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
   

   View Slide

2. @jossiwolf
   @jossiwolf
   

   View Slide

3. @jossiwolf
   Live Data, LiveData everywhere
   

   View Slide

4. @jossiwolf
   ªġƛŸƦŊƴŸƞǛ
   ßŊġǕwŸĚġŢ
   mŊǔġ(öƴö
   ÇXmöǛġƞ
   XŭǔŸŞġƦ
   mŊǔġ(öƴö
   XŭǔŸŞġƦ
   

   View Slide

5. @jossiwolf
   ”LiveData is an observable data
   holder class”
   - Android Developers Documentation
   

   View Slide

6. @jossiwolf
   class HomeViewModel {
   
   val latestData: LiveData = MutableLiveData()
   
   }
   LiveData - an example
   

   View Slide

7. @jossiwolf
   class HomeViewModel {
   
   val latestData: LiveData = MutableLiveData()
   
   fun refresh() {
   
   val data = fetchData()
   
   latestData.post(data)
   
   }
   
   }
   LiveData - an example
   

   View Slide

8. @jossiwolf
   class HomeFragment: Fragment() {
   
   fun setupUI() {
   
   latestData.observe(viewLifecycleOwner) { data #->
   
   ##...
   
   }
   
   }
   
   }
   LiveData - an example
   

   View Slide

9. @jossiwolf
   # LiveData is great!
   *when used properly
   

   View Slide

10. @jossiwolf
    # LiveData &
    Execution Context
    

    View Slide

11. @jossiwolf
    class SupermarketRepository {
    
    fun fetchOpeningHours(
    
    id: SupermarketId
    
    ): LiveData = ##...
    
    }
    Our Repository
    

    View Slide

12. @jossiwolf
    class SupermarketRepository {
    
    fun fetchOpeningHours(
    
    id: SupermarketId
    
    ): LiveData = ##...
    
    }
    Our Repository
    

    View Slide

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
    

    View Slide

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
    

    View Slide

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
    

    View Slide

16. @jossiwolf
    class HelpViewModel(
    
    private val supermarketRepo: SupermarketRepository
    
    ): ViewModel() {
    
    fun fetchSupermarketOpeningHours(id: SupermarketId) =
    
    supermarketRepo.fetchOpeningHours(id).map { openingHours #->
    
    openingHours.hours.filter { it #!= null }
    
    }
    
    }
    

    View Slide

17. @jossiwolf
    class HelpViewModel(
    
    private val supermarketRepo: SupermarketRepository
    
    ): ViewModel() {
    
    fun fetchSupermarketOpeningHours(id: SupermarketId) =
    
    supermarketRepo.fetchOpeningHours(id).map { openingHours #->
    
    openingHours.hours.filter { it #!= null }
    
    }
    
    }
    

    View Slide

18. @jossiwolf
    class HelpViewModel(
    
    private val supermarketRepo: SupermarketRepository
    
    ): ViewModel() {
    
    fun fetchSupermarketOpeningHours(id: SupermarketId) =
    
    supermarketRepo.fetchOpeningHours(id).map { openingHours #->
    
    openingHours.hours.filter { it #!= null }
    
    }
    
    }
    

    View Slide

19. @jossiwolf
    LiveData map(
    
    LiveData source,
    
    Function mapFunction
    
    ) {
    
    MediatorLiveData 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
    

    View Slide

20. @jossiwolf
    LiveData map(
    
    LiveData source,
    
    Function mapFunction
    
    ) {
    
    MediatorLiveData 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
    

    View Slide

21. @jossiwolf
    LiveData map(
    
    LiveData source,
    
    Function mapFunction
    
    ) {
    
    MediatorLiveData 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
    

    View Slide

22. @jossiwolf
    LiveData map(
    
    LiveData source,
    
    Function mapFunction
    
    ) {
    
    MediatorLiveData 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
    

    View Slide

23. @jossiwolf
    LiveData map(
    
    LiveData source,
    
    Function mapFunction
    
    ) {
    
    MediatorLiveData 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
    

    View Slide

24. @jossiwolf
    ”The events are dispatched on the
    main thread.”
    - Android Developers Documentation
    

