Android - 如何从 kotlin 流中读取值？

我有来自房间数据库的剧集流程。 我可以毫无问题地观察该流程的实时数据。

但我还想在用户单击按钮时读取此流程中的最后一个值。我尝试使用first()终端流运算符，但它无法编译。您能帮忙或提出其他建议吗？

非编译尝试从流中读取：

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

该流程来自 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)

视图模型 - 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“解决方法”，我将剧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 ==================

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

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)
}

您可以将流量转换为状态流 https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/-state-flow/index.html by stateIn https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/state-in.html扩展名，然后使用它的value属性以获取最新值。

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