Migrating to Kotlin State & Shared Flows

Transcript

1. Mohit Sarveiya
   Migrating to Kotlin State & Shared Flows
   @heyitsmohit
   

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2. Migrating to Kotlin State & Shared Flows
   ● State & Shared Flow APIs
   
   ● Migrating from broadcast channels
   
   ● Flow Sharing Strategies
   
   ● Manage Backpressure
   

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3. Cold vs Hot Flows
   

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4. What is a cold stream?
   ● Flow completes
   
   ● Triggers same code for every new subscriber
   

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5. Cold Flows
   val flow = flowOf(1, 2, 3)
   

   .map { it + 1 }
   

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6. Cold Flows
   val flow = flowOf(1, 2, 3)
   

   .map { it + 1 }
   flow.collect {
   
   ...
   
   
   }
   
   

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7. Cold Flows
   val flow = flowOf(1, 2, 3)
   

   .map { it + 1 }
   flow.collect {
   
   ...
   
   
   }
   
   

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8. Cold Flows
   val flow = flowOf(1, 2, 3)
   

   .map { it + 1 }
   flow.collect {
   
   ...
   
   
   }
   
   

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9. Cold Flows
   val flow = flowOf(1, 2, 3)
   

   .map { it + 1 }
   flow.collect {
   
   //
   2, 3, 4
   
   }
   
   

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10. Cold Flows
    val flow = flowOf(1, 2, 3)
    

    .map { it + 1 }
    flow.collect {
    
    ...
    
    
    }
    
    flow.collect {
    
    ...
    
    
    }
    
    

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11. Cold Flows
    val flow = flowOf(1, 2, 3)
    

    .map { it + 1 }
    flow.collect {
    
    ...
    
    
    }
    
    flow.collect {
    
    ...
    
    
    }
    
    

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12. Cold Flows
    val flow = flowOf(1, 2, 3)
    

    .map { it + 1 }
    flow.collect {
    
    ...
    
    
    }
    
    flow.collect {
    
    ...
    
    
    }
    
    

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13. Cold Flows
    val flow = flowOf(1, 2, 3)
    

    .map { it + 1 }
    flow.collect {
    
    ...
    
    
    }
    
    flow.collect {
    
    //
    2, 3, 4
    
    }
    
    

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14. What is a hot stream?
    ● Never completes normally
    
    ● Exists independently of subscribers.
    

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15. Hot Streams
    State Flow Shared Flow
    

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16. State Flow
    ● Different ways to create state flows
    
    ● Emit, collect
    
    ● Conflation
    

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17. State Flow
    View View Model
    

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18. State Flow
    View View Model
    State
    

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19. State Flow
    sealed class UiState {
    

    object Loading: UiState()
    

    data class Success(…): UiState()
    
    data class Error(…): UiState()
    
    }
    

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20. State Flow
    sealed class UiState {
    

    object Loading: UiState()
    

    data class Success(…): UiState()
    
    data class Error(…): UiState()
    
    }
    

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21. State Flow
    sealed class UiState {
    

    object Loading: UiState()
    

    data class Success(…): UiState()
    
    data class Error(…): UiState()
    
    }
    

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22. State Flow
    val stateFlow = MutableStateFlow()
    
    

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23. val stateFlow = MutableStateFlow(
    
    UiState.Loading
    
    )
    
    State Flow
    

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24. State Flow
    val stateFlow = MutableStateFlow(
    . ..
    )
    
    stateFlow.emit(
    

    UIState.Success(
    ...
    )
    

    )
    
    

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25. State Flow
    val stateFlow = MutableStateFlow(
    . ..
    )
    
    stateFlow.value = UIState.Success(
    ...
    )
    

    

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26. State Flow
    val stateFlow = MutableStateFlow(
    . ..
    )
    
    stateFlow.update {
    
    UIState.Success(
    .. .
    )
    
    }
    
    

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27. State Flow
    val stateFlow = MutableStateFlow(
    . ..
    )
    
    stateFlow.collect {
    .. .
    }
    

    

    stateFlow.collect {
    .. .
    }
    Latest value is received
    

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28. State Flow Conflation
    val stateFlow = MutableStateFlow(
    . ..
    )
    
    stateFlow.value = UIState.Success(
    ...
    )
    

