From LiveData to Coroutines & Flow - Android Summit

Slides from my talk from Android Summit 2020 about converting LiveData to Coroutines & Flow, with a small section about StateFlow.

Transcript

1. Jossi Wolf @jossiwolf From LiveData to Coroutines Flow #session-chat-d1-s2-t1
2. @jossiwolf @jossiwolf
3. @jossiwolf Live Data, LiveData everywhere
4. @jossiwolf ªġƛŸƦŊƴŸƞǛ ßŊġǕwŸĚġŢ mŊǔġ(öƴö ÇXmöǛġƞ XŭǔŸŞġƦ mŊǔġ(öƴö XŭǔŸŞġƦ
5. @jossiwolf ”LiveData is an observable data holder class” - Android Developers Documentation
6. @jossiwolf class HomeViewModel { val latestData: LiveData<String> = MutableLiveData() } LiveData - an example
7. @jossiwolf class HomeViewModel { val latestData: LiveData<String> = MutableLiveData() fun refresh() { val data = fetchData() latestData.post(data) } } LiveData - an example
8. @jossiwolf class HomeFragment: Fragment() { fun setupUI() { latestData.observe(viewLifecycleOwner) { data #-> ##... } } } LiveData - an example
9. @jossiwolf # LiveData is great! *when used properly
10. @jossiwolf # LiveData & Execution Context
11. @jossiwolf class SupermarketRepository { fun fetchOpeningHours( id: SupermarketId ): LiveData<OpenHoursInformation> = ##... } Our Repository
12. @jossiwolf class SupermarketRepository { fun fetchOpeningHours( id: SupermarketId ): LiveData<OpenHoursInformation> = ##... } Our Repository
13. @jossiwolf class HelpViewModel( private val supermarketRepo: SupermarketRepository ): ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) = supermarketRepo.fetchOpeningHours(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } } Our ViewModel
14. @jossiwolf class HelpViewModel( private val supermarketRepo: SupermarketRepository ): ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) = supermarketRepo.fetchOpeningHours(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } } Our ViewModel
15. @jossiwolf class HelpViewModel( private val supermarketRepo: SupermarketRepository ): ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) = supermarketRepo.fetchOpeningHours(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } } Our ViewModel
16. @jossiwolf class HelpViewModel( private val supermarketRepo: SupermarketRepository ): ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) = supermarketRepo.fetchOpeningHours(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } }
17. @jossiwolf class HelpViewModel( private val supermarketRepo: SupermarketRepository ): ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) = supermarketRepo.fetchOpeningHours(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } }
18. @jossiwolf class HelpViewModel( private val supermarketRepo: SupermarketRepository ): ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) = supermarketRepo.fetchOpeningHours(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } }
19. @jossiwolf <X, Y> LiveData<Y> map( LiveData<X> source, Function<X, Y> mapFunction ) { MediatorLiveData<Y> result = new MediatorLiveData#<>(); result.addSource(source, (x) #-> { result.setValue(mapFunction.apply(x)); }); return result; } cs.android.com/androidx/platform/frameworks/support/+/androidx-master-dev:lifecycle/lifecycle-livedata/src/main/java/androidx/lifecycle/Transformations.java androidx.lifecycle.Transformations#map
20. @jossiwolf <X, Y> LiveData<Y> map( LiveData<X> source, Function<X, Y> mapFunction ) { MediatorLiveData<Y> result = new MediatorLiveData#<>(); result.addSource(source, (x) #-> { result.setValue(mapFunction.apply(x)); }); return result; } cs.android.com/androidx/platform/frameworks/support/+/androidx-master-dev:lifecycle/lifecycle-livedata/src/main/java/androidx/lifecycle/Transformations.java androidx.lifecycle.Transformations#map
21. @jossiwolf <X, Y> LiveData<Y> map( LiveData<X> source, Function<X, Y> mapFunction ) { MediatorLiveData<Y> result = new MediatorLiveData#<>(); result.addSource(source, (x) #-> { result.setValue(mapFunction.apply(x)); }); return result; } cs.android.com/androidx/platform/frameworks/support/+/androidx-master-dev:lifecycle/lifecycle-livedata/src/main/java/androidx/lifecycle/Transformations.java androidx.lifecycle.Transformations#map
