为什么房间Livedata不返回值？

Question

我对返回livedata的房间有问题。

我创建dao，并使用功能返回数据列表。我想返回为livedata。但是，它无法正常工作。

dao函数

@Transaction
@Query("SELECT * FROM AllocationPercentage WHERE id IN (:ids)")
fun getByIds(ids: List<Long>): LiveData<List<AllocationPercentageWithDetails>>

这是我在ViewModel中观察它的方式：

class AllocationViewModel(
    private val getAllocationByIdUseCase: GetAllocationByIdUseCase,
    private val getDetailByIdUseCase: GetAllocationPercentageByIdUseCase
) : ViewModel() {

    var allocationUiState: LiveData<AllocationUiState> = MutableLiveData()
        private set

    var allocationPercentageUiState: LiveData<List<AllocationPercentageUiState>> = MutableLiveData()
        private set

    val mediatorLiveData = MediatorLiveData<List<AllocationPercentageUiState>>()

    fun getAllocationById(allocationId: Long) = viewModelScope.launch(Dispatchers.IO) {
        val result = getAllocationByIdUseCase(allocationId) // LiveData

        allocationUiState = Transformations.map(result) {
            AllocationUiState(allocation = it.allocation)
        }

        mediatorLiveData.addSource(result) { allocation ->
            Log.d(TAG, "> getAllocationById")
            val ids = allocation.percentages.map { percentage -> percentage.id }
            val detailResult: LiveData<List<AllocationPercentageWithDetails>> =
                getDetailByIdUseCase(ids) // LiveData
            allocationPercentageUiState = Transformations.map(detailResult) { details ->
                Log.d(TAG, ">> Transform : $details")
                details.map {
                    AllocationPercentageUiState(
                        id = it.allocationPercentage.id,
                        percentage = it.allocationPercentage.percentage,
                        description = it.allocationPercentage.description,
                        currentProgress = it.allocationPercentage.currentProgress
                    )
                }
            }
        }
    }
}

AllocationPercentAgeUistate通过fragment观察到。

    Log.d(TAG, "observeViewModel: ${it?.size}")
    val percentages = it ?: return@observe
    setAllocationPercentages(percentages) // update UI
}

allocationViewModel.mediatorLiveData.observe(viewLifecycleOwner) {}

getDetailbyIdUsecase只是一个直接返回DAO结果的函数。

class GetAllocationPercentageByIdUseCase(private val repository: AllocationPercentageRepository) {
    operator fun invoke(ids: List<Long>): LiveData<List<AllocationPercentageWithDetails>> {
        return repository.getAllocationPercentageByIds(ids)
    }
}

知道为什么吗？谢谢。

Answer 1

将var与livedata或mutableLivedata结合在一起是没有意义的。它打败了使用Livedata的目的。如果出现某些东西并观察您在该物业中拥有的原始Livedata，它将永远不会收到任何东西。它将无法知道有一个新的livedata实例应该观察到。

我无法确切地告诉您如何修复它，因为您上面的代码不完整，因此我无法分辨您在映射功能中要做什么，或者是否在某些函数中与在ViewModel初始化过程中进行调用。