    

    stateFlow.value = UIState.Error(
    .. .
    ) Conflate
    

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29. State Flow Conflation
    val stateFlow = MutableStateFlow(
    . ..
    )
    
    stateFlow.value = UIState.Success(
    ...
    )
    

    

    stateFlow.value = UIState.Error(
    .. .
    )
    

    

    stateFlow.collect {
    .. .
    } Error
    

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30. State Flow
    View View Model
    State
    

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31. State Flow
    State Flow
    Flow 1 Flow 2 Flow 3
    Combined Flow
    

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32. Flow Combine
    1
    A
    1A
    Flow 1
    Flow 2
    Combined
    

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33. Flow Combine
    1 2
    A
    1A 2A
    Flow 1
    Flow 2
    Combined
    

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34. Flow Combine
    1 2
    A
    1A 2A
    Flow 1
    Flow 2
    Combined
    B
    2B
    

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35. val stateFlow = MutableStateFlow(UIState.Loading)
    

    flow1.combine(flow2) { a, b
    - >
    
    
    }
    
    State Flow
    

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36. val stateFlow = MutableStateFlow(UIState.Loading)
    

    flow1.combine(flow2) { a, b
    - >
    
    
    combineItems(a, b)
    
    }
    
    State Flow
    

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37. val stateFlow = MutableStateFlow(UIState.Loading)
    

    flow1.combine(flow2) { a, b
    - >
    
    
    combineItems(a, b)
    
    }.collect {
    
    stateFlow.emit(it)
    
    }
    
    State Flow
    

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38. Flow Marbles
    

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39. State Flow
    View View Model
    State
    

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40. Cash App - Molecule
    

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41. Molecule
    Purpose
    

    Build a StateFlow using Jetpack Compose
    

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42. Approach
    View Molecule
    Presenter
    

    (Composable)
    Events
    

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43. Presenter
    @Composable
    

    fun Presenter(eventFlow: Flow): UiState {
    

    val event by eventFlow.collectAsState(null)
    
    return if (event
    = =
    null) {
    
    UiState.Loading
    
    } else {
    
    UiState.Data(…)
    
    }
    
    }
    

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44. Presenter
    @Composable
    

    fun Presenter(eventFlow: Flow): UiState {
    

    val event by eventFlow.collectAsState(null)
    
    return if (event
    = =
    null) {
    
    UiState.Loading
    
    } else {
    
    UiState.Data(…)
    
    }
    
    }
    

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45. Presenter
    @Composable
    

    fun Presenter(eventFlow: Flow): UiState {
    

    val event by eventFlow.collectAsState(null)
    
    return if (event
    = =
    null) {
    
    UiState.Loading
    
    } else {
    
    UiState.Data(…)
    
    }
    
    }
    

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46. Setup
    Molecule
    Presenter
    

    (Composable)
    

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47. Setup
    Molecule
    Presenter
    

    (Composable)
    StateFlow
    

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48. Launching Molecule
    val scope = CoroutineScope(Dispatchers.Main)
    
    val models: StateFlow = scope.launchMolecule {
    
    userPresenter(postsFlow, likesFlow)
    
    }
    
    

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49. Launching Molecule
    val flow: StateFlow = scope.launchMolecule {
    
    presenter(eventFlow)
    
    }
    
    

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50. Setup
    View Molecule
    

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51. Setup
    View View Model
    

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52. View Model
    class MyViewModel: ViewModel() {
    

    

    val stateFlow = moleculeScope.launchMolecule {
    
    val event by eventFlow.collectAsState(null)
    
    return if (event
    ==
    null) {
    
    UiState.Loading
    
    } else {
    
    UiState.Success(…)
    
    }
    
    }
    
    

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53. Learn More
    https:
    /
    /
    youtu.be/rUpZSZedoHI
    

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54. State Flow
    ● Different ways to create state flows
    
    ● Emit, collect
    

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55. Shared Flow
    

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56. Shared Flow
    

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57. Shared Flow
    Consumer 1 Consumer 2
    

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58. Shared Flow
    Consumer 1 Consumer 2
    

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59. Shared Flow
    Consumer 1 Consumer 2
    Event Event
    

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60. Shared Flow
    Consumer 1 Consumer 2
    Replay Replay
    

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61. 