22. @jossiwolf <X, Y> LiveData<Y> map( LiveData<X> source, Function<X, Y> mapFunction ) { MediatorLiveData<Y> result = new MediatorLiveData#<>(); result.addSource(source, (x) #-> { result.setValue(mapFunction.apply(x)); }); return result; } cs.android.com/androidx/platform/frameworks/support/+/androidx-master-dev:lifecycle/lifecycle-livedata/src/main/java/androidx/lifecycle/Transformations.java androidx.lifecycle.Transformations#map
23. @jossiwolf <X, Y> LiveData<Y> map( LiveData<X> source, Function<X, Y> mapFunction ) { MediatorLiveData<Y> result = new MediatorLiveData#<>(); result.addSource(source, (x) #-> { result.setValue(mapFunction.apply(x)); }); return result; } cs.android.com/androidx/platform/frameworks/support/+/androidx-master-dev:lifecycle/lifecycle-livedata/src/main/java/androidx/lifecycle/Transformations.java androidx.lifecycle.Transformations#map
24. @jossiwolf ”The events are dispatched on the main thread.” - Android Developers Documentation
25. @jossiwolf # LiveData Observers are always called on the main thread
26. @jossiwolf # It’s easy to let things slide when using LiveData
27. @jossiwolf Replace LiveData Make sure threading is done properly
28. @jossiwolf Replace LiveData @jossiwolf
29. @jossiwolf One-Shot Streams of Data
30. @jossiwolf Coroutines! One-Shot Streams of Data
31. @jossiwolf Coroutines! Flow One-Shot Streams of Data
32. @jossiwolf # Converting to Coroutines
33. @jossiwolf fun fetchOpeningHours(id: SupermarketId): LiveData<OpenHoursInformation> { val liveData = MutableLiveData<OpenHoursInformation>() val openingHours = poiService.downloadOpeningHours(poiType = SUPERMARKET, id).enqueue( object: Callback<OpenHoursInformation>() { override fun onResponse(call: Call<OpenHoursInformation>, response: Response<OpenHoursInformation>) { liveData.postValue(response.body()) } ##... } ) liveData.postValue(openingHours) return liveData } Repository with LiveData + Callbacks
34. @jossiwolf suspend fun fetchOpeningHours(id: SupermarketId): LiveData<OpenHoursInformation> { val liveData = MutableLiveData<OpenHoursInformation>() val openingHours = poiService.downloadOpeningHours(poiType = SUPERMARKET, id) liveData.postValue(openingHours) return liveData } Repository with LiveData + Coroutines
35. @jossiwolf suspend fun fetchOpeningHours(id: SupermarketId) = poiService.downloadOpeningHours(poiType = SUPERMARKET, id) That method as Coroutine…
36. @jossiwolf With Migration Helper suspend fun fetchOpeningHours(id: SupermarketId) = poiService.downloadOpeningHours(poiType = SUPERMARKET, id) @Deprecated fun fetchOpeningHoursAsLiveData(id: SupermarketId) = liveData { emit(fetchOpeningHours(id)) }
37. @jossiwolf With Migration Helper suspend fun fetchOpeningHours(id: SupermarketId) = poiService.downloadOpeningHours(poiType = SUPERMARKET, id) @Deprecated fun fetchOpeningHoursAsLiveData(id: SupermarketId) = liveData { emit(fetchOpeningHours(id)) }
38. @jossiwolf With Migration Helper suspend fun fetchOpeningHours(id: SupermarketId) = poiService.downloadOpeningHours(poiType = SUPERMARKET, id) @Deprecated fun fetchOpeningHoursAsLiveData(id: SupermarketId) = liveData { emit(fetchOpeningHours(id)) }
39. @jossiwolf With Migration Helper suspend fun fetchOpeningHours(id: SupermarketId) = poiService.downloadOpeningHours(poiType = SUPERMARKET, id) @Deprecated fun fetchOpeningHoursAsLiveData(id: SupermarketId) = liveData { emit(fetchOpeningHours(id)) }
40. @jossiwolf With Migration Helper suspend fun fetchOpeningHours(id: SupermarketId) = poiService.downloadOpeningHours(poiType = SUPERMARKET, id) @Deprecated fun fetchOpeningHoursAsLiveData(id: SupermarketId) = liveData(context = Dispatchers.IO) { emit(fetchOpeningHours(id)) }
41. @jossiwolf class HelpViewModel( private val repo: SupermarketRepository ): ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) = repo.fetchOpeningHours(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } } Our ViewModel
