不确定如何使用 DAO 将 Cursor 转换为此方法的返回类型

Question

I have an issue with return type in DAO interface. I have made research and I have tried make it without/with suspend and still the same.

Error: Not sure how to convert a Cursor to this method's return type ...

Dao

    @Query("SELECT * FROM notification_list ORDER BY id ASC")
    abstract fun readAllDataState(): DataState<List<NotificationItemsResponse.NotificationItemData>>

My UseCase:

class GetNotificationListItemDetailsUseCase @Inject constructor(private val notificationDao: NotificationDao): BaseFlowUseCase<Unit, DataState<List<NotificationItemsResponse.NotificationItemData>>>() {
    override fun create(params: Unit): Flow<DataState<List<NotificationItemsResponse.NotificationItemData>>> {
        return flow{
            emit(DataState.Loading)

            try {
                emit(DataState.Success(notificationDao.readAllDataState()))
            } catch(e: Exception) {
                emit(DataState.Error(e)) // error, and send the exception
            }

ViewModel

    val notificationData = MutableStateFlow<List<NotificationItemsResponse.NotificationItemData>>(emptyList())

[..]
    fun getActualState() {
        viewModelScope.launch {
            getNotificationListItemDetailsUseCase.build(Unit).collect {
                notificationData.value = it
                Log.d("test2", "${notificationData.value}")

            }
        }
    }

Fragment

        GlobalScope.launch { collectNotificationItems() }

[..]

    private suspend fun collectNotificationItems() {
        vm.notificationData.collectLatest { dataState ->
            when(dataState) {
                is DataState.Error -> {
                    collectErrorState()
                    Log.d("collectNotificationItems", "Collect ErrorState")
                }
                DataState.Loading -> {
                    Log.d("collectNotificationItems", "Collect Loading")
                }
                is DataState.Success<*> -> {
                    vm.notificationData.collectWith(viewLifecycleOwner) {
                        notificationAdapter.items = it
                        notificationAdapter.notifyDataSetChanged()
                        Log.d("collectNotificationItems", "Collect Sucess")
                    }
                }
            }
        }

There are all details above I hope it is enough to solve this issue.

Answer 1

Room doesn't know how to return a DataState type. Change readAllDataState() 's return type to List<NotificationItemsResponse.NotificationItemData> and make the function suspend , that way, you'll get only one value for each call. Then, just wrap the returned value in emit(DataState.Success(returnedValue)) . Like this:

@Query("SELECT * FROM notification_list ORDER BY id ASC")
abstract suspend fun readAllDataState(): List<NotificationItemsResponse.NotificationItemData>

class GetNotificationListItemDetailsUseCase @Inject constructor(private val notificationDao: NotificationDao): BaseFlowUseCase<Unit, DataState<List<NotificationItemsResponse.NotificationItemData>>>() {

  override fun create(params: Unit): Flow<DataState<List<NotificationItemsResponse.NotificationItemData>>> = 
    flow {
        emit(DataState.Loading)
        try {
          emit(DataState.Success(notificationDao.readAllDataState()))
        } catch(e: Exception) {
          emit(DataState.Error(e)) // error, and send the exception
        }
    }