    Shared Flow
    Buffer
    

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62. Shared Flow
    val flow = MutableSharedFlow()
    
    

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63. Shared Flow
    val flow = MutableSharedFlow()
    
    launch { flow.collect { } }
    
    

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64. Shared Flow
    val flow = MutableSharedFlow()
    
    

    launch { flow.emit("Event 1") }
    
    launch { flow.collect { } }
    
    

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65. Shared Flow
    val flow = MutableSharedFlow()
    
    

    launch { flow.emit("Event 1") }
    
    launch { flow.collect { } }
    Event 1
    

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66. Shared Flow
    val flow = MutableSharedFlow()
    
    

    launch { flow.emit("Event 1”) }
    
    launch { delay(2000); flow.collect { } }
    
    

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67. Shared Flow
    val flow = MutableSharedFlow()
    
    

    launch { delay(2000); flow.collect { } }
    
    launch { flow.emit("Event 1”) }
    
    

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68. Shared Flow
    val flow = MutableSharedFlow()
    
    

    launch { delay(2000); flow.collect { } }
    
    launch { flow.emit("Event 1”) }
    
    

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69. Shared Flow
    val flow = MutableSharedFlow()
    
    

    launch { delay(2000); flow.collect { } }
    No value is received
    launch { flow.emit("Event 1”) }
    
    

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70. Shared Flow
    val flow = MutableSharedFlow(replay = 1)
    
    

    launch { flow.emit("Event 1”) }
    
    launch { delay(2000); flow.collect { } }
    
    

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71. Shared Flow
    val flow = MutableSharedFlow(replay = 1)
    
    

    launch { delay(2000); flow.collect { } }
    
    launch { flow.emit("Event 1”) }
    
    Event 1
    

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72. Shared Flow
    val flow = MutableSharedFlow(replay = 1)
    
    Shared Flow does not complete normally
    launch { flow.collect { } }
    
    

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73. Cold Flows
    val flow = flowOf(1, 2, 3)
    

    flow
    
    .onCompletion { }
    
    .collect {
    
    ...
    
    
    }
    
    Flow completes normally
    

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74. Shared Flow
    val flow = MutableSharedFlow(replay = 1)
    
    val job = launch { flow.collect { } }
    
    job.cancel()
    
    

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75. Shared Flow
    val flow = MutableSharedFlow(replay = 1)
    
    val job = launch { flow.onCompletion { }.collect { } }
    
    job.cancel()
    
    Flow completes exceptionally
    

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76. Shared Flow
    ● Setup
    
    ● Replay and emit
    
    ● Cancellation
    

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77. Broadcast Channel vs Shared Flow
    

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78. Broadcast
    Channel
    Shared Flow
    Channel APIs
    Replay
    Buffer
    Closed
    

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79. val channel = BroadcastChannel(10)
    
    Broadcast Channel
    

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80. val channel = BroadcastChannel(10)
    
    channel.send(
    ...
    )
    Broadcast Channel
    

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81. val channel = BroadcastChannel(10)
    
    channel.send(
    ...
    )
    
    channel.close()
    Broadcast Channel
    

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82. val flow = MutableSharedFlow()
    
    flow.emit(
    ...
    )
    
    Shared Flow
    

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83. Broadcast
    Channel
    Shared Flow
    Channel APIs
    Replay
    Buffer
    Closed
    

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84. val flow = MutableSharedFlow(replay = 2)
    
    flow.emit(
    ...
    )
    
    Shared Flow
    

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85. Broadcast
    Channel
    Shared Flow
    Channel APIs
    Replay
    Buffer
    Closed
    

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86. val channel = BroadcastChannel(capacity = 10)
    
    channel.send(
    ...
    )
    Broadcast Channel
    

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87. val flow = MutableSharedFlow(
    
    replay = 2,
    
    extraBufferCapacity = 10
    
    )
    
    flow.emit(
    ...
    )
    
    Shared Flow
    

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88. Broadcast
    Channel
    Shared Flow
    Channel APIs
    Replay
    Buffer
    Closed
    