42. @jossiwolf class HelpViewModel( private val repo: SupermarketRepository ): ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) = repo.fetchOpeningHoursAsLiveData(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } } Our ViewModel
43. @jossiwolf class HelpViewModel( private val repo: SupermarketRepository ): ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) = repo.fetchOpeningHoursAsLiveData(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } } Our ViewModel
44. @jossiwolf Our ViewModel class HelpViewModel( private val repo: SupermarketRepository ): ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) = repo.fetchOpeningHoursAsLiveData(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } }
45. @jossiwolf Repository with Migration Helper suspend fun fetchOpeningHours(id: SupermarketId) = poiService.downloadOpeningHours(poiType = SUPERMARKET, id) @Deprecated fun fetchOpeningHoursAsLiveData(id: SupermarketId) = liveData(context = Dispatchers.IO) { emit(fetchOpeningHours(id)) }
46. @jossiwolf Our ViewModel class HelpViewModel( private val repo: SupermarketRepository ): ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) = repo.fetchOpeningHoursAsLiveData(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } }
47. @jossiwolf Our ViewModel class HelpViewModel( private val repo: SupermarketRepository ): ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) = liveData(context = Dispatchers.IO) { val openingHours = repo.fetchOpeningHours(id) val filteredHours = openingHours.hours.filter { it #!= null } emit(filteredHours) } }
48. @jossiwolf # Converting to Flow
49. @jossiwolf suspend fun fetchSupermarketDetails( id: SupermarketId ): LiveData<SupermarketDetails> { val liveData = MutableLiveData<OpenHoursInformation>() val cachedDetails = detailsCache[id] if (cachedDetails #!= null) liveData.postValue(cachedDetails) val freshDetails = poiService.downloadDetails(SUPERMARKET, id) liveData.postValue(freshDetails) return liveData } With LiveData
50. @jossiwolf suspend fun fetchSupermarketDetails( id: SupermarketId ): LiveData<SupermarketDetails> { val liveData = MutableLiveData<OpenHoursInformation>() val cachedDetails = detailsCache[id] if (cachedDetails #!= null) liveData.postValue(cachedDetails) val freshDetails = poiService.downloadDetails(SUPERMARKET, id) liveData.postValue(freshDetails) return liveData } With LiveData
51. @jossiwolf suspend fun fetchSupermarketDetails( id: SupermarketId ): LiveData<SupermarketDetails> { val liveData = MutableLiveData<OpenHoursInformation>() val cachedDetails = detailsCache[id] if (cachedDetails #!= null) liveData.postValue(cachedDetails) val freshDetails = poiService.downloadDetails(SUPERMARKET, id) liveData.postValue(freshDetails) return liveData } With LiveData
52. @jossiwolf fun fetchSupermarketDetails( id: SupermarketId ): Flow<SupermarketDetails> = flow { val cachedDetails = detailsCache[id] if (cachedDetails #!= null) emit(cachedDetails) val freshDetails = poiService.downloadDetails(SUPERMARKET, id) emit(freshDetails) } With Flow
53. @jossiwolf fun fetchSupermarketDetails( id: SupermarketId ): Flow<SupermarketDetails> = flow { val cachedDetails = detailsCache[id] if (cachedDetails #!= null) emit(cachedDetails) val freshDetails = poiService.downloadDetails(SUPERMARKET, id) emit(freshDetails) } With Flow
54. @jossiwolf fun fetchSupermarketDetails( id: SupermarketId ): Flow<SupermarketDetails> = flow { val cachedDetails = detailsCache[id] if (cachedDetails #!= null) emit(cachedDetails) val freshDetails = poiService.downloadDetails(SUPERMARKET, id) emit(freshDetails) } With Flow
55. @jossiwolf fun fetchSupermarketDetails( id: SupermarketId ): Flow<SupermarketDetails> = flow { val cachedDetails = detailsCache[id] if (cachedDetails #!= null) emit(cachedDetails) val freshDetails = poiService.downloadDetails(SUPERMARKET, id) emit(freshDetails) } With Flow
