[移动开发]Kotlin Flow

Flow 非常适合实时数据更新和无休止的数据流，Flow 已经被集成到许多 Jetpack 库中

@Dao
abstract class ExampleDao {
    @Query("SELECT * FROM Example")
    abstract fun getExamples(): Flow<List<Example>>
}

创建 Flow 数据流

class NewsRemoteDataSource(
    private val newsApi: NewsApi,
    private val refreshIntervalMs: Long = 5000
) {
    val latestNews: Flow<List<ArticleHeadline>> = flow {
        while (true) {
            val latestNews = newsApi.fetchLatestNews()
            emit(latestNews) // Emits the result of the request to the flow
            delay(refreshIntervalMs) // Suspends the coroutine for some time
        }
    }.map { it }
        .flowOn(Dispatchers.IO)
        .catch { exception -> notifyError(exception) }
}

interface NewsApi {
    suspend fun fetchLatestNews(): List<ArticleHeadline>
}

订阅 Flow 数据流

class LatestNewsViewModel(
    private val newsRepository: NewsRepository
) : ViewModel() {
    init {
        viewModelScope.launch {
            // Trigger the flow and consume its elements using collect
            newsRepository.favoriteLatestNews.collect { favoriteNews ->
                // Update View with the latest favorite news
            }
        }
    }
}

在 默认的 Flow Builder 中，我们通过 emit 发射数据，而 callbackFlow / channelFlow 允许我们从不同的 CoroutineContext 中或者在协程之外通过 trySend 发射数据。callbackFlow / channelFlow 内部通过 Channel 实现，默认的缓冲区大小为64，如果超过该大小，则 trySend 返回 false

val latestNews: Flow<List<Int>> = channelFlow {
    newsApi.fetchLatestNews(object : NewsApiCallback {
        override fun onSuccess(list: List<Int>) {
            if (trySend(listOf(i)).isSuccess) {
                Log.e("scrutiny", "source trySend success")
            } else {
                Log.e("scrutiny", "source trySend failed")
            }
        }
        override fun onError() {}
    })
    delay(refreshIntervalMs)
    withContext(Dispatchers.IO) {
        trySend(listOf<Int>())
    }
    awaitClose {
        Log.e("scrutiny", "channelFlow close")
    }
}

冷数据流 VS 热数据流

相比于热数据流（StateFlow、SharedFlow），通过 flow / callbackFlow / channelFlow 创建的数据流是冷数据流，Consumer 每次 collect 都会重新执行发射逻辑

StateFlow

持有状态的 Flow，它向 Consumer 发出当前和新状态更新。 当前状态值也可以通过其 value 属性读取。 要更新状态并将其发送到流，请为 MutableStateFlow 类的 value 属性分配一个新值

通过 repeatOnlifecycle 可以用来替换 LiveData，用于解决复杂场景的线程切换问题

override fun onCreate(savedInstanceState: Bundle?) {
    // Start a coroutine in the lifecycle scope
    lifecycleScope.launch {
        // repeatOnLifecycle launches the block in a new coroutine every time the
        // lifecycle is in the STARTED state (or above) and cancels it when it's STOPPED.
        repeatOnLifecycle(Lifecycle.State.STARTED) {
            // Trigger the flow and start listening for values.
            // Note that this happens when lifecycle is STARTED and stops
            // collecting when the lifecycle is STOPPED
            latestNewsViewModel.uiState.collect { uiState ->
                // New value received
                when (uiState) {
                    is LatestNewsUiState.Success -> showFavoriteNews(uiState.news)
                    is LatestNewsUiState.Error -> showError(uiState.exception)
                }
            }
        }
    }
}

To convert any flow to a StateFlow, use the stateIn intermediate operator.

StateFlow vs LiveData

StateFlow and LiveData have similarities. Both are observable data holder classes, and both follow a similar pattern when used in your app architecture. Note, however, that StateFlow and LiveData do behave differently:

• StateFlow requires an initial state to be passed in to the constructor, while LiveData does not.
• LiveData.observe automatically unregisters the consumer when the view goes to the STOPPED state, whereas collecting from a StateFlow or any other flow does not stop collecting automatically. To achieve the same behavior,you need to collect the flow from a Lifecycle.repeatOnLifecycle block.
• StateFlow 配合协程可以完成复杂场景的线程切换

?SharedFlow

通过 shareIn 函数可以将冷流 Flow 转换为热流 SharedFlow， SharedFlow 是 StateFlow 的高度可配置的泛化，具体参数有：

• A CoroutineScope that is used to share the flow. This scope should live longer than any consumer to keep the shared flow alive as long as needed.
• The number of items to replay to each new collector.
• The start behavior policy.

val latestNews: Flow<List<ArticleHeadline>> = flow {
    ...
}.shareIn(
    externalScope,
    replay = 1,
    started = SharingStarted.WhileSubscribed()
)

SharingStarted 参数

public companion object {
    /**
     * Sharing is started immediately and never stops.
     */
    public val Eagerly: SharingStarted = StartedEagerly()

    /**
     * Sharing is started when the first subscriber appears and never stops.
     */
    public val Lazily: SharingStarted = StartedLazily()

    /**
     * Sharing is started when the first subscriber appears, immediately stops when the last
     * subscriber disappears (by default), keeping the replay cache forever (by default).
     *
     * It has the following optional parameters:
     *
     * * [stopTimeoutMillis] configures a delay (in milliseconds) between the disappearance of the last
     *   subscriber and the stopping of the sharing coroutine. It defaults to zero (stop immediately).
     * * [replayExpirationMillis] configures a delay (in milliseconds) between the stopping of
     *   the sharing coroutine and the resetting of the replay cache
     *   (which makes the cache empty for the [shareIn] operator and resets the cached value
     *   to the original `initialValue` for the [stateIn] operator).
     *   It defaults to `Long.MAX_VALUE` (keep replay cache forever, never reset buffer).
     *   Use zero value to expire the cache immediately.
     */
    @Suppress("FunctionName")
    public fun WhileSubscribed(
        stopTimeoutMillis: Long = 0,
        replayExpirationMillis: Long = Long.MAX_VALUE
    ): SharingStarted = StartedWhileSubscribed(stopTimeoutMillis, replayExpirationMillis)
}