Migrating to Kotlin State & Shared Flows

Transcript

1. Mohit Sarveiya Migrating to Kotlin State & Shared Flows @heyitsmohit
2. Migrating to Kotlin State & Shared Flows • State & Shared Flow APIs • Migrating from broadcast channels • Flow Sharing Strategies • Manage Backpressure
3. Cold vs Hot Flows
4. What is a cold stream? • Flow completes • Triggers same code for every new subscriber
5. Cold Flows val flow = flowOf(1, 2, 3) 
 .map { it + 1 }
6. Cold Flows val flow = flowOf(1, 2, 3) 
 .map { it + 1 } flow.collect { ... }
7. Cold Flows val flow = flowOf(1, 2, 3) 
 .map { it + 1 } flow.collect { ... }
8. Cold Flows val flow = flowOf(1, 2, 3) 
 .map { it + 1 } flow.collect { ... }
9. Cold Flows val flow = flowOf(1, 2, 3) 
 .map { it + 1 } flow.collect { // 2, 3, 4 }
10. Cold Flows val flow = flowOf(1, 2, 3) 
 .map { it + 1 } flow.collect { ... } flow.collect { ... }
11. Cold Flows val flow = flowOf(1, 2, 3) 
 .map { it + 1 } flow.collect { ... } flow.collect { ... }
12. Cold Flows val flow = flowOf(1, 2, 3) 
 .map { it + 1 } flow.collect { ... } flow.collect { ... }
13. Cold Flows val flow = flowOf(1, 2, 3) 
 .map { it + 1 } flow.collect { ... } flow.collect { // 2, 3, 4 }
14. What is a hot stream? • Never completes normally • Exists independently of subscribers.
15. Hot Streams State Flow Shared Flow
16. State Flow • Different ways to create state flows • Emit, collect • Conflation
17. State Flow View View Model
18. State Flow View View Model State
19. State Flow sealed class UiState { 
 object Loading: UiState() 
 data class Success(…): UiState() data class Error(…): UiState() }
20. State Flow sealed class UiState { 
 object Loading: UiState() 
 data class Success(…): UiState() data class Error(…): UiState() }
21. State Flow sealed class UiState { 
 object Loading: UiState() 
 data class Success(…): UiState() data class Error(…): UiState() }
22. State Flow val stateFlow = MutableStateFlow()
23. val stateFlow = MutableStateFlow( UiState.Loading ) State Flow
24. State Flow val stateFlow = MutableStateFlow( . .. ) stateFlow.emit( 
 UIState.Success( ... ) 
 )
25. State Flow val stateFlow = MutableStateFlow( . .. ) stateFlow.value = UIState.Success( ... ) 

26. State Flow val stateFlow = MutableStateFlow( . .. ) stateFlow.update { UIState.Success( .. . ) }
27. State Flow val stateFlow = MutableStateFlow( . .. ) stateFlow.collect { .. . } 
 
 stateFlow.collect { .. . } Latest value is received
28. State Flow Conflation val stateFlow = MutableStateFlow( . .. ) stateFlow.value = UIState.Success( ... ) 
 
 stateFlow.value = UIState.Error( .. . ) Conflate
29. State Flow Conflation val stateFlow = MutableStateFlow( . .. ) stateFlow.value = UIState.Success( ... ) 
 
 stateFlow.value = UIState.Error( .. . ) 
 
