Android

Question

我有来自房间数据库的情节流。我可以毫无问题地观察到这个流的实时数据。

但我还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。你能帮忙或提出其他建议吗？

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()
    }
}

此流量来自 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”，我将剧集保存在这样的实例变量中，但我想尽可能避免这样做：

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()
}

=================== UPDATE/SOLUTION 2015 年 1 月 21 日 ================ ==

受 beigirad 启发的解决方案（请参阅下面的 post）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)
}

Answer 1

您粘贴的代码应该可以工作，因为 launch 的回调已经在协同程序中运行，您可以从那里调用 suspend 函数。

非阻塞解决方案可能是您使用 LiveData 或 StateFlow。在两者上，您都可以检查属性 value 如果没有发出任何值或将包含最后发出的值，它将是 null。

您能否提供更多 ViewModel 代码？

Answer 2

您可以将流转换为 StateFlow by stateIn 扩展，然后使用其 value 属性获取最新值。

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

Android - 如何从 kotlin 流中读取值？

Android - How to read a value from a kotlin flow?

kotlin-coroutines

kotlin-flow

kotlin-stateflow