Android |科特林 |流 - 无法转换为 kotlinx.coroutines.flow.StateFlow

Question

我正在努力让自己熟悉 DataStore，所以在我当前的项目中，我正在尝试使用它。

在我的依赖中。我已经添加了：

    implementation "androidx.datastore:datastore-preferences:1.0.0-alpha06"

然后我创建了这个 class 来处理数据存储：

class BasicDataStore(context: Context) :
    PrefsDataStore(
        context,
        PREF_FILE_BASIC
    ),
    BasicImpl {
    override val serviceRunning: Flow<Boolean>
        get() = dataStore.data.map { preferences ->
            preferences[SERVICE_RUNNING_KEY] ?: false
        }
    override suspend fun setServiceRunningToStore(serviceRunning: Boolean) {
        dataStore.edit { preferences ->
            preferences[SERVICE_RUNNING_KEY] = serviceRunning
        }
    }
    companion object {
        private const val PREF_FILE_BASIC = "basic_preference"
        private val SERVICE_RUNNING_KEY = booleanPreferencesKey("service_running")
    }
}
@Singleton
interface BasicImpl {
    val serviceRunning: Flow<Boolean>
    suspend fun setServiceRunningToStore(serviceRunning: Boolean)
}

在我的视图模型中，我尝试使用该值，如下所示：

class MainViewModel(application: Application) : AndroidViewModel(application) {
    ...
    private val basicDataStore = BasicDataStore(application)
    val serviceRunning
            : StateFlow<Boolean> get()
            = basicDataStore.serviceRunning as StateFlow<Boolean>
    fun setServiceRunning(serviceRunning: Boolean) {
        viewModelScope.launch(IO) {
            basicDataStore.setServiceRunningToStore(serviceRunning)
        }
    }
}

但它给了我以下错误：

Caused by: java.lang.ClassCastException: com.mua.roti.data.datastore.BasicDataStore$serviceRunning$$inlined$map cannot be cast to kotlinx.coroutines.flow.StateFlow
        at com.mua.roti.viewmodel.MainViewModel.getServiceRunning(MainViewModel.kt:33)
...

在xml中，在UI部分：


        <TextView
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="@{main.serviceRunning.value.toString()}" />

有了 viewmodel，一切都变得如此酷和简单，易于阅读和实施。现在我对 Flow 感到困惑。谢谢。

Answer 1

流的层次结构如下：StateFlow -> SharedFlow -> Flow

所以您不能真正转换它，如果您想将冷流转换为热 StateFlow，则应该使用 stateIn() 运算符。在你的情况下：

val serviceRunning: StateFlow<Boolean> 
    get() = basicDataStore.serviceRunning.stateIn(viewModelScope, SharingStarted.Lazily, false)

您可以调整状态流的 SharingStarted and/or 初始值

Android |科特林 |流 - 无法转换为 kotlinx.coroutines.flow.StateFlow

Android | Kotlin | Flow - cannot be cast to kotlinx.coroutines.flow.StateFlow

android

android-lifecycle

android-architecture-components

kotlin-coroutines

android-jetpack-datastore