🧖 👲🏽 🍄 LiveData から Kotlin のフローへの移行 😍 🌏 🐢

2017 年には LiveData が必要でした。Observer パターンは私たちの生活を楽にしてくれましたが、RxJava のようなオプションは当時の初心者には複雑すぎました。Architecture Components チームは LiveData を作成しました。これは、Android 向けに開発された非常に信頼できる監視可能なデータストアクラスです。開始を容易にするのは簡単で、より複雑なリアクティブスレッドの場合には、RxJava と RxJava の統合を利用して、RxJava を使用することが推奨されました。

デッドデータ?

LiveData - Java-, . , Kotlin Flows. Flows () , Kotlin, Jetbrains; , Compose, .

Flows , ViewModel. , Android, .

, Flows , , .

Flow: , —

LiveData : , Android. , , .

LiveData Flow:

#1:

, :

Mutable データホルダー (LiveData) を使用した 1 回の操作の結果の表示 — (Mutable) (LiveData)

<!-- Copyright 2020 Google LLC.	
   SPDX-License-Identifier: Apache-2.0 -->

class MyViewModel {
    private val _myUiState = MutableLiveData<Result<UiState>>(Result.Loading)
    val myUiState: LiveData<Result<UiState>> = _myUiState

    // Load data from a suspend fun and mutate state
    init {
        viewModelScope.launch { 
            val result = ...
            _myUiState.value = result
        }
    }
}

, StateFlow:

class MyViewModel {
    private val _myUiState = MutableStateFlow<Result<UiState>>(Result.Loading)
    val myUiState: StateFlow<Result<UiState>> = _myUiState

    // Load data from a suspend fun and mutate state
    init {
        viewModelScope.launch { 
            val result = ...
            _myUiState.value = result
        }
    }
}

StateFlow — SharedFlow ( Flow), LiveData:

.
.
( ).
, .

StateFlow. , .

#2:

, .

LiveData liveData:

class MyViewModel(...) : ViewModel() {
    val result: LiveData<Result<UiState>> = liveData {
        emit(Result.Loading)
        emit(repository.fetchItem())
    }
}

, UI- - Result

, , Loading

, Success

Error

.

Flow , :

class MyViewModel(...) : ViewModel() {
    val result: StateFlow<Result<UiState>> = flow {
        emit(repository.fetchItem())
    }.stateIn(
        scope = viewModelScope, 
        started = WhileSubscribed(5000), // Or Lazily because it's a one-shot
        initialValue = Result.Loading
    )
}

stateIn

— Flow, StateFlow

. , .

#3:

, , ID , AuthManager

, Flow:

LiveData :

class MyViewModel(authManager..., repository...) : ViewModel() {
    private val userId: LiveData<String?> = 
        authManager.observeUser().map { user -> user.id }.asLiveData()

    val result: LiveData<Result<Item>> = userId.switchMap { newUserId ->
        liveData { emit(repository.fetchItem(newUserId)) }
    }
}

switchMap

— , , userId

.

, userId

LiveData, Flow LiveData.

class MyViewModel(authManager..., repository...) : ViewModel() {
    private val userId: Flow<UserId> = authManager.observeUser().map { user -> user.id }

    val result: LiveData<Result<Item>> = userId.mapLatest { newUserId ->
       repository.fetchItem(newUserId)
    }.asLiveData()
}

Flows :

パラメータを使用した 1 回限りのデータロード (StateFlow) — (StateFlow)

class MyViewModel(authManager..., repository...) : ViewModel() {
    private val userId: Flow<UserId> = authManager.observeUser().map { user -> user.id }

    val result: StateFlow<Result<Item>> = userId.mapLatest { newUserId ->
        repository.fetchItem(newUserId)
    }.stateIn(
        scope = viewModelScope, 
        started = WhileSubscribed(5000), 
        initialValue = Result.Loading
    )
}

, , transformLatest

emit

:

	val result = userId.transformLatest { newUserId ->
        emit(Result.LoadingData)
        emit(repository.fetchItem(newUserId))
    }.stateIn(
        scope = viewModelScope, 
        started = WhileSubscribed(5000), 
        initialValue = Result.LoadingUser // Note the different Loading states
    )

#4:

. , , .

: fetchItem

observeItem

, Flow.

