问题描述
我有一个房间数据库中的剧集流。 我可以毫无问题地将此流程作为实时数据进行观察。
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。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()
}
}
这个流程来自 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,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,Snackbar.LENGTH_SHORT).show()
}
================== 更新/解决方案 21/01/15 ==================>
受 beigirad 启发的解决方案(见下面他的帖子)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)
}
解决方法
您可以通过 StateFlow 扩展将流转换为 stateIn,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)
,
您粘贴的代码应该可以工作,因为 launch
的回调已经在协程内运行,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,则该属性将为 null
。
能否提供更多的 ViewModel
代码?