android - 如何将 Flow 转换为 StateFlow？ - 安卓 Kotlin

Question

我是学Android开发的，看到很多话题都说LiveData不推荐用了。我的意思是它不是最新的，我们应该改用 Flows。

我正在尝试使用 Flows 从 ROOM 数据库获取数据，然后将它们转换为 StateFlow，因为我知道它们是可观察的，而且我还想向它们添加 UI 状态。比如当我成功获取数据时，状态会变为 Success，如果失败，则变为 Error。

我有一个简单的练习应用程序。它存储订阅者的姓名和电子邮件，并在回收站 View 中显示它们。

我查了很多网站，如何使用stateIn方法，如何使用StateFlows和Flows但没有成功。执行此操作的最佳方法是什么？

还有什么是更新 recyclerview 适配器的正确方法？一直在MainActivity中换一个新的adapter可以吗？

这是项目(SubscriberViewModel.kt - 第 30 行): Project link

如果我做错了其他事情，请告诉我，我想学习。我感谢任何形式的帮助。

道:

import androidx.room.*
import kotlinx.coroutines.flow.Flow

@Dao
interface SubscriberDAO {

@Insert
suspend fun insertSubscriber(subscriber : Subscriber) : Long

@Update
suspend fun updateSubscriber(subscriber: Subscriber) : Int

@Delete
suspend fun deleteSubscriber(subscriber: Subscriber) : Int

@Query("DELETE FROM subscriber_data_table")
suspend fun deleteAll() : Int

@Query("SELECT * FROM subscriber_data_table")
fun getAllSubscribers() : Flow<List<Subscriber>>

@Query("SELECT * FROM subscriber_data_table WHERE :id=subscriber_id")
fun getSubscriberById(id : Int) : Flow<Subscriber>

}

View 模型:

class SubscriberViewModel(private val repository: SubscriberRepository) : ViewModel() {

private var isUpdateOrDelete = false
private lateinit var subscriberToUpdateOrDelete: Subscriber

val inputName = MutableStateFlow("")
val inputEmail = MutableStateFlow("")

private val _isDataAvailable = MutableStateFlow(false)
val isDataAvailable : StateFlow<Boolean>
    get() = _isDataAvailable

val saveOrUpdateButtonText = MutableStateFlow("Save")
val deleteOrDeleteAllButtonText = MutableStateFlow("Delete all")

/*
//TODO - How to implement this as StateFlow<SubscriberListUiState> ??
//private val _subscribers : MutableStateFlow<SubscriberListUiState>
//val subscribers : StateFlow<SubscriberListUiState>
    get() = _subscribers
*/

private fun clearInput() {
    inputName.value = ""
    inputEmail.value = ""
    isUpdateOrDelete = false
    saveOrUpdateButtonText.value = "Save"
    deleteOrDeleteAllButtonText.value = "Delete all"
}

fun initUpdateAndDelete(subscriber: Subscriber) {
    inputName.value = subscriber.name
    inputEmail.value = subscriber.email
    isUpdateOrDelete = true
    subscriberToUpdateOrDelete = subscriber
    saveOrUpdateButtonText.value = "Update"
    deleteOrDeleteAllButtonText.value = "Delete"
}

fun saveOrUpdate() {
    if (isUpdateOrDelete) {
        subscriberToUpdateOrDelete.name = inputName.value
        subscriberToUpdateOrDelete.email = inputEmail.value
        update(subscriberToUpdateOrDelete)
    } else {
        val name = inputName.value
        val email = inputEmail.value
        if (name.isNotBlank() && email.isNotBlank()) {
            insert(Subscriber(0, name, email))
        }
        inputName.value = ""
        inputEmail.value = ""
    }
}

fun deleteOrDeleteAll() {
    if (isUpdateOrDelete) {
        delete(subscriberToUpdateOrDelete)
    } else {
        deleteAll()
    }
}

private fun insert(subscriber: Subscriber) = viewModelScope.launch(Dispatchers.IO) {
    repository.insert(subscriber)
    _isDataAvailable.value = true
}

private fun update(subscriber: Subscriber) = viewModelScope.launch(Dispatchers.IO) {
    repository.update(subscriber)
    clearInput()
}

private fun delete(subscriber: Subscriber) = viewModelScope.launch(Dispatchers.IO) {
    repository.delete(subscriber)
    clearInput()
}

private fun deleteAll() = viewModelScope.launch(Dispatchers.IO) {
    repository.deleteAll()
    //_subscribers.value = SubscriberListUiState.Success(emptyList())
    _isDataAvailable.value = false
}

sealed class SubscriberListUiState {
    data class Success(val list : List<Subscriber>) : SubscriberListUiState()
    data class Error(val msg : String) : SubscriberListUiState()
}
}

主要 Activity :

class MainActivity : AppCompatActivity() {
private lateinit var binding: ActivityMainBinding
private lateinit var viewModel: SubscriberViewModel
private lateinit var viewModelFactory: SubscriberViewModelFactory

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    binding = DataBindingUtil.setContentView(this, R.layout.activity_main)
    val dao = SubscriberDatabase.getInstance(application).subscriberDAO
    viewModelFactory = SubscriberViewModelFactory(SubscriberRepository(dao))
    viewModel = ViewModelProvider(this, viewModelFactory)[SubscriberViewModel::class.java]
    binding.viewModel = viewModel
    binding.lifecycleOwner = this
    initRecycleView()
}

private fun initRecycleView() {
    binding.recyclerViewSubscribers.layoutManager = LinearLayoutManager(
        this@MainActivity,
        LinearLayoutManager.VERTICAL, false
    )
    displaySubscribersList()
}

private fun displaySubscribersList() {
    /*
    lifecycleScope.launch {
        repeatOnLifecycle(Lifecycle.State.STARTED) {
            viewModel.subscribers.collect { uiState ->
                when (uiState) {
                    is SubscriberViewModel.SubscriberListUiState.Success -> {
                        binding.recyclerViewSubscribers.adapter = SubscriberRecyclerViewAdapter(uiState.list) {
                            subscriber: Subscriber -> listItemClicked(subscriber)
                        }
                    }
                    is SubscriberViewModel.SubscriberListUiState.Error -> {
                        Toast.makeText(applicationContext,uiState.msg, Toast.LENGTH_LONG).show()
                    }
                }
            }
        }
    }*/
}

private fun listItemClicked(subscriber: Subscriber) {
    Toast.makeText(this, "${subscriber.name} is selected", Toast.LENGTH_SHORT).show()
    viewModel.initUpdateAndDelete(subscriber)
}

}

Answer 1

您可以使用 stateIn 方法将 Flow 类型转换为 StateFlow。

 private val coroutineScope = CoroutineScope(Job())
 private val flow: Flow<CustomType>
 val stateFlow = flow.stateIn(scope = coroutineScope)

为了将 CustomType 转换为 UIState，您可以在 Flow 上使用 transformLatest 方法。它将类似于以下内容:

stateFlow.transformLatest { customType ->
    customType.toUiState()
}

您可以在其中创建扩展函数以将 CustomType 转换为 UiState，如下所示:

fun CustomType.toUiState() = UiState(
x = x,
y = y... and so on.
)