 stateFlow.collect { .. . } Error
30. State Flow View View Model State
31. State Flow State Flow Flow 1 Flow 2 Flow 3 Combined Flow
32. Flow Combine 1 A 1A Flow 1 Flow 2 Combined
33. Flow Combine 1 2 A 1A 2A Flow 1 Flow 2 Combined
34. Flow Combine 1 2 A 1A 2A Flow 1 Flow 2 Combined B 2B
35. val stateFlow = MutableStateFlow(UIState.Loading) 
 flow1.combine(flow2) { a, b - > } State Flow
36. val stateFlow = MutableStateFlow(UIState.Loading) 
 flow1.combine(flow2) { a, b - > combineItems(a, b) } State Flow
37. val stateFlow = MutableStateFlow(UIState.Loading) 
 flow1.combine(flow2) { a, b - > combineItems(a, b) }.collect { stateFlow.emit(it) } State Flow
38. Flow Marbles
39. State Flow View View Model State
40. Cash App - Molecule
41. Molecule Purpose 
 Build a StateFlow using Jetpack Compose
42. Approach View Molecule Presenter 
 (Composable) Events
43. Presenter @Composable 
 fun Presenter(eventFlow: Flow<Event>): UiState { 
 val event by eventFlow.collectAsState(null) return if (event = = null) { UiState.Loading } else { UiState.Data(…) } }
44. Presenter @Composable 
 fun Presenter(eventFlow: Flow<Event>): UiState { 
 val event by eventFlow.collectAsState(null) return if (event = = null) { UiState.Loading } else { UiState.Data(…) } }
45. Presenter @Composable 
 fun Presenter(eventFlow: Flow<Event>): UiState { 
 val event by eventFlow.collectAsState(null) return if (event = = null) { UiState.Loading } else { UiState.Data(…) } }
46. Setup Molecule Presenter 
 (Composable)
47. Setup Molecule Presenter 
 (Composable) StateFlow
48. Launching Molecule val scope = CoroutineScope(Dispatchers.Main) val models: StateFlow<UsersModel> = scope.launchMolecule { userPresenter(postsFlow, likesFlow) }
49. Launching Molecule val flow: StateFlow<UiState> = scope.launchMolecule { presenter(eventFlow) }
50. Setup View Molecule
51. Setup View View Model
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(…) } }
53. Learn More https: / / youtu.be/rUpZSZedoHI
54. State Flow • Different ways to create state flows • Emit, collect
55. Shared Flow
56. Shared Flow
57. Shared Flow Consumer 1 Consumer 2
58. Shared Flow Consumer 1 Consumer 2
59. Shared Flow Consumer 1 Consumer 2 Event Event
60. Shared Flow Consumer 1 Consumer 2 Replay Replay
61. 
 Shared Flow Buffer
62. Shared Flow val flow = MutableSharedFlow<String>()
63. Shared Flow val flow = MutableSharedFlow<String>() launch { flow.collect { } }
64. Shared Flow val flow = MutableSharedFlow<String>() 
 launch { flow.emit("Event 1") } launch { flow.collect { } }
65. Shared Flow val flow = MutableSharedFlow<String>() 
 launch { flow.emit("Event 1") } launch { flow.collect { } } Event 1
66. Shared Flow val flow = MutableSharedFlow<String>() 
 launch { flow.emit("Event 1”) } launch { delay(2000); flow.collect { } }
67. Shared Flow val flow = MutableSharedFlow<String>() 
 launch { delay(2000); flow.collect { } } launch { flow.emit("Event 1”) }
68. Shared Flow val flow = MutableSharedFlow<String>() 
 launch { delay(2000); flow.collect { } } launch { flow.emit("Event 1”) }
69. Shared Flow val flow = MutableSharedFlow<String>() 
 launch { delay(2000); flow.collect { } } No value is received launch { flow.emit("Event 1”) }
70. Shared Flow val flow = MutableSharedFlow<String>(replay = 1) 
 launch { flow.emit("Event 1”) } launch { delay(2000); flow.collect { } }
71. Shared Flow val flow = MutableSharedFlow<String>(replay = 1) 
 launch { delay(2000); flow.collect { } } launch { flow.emit("Event 1”) } Event 1
72. Shared Flow val flow = MutableSharedFlow<String>(replay = 1) Shared Flow does not complete normally launch { flow.collect { } }
73. Cold Flows val flow = flowOf(1, 2, 3) 
 flow .onCompletion { } .collect { ... } Flow completes normally
74. Shared Flow val flow = MutableSharedFlow<String>(replay = 1) val job = launch { flow.collect { } } job.cancel()
75. Shared Flow val flow = MutableSharedFlow<String>(replay = 1) val job = launch { flow.onCompletion { }.collect { } } job.cancel() Flow completes exceptionally
76. Shared Flow • Setup • Replay and emit • Cancellation
77. Broadcast Channel vs Shared Flow
78. Broadcast Channel Shared Flow Channel APIs Replay Buffer Closed
79. val channel = BroadcastChannel<Int>(10) Broadcast Channel
80. val channel = BroadcastChannel<Int>(10) channel.send( ... ) Broadcast Channel
81. val channel = BroadcastChannel<Int>(10) channel.send( ... ) channel.close() Broadcast Channel
82. val flow = MutableSharedFlow() flow.emit( ... ) Shared Flow
83. Broadcast Channel Shared Flow Channel APIs Replay Buffer Closed
84. val flow = MutableSharedFlow(replay = 2) flow.emit( ... ) Shared Flow
85. Broadcast Channel Shared Flow Channel APIs Replay Buffer Closed
86. val channel = BroadcastChannel<Int>(capacity = 10) channel.send( ... ) Broadcast Channel
87. val flow = MutableSharedFlow( replay = 2, extraBufferCapacity = 10 ) flow.emit( ... ) Shared Flow
88. Broadcast Channel Shared Flow Channel APIs Replay Buffer Closed
89. Broadcast Channel Shared Flow
90. val channel = BroadcastChannel<Int>(capacity) val flow = MutableSharedFlow<String>(extraBufferCapacity)
91. channel.send( ... ) channel.trySend( .. . ) flow.emit( ... ) flow.tryEmit( ... )
92. Broadcast Channel Shared Flow Channel APIs Replay Buffer Closed
93. Flow Sharing Strategies
94. Cold Flow Hot Flow Convert
