From LiveData to Coroutines & Flow - DevFest ZA

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Developers Documentation

@jossiwolf class HomeViewModel { val latestData: LiveData<String> = MutableLiveData() }

LiveData - an example

@jossiwolf class HomeViewModel { val latestData: LiveData<String> = MutableLiveData() fun

refresh() { val data = fetchData() latestData.post(data) } } LiveData - an example

@jossiwolf class HomeFragment: Fragment() { fun setupUI() { latestData.observe(viewLifecycleOwner) {

data #-> ##... } } } LiveData - an example

@jossiwolf # LiveData is great! *when used properly

@jossiwolf # LiveData & Execution Context

@jossiwolf class SupermarketRepository { fun fetchOpeningHours( id: SupermarketId ): LiveData<OpenHoursInformation>

= ##... } Our Repository

@jossiwolf class SupermarketRepository { fun fetchOpeningHours( id: SupermarketId ): LiveData<OpenHoursInformation>

= ##... } Our Repository

@jossiwolf class HelpViewModel( private val supermarketRepo: SupermarketRepository ): ViewModel() {

fun fetchSupermarketOpeningHours(id: SupermarketId) = supermarketRepo.fetchOpeningHours(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } } Our ViewModel

@jossiwolf class HelpViewModel( private val supermarketRepo: SupermarketRepository ): ViewModel() {

fun fetchSupermarketOpeningHours(id: SupermarketId) = supermarketRepo.fetchOpeningHours(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } }

@jossiwolf class HelpViewModel( private val supermarketRepo: SupermarketRepository ): ViewModel() {

@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

@jossiwolf <X, Y> LiveData<Y> map( LiveData<X> source, Function<X, Y> mapFunction

@jossiwolf ”The events are dispatched on the main thread.” -

Android Developers Documentation

@jossiwolf # LiveData Observers are always called on the main

thread

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

LiveData

@jossiwolf Replace LiveData Make sure threading is done properly

@jossiwolf Replace LiveData @jossiwolf

@jossiwolf One-Shot Streams of Data

@jossiwolf Coroutines! One-Shot Streams of Data

@jossiwolf Coroutines! Flow One-Shot Streams of Data

@jossiwolf # Converting to Coroutines

@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

@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

@jossiwolf suspend fun fetchOpeningHours(id: SupermarketId) = poiService.downloadOpeningHours(poiType = SUPERMARKET, id)

That method as Coroutine…

@jossiwolf With Migration Helper suspend fun fetchOpeningHours(id: SupermarketId) = poiService.downloadOpeningHours(poiType

= SUPERMARKET, id) @Deprecated fun fetchOpeningHoursAsLiveData(id: SupermarketId) = liveData { emit(fetchOpeningHours(id)) }

@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)) }

@jossiwolf class HelpViewModel( private val repo: SupermarketRepository ): ViewModel() {

fun fetchSupermarketOpeningHours(id: SupermarketId) = repo.fetchOpeningHours(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } } Our ViewModel

@jossiwolf class HelpViewModel( private val repo: SupermarketRepository ): ViewModel() {

fun fetchSupermarketOpeningHours(id: SupermarketId) = repo.fetchOpeningHoursAsLiveData(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } } Our ViewModel

@jossiwolf class HelpViewModel( private val repo: SupermarketRepository ): ViewModel() {

fun fetchSupermarketOpeningHours(id: SupermarketId) = repo.fetchOpeningHoursAsLiveData(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } } Our ViewModel

@jossiwolf Our ViewModel class HelpViewModel( private val repo: SupermarketRepository ):

ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) = repo.fetchOpeningHoursAsLiveData(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } }

@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)) }

@jossiwolf Our ViewModel class HelpViewModel( private val repo: SupermarketRepository ):

ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) = repo.fetchOpeningHoursAsLiveData(id).map { openingHours #-> openingHours.hours.filter { it #!= null } } }

@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) } }

@jossiwolf # Converting to Flow

@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

@jossiwolf suspend fun fetchSupermarketDetails( id: SupermarketId ): LiveData<SupermarketDetails> { val

@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

@jossiwolf fun fetchSupermarketDetails( id: SupermarketId ): Flow<SupermarketDetails> = flow {

@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()

@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)

@jossiwolf With Flow + Migration Helper fun fetchSupermarketDetails( id: SupermarketId

@jossiwolf # StateFlow

@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

@jossiwolf Our ViewModel class HelpViewModel( private val repo: SupermarketRepository ):

ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) : LiveData<OpeningHoursState> }

@jossiwolf Our ViewModel class HelpViewModel( private val repo: SupermarketRepository ):

ViewModel() { fun fetchSupermarketOpeningHours(id: SupermarketId) : Unit }

@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 } } }

@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 } } }

@jossiwolf Our ViewModel class HelpViewModel(…): ViewModel() { private 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 } } }

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

}

@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

From LiveData to Coroutines & Flow - DevFest ZA

From LiveData to Coroutines & Flow - DevFest ZA

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