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android - Android - 如何从 kotlin 流中读取值？

问题描述

我有来自房间数据库的情节流。我可以毫无问题地观察这个流的实时数据。

但我也想在用户单击按钮时从该流程中读取最后一个值。我尝试使用 first() 终端流运算符，但它无法编译。你能帮忙或建议别的吗？

从流中读取的非编译尝试：

bd.buttonNext.setOnClickListener {
    lifecycleScope.launch {
        val episode: Episode? = viewModel.episodeFlow().first()           <=== Compile ERROR
        Snackbar.make(bd.root, "episode ${episode?.name}", Snackbar.LENGTH_SHORT).show()
    }
}

此流程来自 ROOM ：

@Query("SELECT * FROM Episode WHERE id = :id")
fun getEpisode(id: Long): Flow<Episode?>

存储库：

fun getEpisode(episodeId: Long): Flow<Episode?> = dao.getEpisode(episodeId)

查看模型 - Id 来自 StateFlow ：

fun episodeFlow(): Flow<Episode?> 
  = episodeIdStateFlow.flatMapLatest { episodeId ->
    repository.getEpisode(episodeId)
  }

编译错误：

Unresolved reference. None of the following candidates is applicable because of receiver type mismatch: 
public fun <T> Array<out TypeVariable(T)>.first(): TypeVariable(T) defined in kotlin.collections
public inline fun <T> Array<out TypeVariable(T)>.first(predicate: (TypeVariable(T)) -> Boolean): TypeVariable(T) defined in kotlin.collections
public fun BooleanArray.first(): Boolean defined in kotlin.collections
public inline fun BooleanArray.first(predicate: (Boolean) -> Boolean): Boolean defined in kotlin.collections
public fun ByteArray.first(): Byte defined in kotlin.collections
public inline fun ByteArray.first(predicate: (Byte) -> Boolean): Byte defined in kotlin.collections
public fun CharArray.first(): Char defined in kotlin.collections
public inline fun CharArray.first(predicate: (Char) -> Boolean): Char defined in kotlin.collections
public fun CharSequence.first(): Char defined in kotlin.text

作为一种“解决方法”，我将这一集保存在这样的实例变量中，但如果可能的话，我想避免这样做：

var episode: Episode? = null
...
viewModel.episodeFlow().asLiveData().observe(this) { episode ->
   this.episode = episode
}
...
bd.buttonNext.setOnClickListener {
   Snackbar.make(bd.root, "episode ${episode?.name}", Snackbar.LENGTH_SHORT).show()
}

=================== 更新/解决方案 21/01/15 ==================

使用 stateIn 受 beigirad 启发的解决方案（请参阅下面的帖子）：

private val _episodeSF  = episodeFlow().stateIn(viewModelScope, SharingStarted.Eagerly, null)
val episodeSF: StateFlow<Episode?>
  get() = _episodeSF

fun episodeFlow(): Flow<Episode?> = episodeIdSF.flatMapLatest { episodeId ->
  repository.episodeFlow(episodeId)
}

标签： androidkotlin-flowkotlin-coroutineskotlin-stateflow

解决方案

您可以通过stateIn扩展将您的流转换为StateFlow，然后使用其属性获取最新值。value

fun episodeFlow(): StateFlow<Episode?> 
  = episodeIdStateFlow.flatMapLatest { episodeId ->
    repository.getEpisode(episodeId)
  }.stateIn(viewModelScope, SharingStarted.Lazily, initialValue)

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