeque
    IsoMutableCollection
    IsoMutableIterator
    IsoMutableList
    IsoMutableMap
    IsoMutableSet
    

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98. fun IsoMutableMap.checkFor2(first: K, second: K, ins: V)=
    access { map ->
    if (map.containsKey(first))
    map.put(first, ins)
    else
    map.put(second, ins)
    }
    

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99. Objective-C/C/C++
    just do what you want!
    

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100. Multiplatform & State
     

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101. Multiplatform State
     

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102. freeze() is native only
     

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103. expect fun T.freeze(): T
     expect val T.isFrozen: Boolean
     expect fun Any.ensureNeverFrozen()
     expect annotation class Throws
     

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104. freeze() on iOS
     threading rules apply
     

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105. Swift/Objc can’t be frozen
     be aware of concurrency issues
     

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106. Concurrency Libraries
     

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107. Official Native kotlinx.coroutines
     single-threaded
     

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108. ktor on native
     called on main thread
     

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109. multithreaded coroutines!
     

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110. multithreaded coroutines?
     

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111. Switching Threads
     use withContext (or equivalents)
     data passed in (or captured) is frozen
     data returned is frozen
     

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112. fun printStuffBg() {
     val someData = SomeData("Hello ", 2)
     background {
     println(someData)
     }
     }
     

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113. suspend fun printStuffBg() {
     val someData = SomeData("Hello ", 2)
     withContext(workerDispatcher) {
     println(someData)
     }
     }
     

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114. suspend fun printStuffBg() {
     val someData = SomeData("Hello ", 2)
     withContext(workerDispatcher) {
     println(someData)
     }
     }
     

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115. suspend fun printStuffForeground() {
     val someData = SomeData("Hello ", 2)
     val someMoreData = withContext(workerDispatcher) {
     SomeData("${someData.s}, Dogs!", someData.i + 1)
     }
     println(someMoreData)
     println("I am frozen? ${someMoreData.isFrozen()}")
     }
     

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116. suspend fun printStuffForeground() {
     val someData = SomeData("Hello ", 2)
     val someMoreData = withContext(workerDispatcher) {
     SomeData("${someData.s}, Dogs!", someData.i + 1)
     }
     println(someMoreData)
     println("I am frozen? ${someMoreData.isFrozen()}")
     }
     

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117. suspend fun printStuffForeground() {
     val someData = SomeData("Hello ", 2)
     val someMoreData = withContext(workerDispatcher) {
     SomeData("${someData.s}, Dogs!", someData.i + 1)
     }
     println(someMoreData)
     println("I am frozen? ${someMoreData.isFrozen()}")
     }
     

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118. SomeData(s=Hello , Dogs!, i=3)
     I am frozen? true
     

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119. Flow, Channel, etc
     all freezable
     

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120. Main Thread
     Other Thread (maybe?)
     Data
     Data
     Data
     Content
     Model
     iOS View
     Model
     

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121. iOS Suspend Functions
     new for 1.4!
     

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122. Stately
     

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123. Stately (Probably) deprecated
     

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124. Stately
     v1! (ish)
     

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125. Modules
     • stately-common
     • stately-collections
     • stately-concurrency
     

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126. Modules
     • stately-isolate
     • stately-iso-collections
     • alpha. Feedback please!
     

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127. github.com/Autodesk/coroutineworker
     

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128. github.com/Badoo/reaktive
     

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129. Next Steps
     

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130. go.touchlab.co/kotlinlang
     

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131. go.touchlab.co/kndevto
     

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132. go.touchlab.co/knthreads
     

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133. JetBrains Links
     • https://github.com/JetBrains/kotlin-native/blob/
     master/IMMUTABILITY.md
     • https://github.com/JetBrains/kotlin-native/blob/
     master/CONCURRENCY.md
     • https://www.youtube.com/watch?v=nw6YTfEyfO0
     

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134. github.com/touchlab/KaMPKit
     

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135. Thanks!
     @kpgalligan
     touchlab.co
     

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