LiveData LiveData emitSource

:

パラメータ付きのストリームの視聴 (LiveData) — (LiveData)

class MyViewModel(authManager..., repository...) : ViewModel() {
    private val userId: LiveData<String?> = 
        authManager.observeUser().map { user -> user.id }.asLiveData()

    val result = userId.switchMap { newUserId ->
        repository.observeItem(newUserId).asLiveData()
    }
}

, , flatMapLatest LiveData:

class MyViewModel(authManager..., repository...) : ViewModel() {
    private val userId: Flow<String?> = 
        authManager.observeUser().map { user -> user?.id }

    val result: LiveData<Result<Item>> = userId.flatMapLatest { newUserId ->
        repository.observeItem(newUserId)
    }.asLiveData()
}

Flow , LiveData:

class MyViewModel(authManager..., repository...) : ViewModel() {
    private val userId: Flow<String?> = 
        authManager.observeUser().map { user -> user?.id }

    val result: StateFlow<Result<Item>> = userId.flatMapLatest { newUserId ->
        repository.observeItem(newUserId)
    }.stateIn(
        scope = viewModelScope, 
        started = WhileSubscribed(5000), 
        initialValue = Result.LoadingUser
    )
}

StateFlow , .

#5 : MediatorLiveData -> Flow.combine

MediatorLiveData ( LiveData) - , . MediatorLiveData:


val liveData1: LiveData<Int> = ...
val liveData2: LiveData<Int> = ...

val result = MediatorLiveData<Int>()

result.addSource(liveData1) { value ->
    result.setValue(liveData1.value ?: 0 + (liveData2.value ?: 0))
}
result.addSource(liveData2) { value ->
    result.setValue(liveData1.value ?: 0 + (liveData2.value ?: 0))
}

Flow :

val flow1: Flow<Int> = ...
val flow2: Flow<Int> = ...

val result = combine(flow1, flow2) { a, b -> a + b }

combineTransform zip.

StateFlow ( stateIn)

stateIn

StateFlow, . -, :

val result: StateFlow<Result<UiState>> = someFlow
    .stateIn(
        scope = viewModelScope, 
        started = WhileSubscribed(5000), 
        initialValue = Result.Loading
    )

, , 5- started

, .

StateIn

3 ( ):

@param scope the coroutine scope in which sharing is started.
@param started the strategy that controls when sharing is started and stopped.
@param initialValue the initial value of the state flow.
This value is also used when the state flow is reset using the [SharingStarted.WhileSubscribed] strategy with the `replayExpirationMillis` parameter.

started

3 :

Lazily

: , , , scope .

Eagerly

: , scope .

WhileSubscribed

: .

Lazily

Eagerly

. , , WhileSubscribed

, , .

WhileSubscribed

WhileSubscribed , . StateFlow, stateIn

, View, , ( ). , , , , .. , , .

WhileSubscribed

:

public fun WhileSubscribed(
    stopTimeoutMillis: Long = 0,
    replayExpirationMillis: Long = Long.MAX_VALUE
)

stopTimeoutMillis

( ) . ( ).

, , . — , .

liveData 5 , , . WhileSubscribed(5000)

:

class MyViewModel(...) : ViewModel() {
    val result = userId.mapLatest { newUserId ->
        repository.observeItem(newUserId)
    }.stateIn(
        scope = viewModelScope, 
        started = WhileSubscribed(5000), 
        initialValue = Result.Loading
    )
}

, , , , .

, , , .

, , , .

replayExpirationMillis

— ( ) ( shareIn

initialValue

stateIn

). Long.MAX_VALUE

( , ). .

StateFlow

, StateFlows ViewModel, . , , , .

, . :

Activity.lifecycleScope.launch

: .

Fragment.lifecycleScope.launch

: .

LaunchWhenStarted, launchWhenResumed...

launch, launchWhenX

, , lifecycleOwner

X, , lifecycleOwner

X. , , .

launch / launchWhenX を使用したスレッドの収集は安全ではありません — launch/launchWhenX

, , , View. , , .

, , StateFlow, ; API.

lifecycle.repeatOnLifecycle

( lifecycle-runtime-ktx 2.4.0-alpha01) , : , .

, :

onCreateView(...) {
    viewLifecycleOwner.lifecycleScope.launch {
        viewLifecycleOwner.lifecycle.repeatOnLifecycle(STARTED) {
            myViewModel.myUiState.collect { ... }
        }
    }
}

, STARTED

, RESUMED

, STOPPED

. Android.

API repeatOnLifecycle

StateFlow .

StateFlow は WhileSubscribe (5000) を使用して公開され、repeatOnLifecycle (STARTED) を使用して収集されます — StateFlow WhileSubscribed(5000) repeatOnLifecycle(STARTED)

: StateFlow, Data Binding, launchWhenCreated

, repeatOnLifecycle

` , .

Data Binding Flows asLiveData()

, . , lifecycle-runtime-ktx 2.4.0

.

ViewModel :

StateFlow

, WhileSubscribed

, . []

repeatOnLifecycle

. [].

, :

"Android Developer. Basic". , . , .

LiveData から Kotlin のフローへの移行