    View Slide

25. @jossiwolf
    # LiveData Observers are always called
    on the main thread
    

    View Slide

26. @jossiwolf
    # It’s easy to let things slide when using
    LiveData
    

    View Slide

27. @jossiwolf
    Replace LiveData
    Make sure threading is done
    properly
    

    View Slide

28. @jossiwolf
    Replace LiveData
    @jossiwolf
    

    View Slide

29. @jossiwolf
    One-Shot Streams of Data
    

    View Slide

30. @jossiwolf
    Coroutines!
    One-Shot Streams of Data
    

    View Slide

31. @jossiwolf
    Coroutines! Flow
    One-Shot Streams of Data
    

    View Slide

32. @jossiwolf
    # Converting to Coroutines
    

    View Slide

33. @jossiwolf
    fun fetchOpeningHours(id: SupermarketId): LiveData {
    
    val liveData = MutableLiveData()
    
    val openingHours = poiService.downloadOpeningHours(poiType = SUPERMARKET, id).enqueue(
    
    object: Callback() {
    
    override fun onResponse(call: Call, response: Response) {
    
    liveData.postValue(response.body())
    
    }
    
    ##...
    
    }
    
    )
    
    liveData.postValue(openingHours)
    
    return liveData
    
    }
    Repository with LiveData + Callbacks
    

    View Slide

34. @jossiwolf
    suspend fun fetchOpeningHours(id: SupermarketId): LiveData {
    
    val liveData = MutableLiveData()
    
    val openingHours = poiService.downloadOpeningHours(poiType = SUPERMARKET, id)
    
    liveData.postValue(openingHours)
    
    return liveData
    
    }
    Repository with LiveData + Coroutines
    

    View Slide

35. @jossiwolf
    suspend fun fetchOpeningHours(id: SupermarketId) =
    
    poiService.downloadOpeningHours(poiType = SUPERMARKET, id)
    That method as Coroutine…
    

    View Slide

36. @jossiwolf
    With Migration Helper
    suspend fun fetchOpeningHours(id: SupermarketId) =
    
    poiService.downloadOpeningHours(poiType = SUPERMARKET, id)
    
    @Deprecated
    
    fun fetchOpeningHoursAsLiveData(id: SupermarketId) =
    
    liveData {
    
    emit(fetchOpeningHours(id))
    
    }
    

    View Slide

37. @jossiwolf
    With Migration Helper
    suspend fun fetchOpeningHours(id: SupermarketId) =
    
    poiService.downloadOpeningHours(poiType = SUPERMARKET, id)
    
    @Deprecated
    
    fun fetchOpeningHoursAsLiveData(id: SupermarketId) =
    
    liveData {
    
    emit(fetchOpeningHours(id))
    
    }
    

    View Slide

38. @jossiwolf
    With Migration Helper
    suspend fun fetchOpeningHours(id: SupermarketId) =
    
    poiService.downloadOpeningHours(poiType = SUPERMARKET, id)
    
    @Deprecated
    
    fun fetchOpeningHoursAsLiveData(id: SupermarketId) =
    
    liveData {
    
    emit(fetchOpeningHours(id))
    
    }
    

    View Slide

39. @jossiwolf
    With Migration Helper
    suspend fun fetchOpeningHours(id: SupermarketId) =
    
    poiService.downloadOpeningHours(poiType = SUPERMARKET, id)
    
    @Deprecated
    
    fun fetchOpeningHoursAsLiveData(id: SupermarketId) =
    
    liveData {
    
    emit(fetchOpeningHours(id))
    
    }
    

    View Slide

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))
    
    }
    

    View Slide

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
    

    View Slide

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
    

    View Slide

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
    

    View Slide

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 }
    
    }
    
    }
    

    View Slide

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))
    
    }
    

    View Slide

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 }
    
    }
    
    }
    

    View Slide

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)
    