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89. Broadcast Channel Shared Flow
    

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90. val channel = BroadcastChannel(capacity)
    val flow = MutableSharedFlow(extraBufferCapacity)
    

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91. channel.send(
    ...
    )
    channel.trySend(
    ..
    .
    )
    flow.emit(
    ...
    )
    flow.tryEmit(
    ...
    )
    

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92. Broadcast
    Channel
    Shared Flow
    Channel APIs
    Replay
    Buffer
    Closed
    

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93. Flow Sharing Strategies
    

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94. Cold Flow Hot Flow
    Convert
    

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95. State Flow
    State Flow
    Flow 1 Flow 2 Flow 3
    Combined Flow
    

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96. Creating State & Shared Flows
    • shareIn
    
    • stateIn
    

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97. Sharing Policies
    • While Subscribed
    
    • Eagerly
    
    • Lazily
    

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98. Sharing Policies
    flow.shareIn(
    
    )
    
    

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99. Sharing Policies
    flow.shareIn(
    
    externalScope,
    
    )
    
    

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100. Sharing Policies
     flow.shareIn(
     
     externalScope,
     
     replay = 1,
     
     )
     
     

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101. Sharing Policies
     flow.shareIn(
     
     externalScope,
     
     replay = 1,
     
     started = SharingStarted.WhileSubscribed()
     
     )
     
     

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102. Sharing Policies
     val sharedFlow = flow.shareIn(
     
     externalScope,
     
     replay = 1,
     
     started = SharingStarted.WhileSubscribed()
     
     )
     
     

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103. Properties
     • Active as long as external scope is alive
     
     • Remains as long as there are collectors.
     

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104. Properties
     Active as long as external scope is alive
     externalScope.launch {
     
     sharedFlow.collect { }
     
     }
     

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105. flow.shareIn(
     
     externalScope,
     
     replay = 1,
     
     started = SharingStarted.WhileSubscribed()
     
     )
     
     Properties
     Active as long as external scope is alive
     

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106. Properties
     Active as long as external scope is alive
     externalScope.cancel()
     externalScope.launch {
     
     sharedFlow.collect { }
     
     }
     

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107. Properties
     Active as long as external scope is alive
     externalScope.cancel()
     externalScope.launch {
     
     sharedFlow.collect { }
     
     }
     Complete Exceptionally
     

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108. Properties
     Active as long as there are collectors.
     

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109. Properties
     Active as long as there are collectors.
     val sharedFlow = flow.onCompletion { }.shareIn(…)
     

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110. Properties
     Active as long as there are collectors.
     val sharedFlow = flow.onCompletion { }.shareIn(…)
     val job = launch { sharedFlow.onCompletion { }.collect { } }
     

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111. Properties
     Active as long as there are collectors.
     val sharedFlow = flow.onCompletion { }.shareIn(…)
     val job = launch { sharedFlow.onCompletion { }.collect { } }
     job.cancel()
     

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112. Properties
     Active as long as there are collectors.
     val sharedFlow = flow.onCompletion { }.shareIn(…)
     val job = launch { sharedFlow.onCompletion { }.collect { } }
     job.cancel()
     

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113. Properties
     Active as long as there are collectors.
     val sharedFlow = flow.onCompletion { }.shareIn(…)
     val job = launch { sharedFlow.onCompletion { }.collect { } }
     job.cancel()
     

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114. Properties
     Active as long as there are collectors.
     val sharedFlow = flow.onCompletion { }.shareIn(…)
     val job1 = launch { sharedFlow.collect { } }
     val job2 = launch { sharedFlow.collect { } }
     

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115. Properties
     Active as long as there are collectors.
     val sharedFlow = flow.onCompletion { }.shareIn(…)
     job1.cancel()
     val job2 = launch { sharedFlow.collect { } }
     

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116. Properties
     Active as long as there are collectors.
     val sharedFlow = flow.onCompletion { }.shareIn(…)
     job1.cancel()
     val job2 = launch { sharedFlow.collect { } }
     Remain Active
     

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117. Properties
     • Active as long as external scope is alive
     
     • Active as long as there are collectors.
     