95. State Flow State Flow Flow 1 Flow 2 Flow 3 Combined Flow
96. Creating State & Shared Flows • shareIn • stateIn
97. Sharing Policies • While Subscribed • Eagerly • Lazily
98. Sharing Policies flow.shareIn( )
99. Sharing Policies flow.shareIn( externalScope, )
100. Sharing Policies flow.shareIn( externalScope, replay = 1, )
101. Sharing Policies flow.shareIn( externalScope, replay = 1, started = SharingStarted.WhileSubscribed() )
102. Sharing Policies val sharedFlow = flow.shareIn( externalScope, replay = 1, started = SharingStarted.WhileSubscribed() )
103. Properties • Active as long as external scope is alive • Remains as long as there are collectors.
104. Properties Active as long as external scope is alive externalScope.launch { sharedFlow.collect { } }
105. flow.shareIn( externalScope, replay = 1, started = SharingStarted.WhileSubscribed() ) Properties Active as long as external scope is alive
106. Properties Active as long as external scope is alive externalScope.cancel() externalScope.launch { sharedFlow.collect { } }
107. Properties Active as long as external scope is alive externalScope.cancel() externalScope.launch { sharedFlow.collect { } } Complete Exceptionally
108. Properties Active as long as there are collectors.
109. Properties Active as long as there are collectors. val sharedFlow = flow.onCompletion { }.shareIn(…)
110. Properties Active as long as there are collectors. val sharedFlow = flow.onCompletion { }.shareIn(…) val job = launch { sharedFlow.onCompletion { }.collect { } }
111. Properties Active as long as there are collectors. val sharedFlow = flow.onCompletion { }.shareIn(…) val job = launch { sharedFlow.onCompletion { }.collect { } } job.cancel()
112. Properties Active as long as there are collectors. val sharedFlow = flow.onCompletion { }.shareIn(…) val job = launch { sharedFlow.onCompletion { }.collect { } } job.cancel()
113. Properties Active as long as there are collectors. val sharedFlow = flow.onCompletion { }.shareIn(…) val job = launch { sharedFlow.onCompletion { }.collect { } } job.cancel()
114. Properties Active as long as there are collectors. val sharedFlow = flow.onCompletion { }.shareIn(…) val job1 = launch { sharedFlow.collect { } } val job2 = launch { sharedFlow.collect { } }
115. Properties Active as long as there are collectors. val sharedFlow = flow.onCompletion { }.shareIn(…) job1.cancel() val job2 = launch { sharedFlow.collect { } }
116. Properties Active as long as there are collectors. val sharedFlow = flow.onCompletion { }.shareIn(…) job1.cancel() val job2 = launch { sharedFlow.collect { } } Remain Active
117. Properties • Active as long as external scope is alive • Active as long as there are collectors.
118. Sharing Policies • While Subscribed • Eagerly • Lazily
119. Eagerly flow.shareIn( externalScope, replay = 1, started = SharingStarted.Eagerly() )
120. Eagerly Start producer eagerly flow .onStart { println("ON START") } .shareIn( ... started = SharingStarted.Eagerly)
121. Eagerly Start producer eagerly flow .onStart { println("ON START") } .shareIn( ... started = SharingStarted.Eagerly)
122. Eagerly Start producer eagerly flow .onStart { println("ON START") } .shareIn( ... started = SharingStarted.Eagerly) // ON START
123. Eagerly Start producer eagerly
124. Sharing Policies • While Subscribed • Eagerly • Lazily
125. Lazily Start sharing after the first subscriber appears
126. Lazily flow.shareIn( externalScope, replay = 1, started = SharingStarted.Lazily )
127. Lazily flow .onStart { println("ON START") } .shareIn(…,started = SharingStarted.Lazily)
128. Lazily flow .onStart { println("ON START") } .shareIn(…,started = SharingStarted.Lazily) launch { sharedFlow.collect { } }
129. Lazily flow .onStart { println("ON START") } .shareIn(…,started = SharingStarted.Lazily) launch { sharedFlow.collect { } } // ON START
130. Lazily Start sharing after the first subscriber appears
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
132. Manage Backpressure
133. 
 Shared Flow Buffer
134. 
 Shared Flow Producer Consumer
135. 
 Shared Flow Producer Consumer Generating events fast
136. 
 Shared Flow Producer Consumer Listening to events 
 with delay
137. 
 Shared Flow Producer Consumer
138. 
 Shared Flow Producer Consumer
139. 
 Shared Flow Producer Consumer What happens when it is full?
140. Buffering Overflow Strategies • Suspend • Drop oldest • Drop latest
141. 
 Shared Flow Producer Consumer Suspend
142. Buffering Overflow Strategies val flow = MutableSharedFlow<String>( extraBufferCapacity = 2, onBufferOverflow = BufferOverflow.SUSPEND ) Buffer + Replay Count
143. Buffering Overflow Strategies val flow = MutableSharedFlow<String>( extraBufferCapacity = 2, onBufferOverflow = BufferOverflow.SUSPEND )
144. Buffering Overflow Strategies launch { flow.emit("Event 1") flow.emit("Event 2") flow.emit("Event 3") } Suspend
145. Buffering Overflow Strategies • Suspend • Drop oldest • Drop latest
146. 
 Shared Flow Producer Consumer Drop Oldest
147. 
 Shared Flow Producer Consumer Drop latest
148. Buffering Overflow Strategies • Suspend • Drop oldest • Drop latest
149. Kotlin State & Shared Flows in Action • State & Shared Flow APIs • Migrating from broadcast channels • Flow Sharing Strategies • Manage Backpressure
150. Thank You! www.codingwithmohit.com @heyitsmohit