    }
    
    }
    

    View Slide

48. @jossiwolf
    # Converting to Flow
    

    View Slide

49. @jossiwolf
    suspend fun fetchSupermarketDetails(
    
    id: SupermarketId
    
    ): LiveData {
    
    val liveData = MutableLiveData()
    
    val cachedDetails = detailsCache[id]
    
    if (cachedDetails #!= null) liveData.postValue(cachedDetails)
    
    val freshDetails = poiService.downloadDetails(SUPERMARKET, id)
    
    liveData.postValue(freshDetails)
    
    return liveData
    
    }
    With LiveData
    

    View Slide

50. @jossiwolf
    suspend fun fetchSupermarketDetails(
    
    id: SupermarketId
    
    ): LiveData {
    
    val liveData = MutableLiveData()
    
    val cachedDetails = detailsCache[id]
    
    if (cachedDetails #!= null) liveData.postValue(cachedDetails)
    
    val freshDetails = poiService.downloadDetails(SUPERMARKET, id)
    
    liveData.postValue(freshDetails)
    
    return liveData
    
    }
    With LiveData
    

    View Slide

51. @jossiwolf
    suspend fun fetchSupermarketDetails(
    
    id: SupermarketId
    
    ): LiveData {
    
    val liveData = MutableLiveData()
    
    val cachedDetails = detailsCache[id]
    
    if (cachedDetails #!= null) liveData.postValue(cachedDetails)
    
    val freshDetails = poiService.downloadDetails(SUPERMARKET, id)
    
    liveData.postValue(freshDetails)
    
    return liveData
    
    }
    With LiveData
    

    View Slide

52. @jossiwolf
    fun fetchSupermarketDetails(
    
    id: SupermarketId
    
    ): Flow = flow {
    
    val cachedDetails = detailsCache[id]
    
    if (cachedDetails #!= null) emit(cachedDetails)
    
    val freshDetails = poiService.downloadDetails(SUPERMARKET, id)
    
    emit(freshDetails)
    
    }
    With Flow
    

    View Slide

53. @jossiwolf
    fun fetchSupermarketDetails(
    
    id: SupermarketId
    
    ): Flow = flow {
    
    val cachedDetails = detailsCache[id]
    
    if (cachedDetails #!= null) emit(cachedDetails)
    
    val freshDetails = poiService.downloadDetails(SUPERMARKET, id)
    
    emit(freshDetails)
    
    }
    With Flow
    

    View Slide

54. @jossiwolf
    fun fetchSupermarketDetails(
    
    id: SupermarketId
    
    ): Flow = flow {
    
    val cachedDetails = detailsCache[id]
    
    if (cachedDetails #!= null) emit(cachedDetails)
    
    val freshDetails = poiService.downloadDetails(SUPERMARKET, id)
    
    emit(freshDetails)
    
    }
    With Flow
    

    View Slide

55. @jossiwolf
    fun fetchSupermarketDetails(
    
    id: SupermarketId
    
    ): Flow = flow {
    
    val cachedDetails = detailsCache[id]
    
    if (cachedDetails #!= null) emit(cachedDetails)
    
    val freshDetails = poiService.downloadDetails(SUPERMARKET, id)
    
    emit(freshDetails)
    
    }
    With Flow
    

    View Slide

56. @jossiwolf
    With Flow + Migration Helper
    fun fetchSupermarketDetails(
    
    id: SupermarketId
    
    ): Flow = flow { … }
    
    @Deprecated("Please use Flows directly.")
    
    fun fetchSupermarketDetailsAsLiveData(
    
    id: SupermarketId
    
    ): LiveData =
    
    fetchSupermarketDetails(id).asLiveData()
    

    View Slide

57. @jossiwolf
    With Flow + Migration Helper
    fun fetchSupermarketDetails(
    
    id: SupermarketId
    
    ): Flow = flow { … }
    
    @Deprecated("Please use Flows directly.")
    
    fun fetchSupermarketDetailsAsLiveData(
    
    id: SupermarketId
    
    ): LiveData =
    
    fetchSupermarketDetails(id).asLiveData(Dispatchers.IO)
    

    View Slide

58. @jossiwolf
    With Flow + Migration Helper
    fun fetchSupermarketDetails(
    
    id: SupermarketId
    
    ): Flow = flow { … }
    
    @Deprecated("Please use Flows directly.")
    