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118. Sharing Policies
     • While Subscribed
     
     • Eagerly
     
     • Lazily
     

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119. Eagerly
     flow.shareIn(
     
     externalScope,
     
     replay = 1,
     
     started = SharingStarted.Eagerly()
     
     )
     
     

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120. Eagerly
     Start producer eagerly
     
     flow
     
     .onStart { println("ON START") }
     
     .shareIn(
     ...
     started = SharingStarted.Eagerly)
     
     

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121. Eagerly
     Start producer eagerly
     
     flow
     
     .onStart { println("ON START") }
     
     .shareIn(
     ...
     started = SharingStarted.Eagerly)
     
     

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122. Eagerly
     Start producer eagerly
     
     flow
     
     .onStart { println("ON START") }
     
     .shareIn(
     ...
     started = SharingStarted.Eagerly)
     
     //
     ON START
     

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123. Eagerly
     Start producer eagerly
     
     

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124. Sharing Policies
     • While Subscribed
     
     • Eagerly
     
     • Lazily
     

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125. Lazily
     Start sharing after the first subscriber appears
     
     

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126. Lazily
     flow.shareIn(
     
     externalScope,
     
     replay = 1,
     
     started = SharingStarted.Lazily
     
     )
     
     

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127. Lazily
     flow
     
     .onStart { println("ON START") }
     
     .shareIn(…,started = SharingStarted.Lazily)
     
     

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128. Lazily
     flow
     
     .onStart { println("ON START") }
     
     .shareIn(…,started = SharingStarted.Lazily)
     
     launch { sharedFlow.collect { } }
     

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129. Lazily
     flow
     
     .onStart { println("ON START") }
     
     .shareIn(…,started = SharingStarted.Lazily)
     
     launch { sharedFlow.collect { } }
     //
     ON START
     

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130. Lazily
     Start sharing after the first subscriber appears
     
     

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131. Sharing Policies
     • While Subscribed
     
     • Active while there are active subscribers.
     

     • Eagerly
     
     • Start producer eagerly and never stop
     

     • Lazily
     
     • Start after the first subscriber appears and never stop
     
     

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132. Manage Backpressure
     

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133. 

     Shared Flow
     Buffer
     

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134. 

     Shared Flow
     Producer Consumer
     

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135. 

     Shared Flow
     Producer Consumer
     Generating events fast
     

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136. 

     Shared Flow
     Producer Consumer
     Listening to events
     

     with delay
     

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137. 

     Shared Flow
     Producer Consumer
     

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138. 

     Shared Flow
     Producer Consumer
     

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139. 

     Shared Flow
     Producer Consumer
     What happens when it is full?
     

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140. Buffering Overflow Strategies
     • Suspend
     
     • Drop oldest
     
     • Drop latest
     

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141. 

     Shared Flow
     Producer Consumer
     Suspend
     

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142. Buffering Overflow Strategies
     val flow = MutableSharedFlow(
     
     extraBufferCapacity = 2,
     
     onBufferOverflow = BufferOverflow.SUSPEND
     
     )
     Buffer + Replay Count
     

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143. Buffering Overflow Strategies
     val flow = MutableSharedFlow(
     
     extraBufferCapacity = 2,
     
     onBufferOverflow = BufferOverflow.SUSPEND
     
     )
     

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144. Buffering Overflow Strategies
     launch {
     
     flow.emit("Event 1")
     
     flow.emit("Event 2")
     
     flow.emit("Event 3")
     
     }
     
     Suspend
     

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145. Buffering Overflow Strategies
     • Suspend
     
     • Drop oldest
     
     • Drop latest
     

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146. 

     Shared Flow
     Producer Consumer
     Drop Oldest
     

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147. 

     Shared Flow
     Producer Consumer
     Drop latest
     

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148. Buffering Overflow Strategies
     • Suspend
     
     • Drop oldest
     
     • Drop latest
     

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149. Kotlin State & Shared Flows in Action
     ● State & Shared Flow APIs
     
     ● Migrating from broadcast channels
     
     ● Flow Sharing Strategies
     
     ● Manage Backpressure
     

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150. Thank You!
     www.codingwithmohit.com
     @heyitsmohit
     

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