56. @jossiwolf With Flow + Migration Helper fun fetchSupermarketDetails( id: SupermarketId ): Flow<SupermarketDetails> = flow { … } @Deprecated("Please use Flows directly.") fun fetchSupermarketDetailsAsLiveData( id: SupermarketId ): LiveData<SupermarketDetails> = fetchSupermarketDetails(id).asLiveData()
57. @jossiwolf With Flow + Migration Helper fun fetchSupermarketDetails( id: SupermarketId ): Flow<SupermarketDetails> = flow { … } @Deprecated("Please use Flows directly.") fun fetchSupermarketDetailsAsLiveData( id: SupermarketId ): LiveData<SupermarketDetails> = fetchSupermarketDetails(id).asLiveData(Dispatchers.IO)
58. @jossiwolf With Flow + Migration Helper fun fetchSupermarketDetails( id: SupermarketId ): Flow<SupermarketDetails> = flow { … } @Deprecated("Please use Flows directly.") fun fetchSupermarketDetailsAsLiveData( id: SupermarketId ): LiveData<SupermarketDetails> = fetchSupermarketDetails(id).asLiveData(Dispatchers.IO)
59. @jossiwolf With Flow + Migration Helper fun fetchSupermarketDetails( id: SupermarketId ): Flow<SupermarketDetails> = flow { … } @Deprecated("Please use Flows directly.") fun fetchSupermarketDetailsAsLiveData( id: SupermarketId ): LiveData<SupermarketDetails> = fetchSupermarketDetails(id).asLiveData(Dispatchers.IO)
60. @jossiwolf # StateFlow
61. @jossiwolf “A Flow that represents a read-only state with a single updatable data value that emits updates to the value to its collectors.” - StateFlow Documentation # StateFlow
62. @jossiwolf Our ViewModel class HelpViewModel( private val repo: SupermarketRepository ): ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) : LiveData<OpeningHoursState> }
63. @jossiwolf Our ViewModel class HelpViewModel( private val repo: SupermarketRepository ): ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) : Unit }
64. @jossiwolf Our ViewModel class HelpViewModel(…): ViewModel() { val _openingHoursState = MutableStateFlow(OpeningHoursState.Empty) val openingHoursState: StateFlow<OpeningHoursState> = _openingHoursState fun fetchSupermarketOpeningHours(id: SupermarketId) { viewModelScope.launch(Dispatchers.IO) { val openingHours = repo.fetchOpeningHours() _openingHoursState.value = openingHours } } }
65. @jossiwolf Our ViewModel class HelpViewModel(…): ViewModel() { val _openingHoursState = MutableStateFlow(OpeningHoursState.Empty) val openingHoursState: StateFlow<OpeningHoursState> = _openingHoursState fun fetchSupermarketOpeningHours(id: SupermarketId) { viewModelScope.launch(Dispatchers.IO) { val openingHours = repo.fetchOpeningHours() _openingHoursState.value = openingHours } } }
66. @jossiwolf Our ViewModel class HelpViewModel(…): ViewModel() { val _openingHoursState = MutableStateFlow(OpeningHoursState.Empty) val openingHoursState: StateFlow<OpeningHoursState> = _openingHoursState fun fetchSupermarketOpeningHours(id: SupermarketId) { viewModelScope.launch(Dispatchers.IO) { val openingHours = repo.fetchOpeningHours() _openingHoursState.value = openingHours } } }
67. @jossiwolf Our UI Layer fun onViewCreated(#..) { viewModel.openingHoursState .onEach(#::renderOpeningHours) .launchIn(lifecycleScope) }
68. @jossiwolf -Single API instead of two APIs Why StateFlow?
69. @jossiwolf All methods of data flow are thread- safe and can be safely invoked from concurrent coroutines without external synchronization. - StateFlow Documentation
70. @jossiwolf -Single API instead of two APIs -More control over execution context Why StateFlow?
71. @jossiwolf -Single API instead of two APIs -More control over execution context -Integrates nicely with Jetpack Compose Why StateFlow?
72. @jossiwolf -Still experimental -Things are a bit unclear :) StateFlow Drawbacks
73. @jossiwolf - developer.android.com/topic/libraries/architecture/livedata - proandroiddev.com/dont-use-livedata-in- repositories-f3bebe502ed3 - medium.com/@elizarov/execution-context-of-kotlin-flows-b8c151c9309b - https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/ kotlinx.coroutines.flow/-state-flow/ Resources
74. @jossiwolf
75. Jossi Wolf @jossiwolf From LiveData to Coroutines Flow