    fun fetchSupermarketDetailsAsLiveData(
    
    id: SupermarketId
    
    ): LiveData =
    
    fetchSupermarketDetails(id).asLiveData(Dispatchers.IO)
    

    View Slide

59. @jossiwolf
    With Flow + Migration Helper
    fun fetchSupermarketDetails(
    
    id: SupermarketId
    
    ): Flow = flow { … }
    
    @Deprecated("Please use Flows directly.")
    
    fun fetchSupermarketDetailsAsLiveData(
    
    id: SupermarketId
    
    ): LiveData =
    
    fetchSupermarketDetails(id).asLiveData(Dispatchers.IO)
    

    View Slide

60. @jossiwolf
    # StateFlow
    

    View Slide

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
    

    View Slide

62. @jossiwolf
    Our ViewModel
    class HelpViewModel(
    
    private val repo: SupermarketRepository
    
    ): ViewModel() {
    
    fun fetchSupermarketOpeningHours(id: SupermarketId)
    
    : LiveData
    
    }
    

    View Slide

63. @jossiwolf
    Our ViewModel
    class HelpViewModel(
    
    private val repo: SupermarketRepository
    
    ): ViewModel() {
    
    fun fetchSupermarketOpeningHours(id: SupermarketId)
    
    : Unit
    
    }
    

    View Slide

64. @jossiwolf
    Our ViewModel
    class HelpViewModel(…): ViewModel() {
    
    val _openingHoursState = MutableStateFlow(OpeningHoursState.Empty)
    
    val openingHoursState: StateFlow
    
    = _openingHoursState
    
    fun fetchSupermarketOpeningHours(id: SupermarketId) {
    
    viewModelScope.launch(Dispatchers.IO) {
    
    val openingHours = repo.fetchOpeningHours()
    
    _openingHoursState.value = openingHours
    
    }
    
    }
    
    }
    

    View Slide

65. @jossiwolf
    Our ViewModel
    class HelpViewModel(…): ViewModel() {
    
    val _openingHoursState = MutableStateFlow(OpeningHoursState.Empty)
    
    val openingHoursState: StateFlow
    
    = _openingHoursState
    
    fun fetchSupermarketOpeningHours(id: SupermarketId) {
    
    viewModelScope.launch(Dispatchers.IO) {
    
    val openingHours = repo.fetchOpeningHours()
    
    _openingHoursState.value = openingHours
    
    }
    
    }
    
    }
    

    View Slide

66. @jossiwolf
    Our ViewModel
    class HelpViewModel(…): ViewModel() {
    
    val _openingHoursState = MutableStateFlow(OpeningHoursState.Empty)
    
    val openingHoursState: StateFlow
    
    = _openingHoursState
    
    fun fetchSupermarketOpeningHours(id: SupermarketId) {
    
    viewModelScope.launch(Dispatchers.IO) {
    
    val openingHours = repo.fetchOpeningHours()
    
    _openingHoursState.value = openingHours
    
    }
    
    }
    
    }
    

    View Slide

67. @jossiwolf
    Our UI Layer
    fun onViewCreated(#..) {
    
    viewModel.openingHoursState
    
    .onEach(#::renderOpeningHours)
    
    .launchIn(lifecycleScope)
    
    }
    

    View Slide

68. @jossiwolf
    -Single API instead of two APIs
    Why StateFlow?
    

    View Slide

69. @jossiwolf
    All methods of data flow are thread-
    safe and can be safely invoked from
    concurrent coroutines without
    external synchronization.
    - StateFlow Documentation
    

    View Slide

70. @jossiwolf
    -Single API instead of two APIs
    -More control over execution context
    Why StateFlow?
    

    View Slide

71. @jossiwolf
    -Single API instead of two APIs
    -More control over execution context
    -Integrates nicely with Jetpack Compose
    Why StateFlow?
    

    View Slide

72. @jossiwolf
    -Still experimental
    -Things are a bit unclear :)
    StateFlow Drawbacks
    

    View Slide

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
    

    View Slide

74. @jossiwolf
    

    View Slide

75. Jossi Wolf
    @jossiwolf
    From LiveData to
    Coroutines Flow
    

    View Slide