Android:使用 kotlin 流时，Firebase 对象为空

Question

我的问题是，当我尝试从数据库中获取文档时，该文档又名对象始终为空。只有当我使用 Kotlin Coroutines 将文档从数据库中取出时，我才会遇到这个问题。对听众使用标准方法确实有效。
电子邮件存储库

interface EmailRepository {
    suspend fun getCalibratePrice(): Flow<EmailEntity?>
    suspend fun getRepairPrice(): Flow<EmailEntity?>
}

电子邮件存储库实现

class EmailRepositoryImpl @Inject constructor(private val db: FirebaseFirestore) : EmailRepository {

    fun hasInternet(): Boolean {
        return true
    }

    // This works! When using flow to write a document, the document is written!
    override fun sendEmail(email: Email)= flow {
        emit(EmailStatus.loading())
        if (hasInternet()) {
            db.collection("emails").add(email).await()
            emit(EmailStatus.success(Unit))
        } else {
            emit(EmailStatus.failed<Unit>("No Email connection"))
        }
    }.catch {
        emit(EmailStatus.failed(it.message.toString()))
    }.flowOn(Dispatchers.Main)


    // This does not work! "EmailEntity" is always null. I checked the document path!
    override suspend fun getCalibratePrice(): Flow<EmailEntity?> = flow {
        val result = db.collection("emailprice").document("Kalibrieren").get().await()
        emit(result.toObject<EmailEntity>())
    }.catch {

    }.flowOn(Dispatchers.Main)


    // This does not work! "EmailEntity" is always null. I checked the document path!
    override suspend fun getRepairPrice(): Flow<EmailEntity?> = flow {
        val result = db.collection("emailprice").document("Reparieren").get().await()
        emit(result.toObject<EmailEntity>())
    }.catch {

    }.flowOn(Dispatchers.Main)
}

我获取数据的 View 模型

init {
        viewModelScope.launch {
            withContext(Dispatchers.IO) {
                if (subject.value != null){
                    when(subject.value) {
                        "Test" -> {
                            emailRepository.getCalibratePrice().collect {
                                emailEntity.value = it
                            }
                        }
                        "Toast" -> {
                            emailRepository.getRepairPrice().collect {
                                emailEntity.value = it
                            }
                        }
                    }
                }
            }
        }
    }

private val emailEntity = MutableLiveData<EmailEntity?>()

private val _subject = MutableLiveData<String>()
val subject: LiveData<String> get() = _subject

分段

@AndroidEntryPoint
class CalibrateRepairMessageFragment() : EmailFragment<FragmentCalibrateRepairMessageBinding>(
    R.layout.fragment_calibrate_repair_message,
) {
    // Get current toolbar Title and send it to the next fragment.
    private val toolbarText: CharSequence by lazy { toolbar_title.text }

    override val viewModel: EmailViewModel by navGraphViewModels(R.id.nav_send_email) { defaultViewModelProviderFactory }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        // Here I set the data from the MutableLiveData "subject". I don't know how to do it better
        viewModel.setSubject(toolbarText.toString())
    }
}

有人会说，Firebase 规则是这里的问题，但这里不应该是这种情况，因为数据库是开放的并且使用监听器方法确实有效。
我得到 subject.value来自我的 CalibrateRepairMessageFragment .当我不检查 if(subject.value != null)我从我的 init 收到 NullPointerException堵塞。
我将使用 emailEntitiy仅在我的 viewModel 中而不是在它之外。
我感谢每一个帮助，谢谢。
编辑
这是我获取数据的新方法。该对象仍然为空!我还添加了 Timber.d我的挂起函数中的消息也永远不会被执行，因此流永远不会引发错误。使用这种新方法，我不再得到 NullPointerException

private val emailEntity = liveData {
    when(subject.value) {
        "Test" -> emailRepository.getCalibratePrice().collect {
            emit(it)
        }
        "Toast" -> emailRepository.getRepairPrice().collect {
            emit(it)
        }
        // Else block is never executed, therefore "subject.value" is either Test or toast and the logic works. Still error when using flow!
        else -> EmailEntity("ERROR", 0F)
    }
}

我用 Timber.d("EmailEntity is ${emailEntity.value}") 检查 emailEntity 是否为空在我的一项职能中。
然后我用 val price = MutableLiveData(emailEntity.value?.basePrice ?: 1000F) 设置价格但是因为 emailentity是 null价格总是1000
编辑 2
我现在进一步研究了这个问题，并向前迈出了一大步。当观察 emailEntity来自像 CalibrateRepairMessageFragment 这样的 fragment 值不再是 null .
此外，当观察 emailEntity 值也不是 null在 viewModel ，但仅当它在一个 fragment 中被观察到时!那么如何观察 emailEntity来自我的 viewModel或从我的 repository 获取值并在我的 viewmodel 中使用它?

Answer 1

好的，我已经解决了我的问题，这是最终的解决方案:
状态等级

sealed class Status<out T> {
    data class Success<out T>(val data: T) : Status<T>()
    class Loading<T> : Status<T>()
    data class Failure<out T>(val message: String?) : Status<T>()

    companion object {
        fun <T> success(data: T) = Success<T>(data)
        fun <T> loading() = Loading<T>()
        fun <T> failed(message: String?) = Failure<T>(message)
    }
}

电子邮件存储库

interface EmailRepository {
    fun sendEmail(email: Email): Flow<Status<Unit>>
    suspend fun getCalibratePrice(): Flow<Status<CalibrateRepairPricing?>>
    suspend fun getRepairPrice(): Flow<Status<CalibrateRepairPricing?>>
}

EmailRepositoryImpl

class EmailRepositoryImpl (private val db: FirebaseFirestore) : EmailRepository {
    fun hasInternet(): Boolean {
        return true
    }

    override fun sendEmail(email: Email)= flow {
        Timber.d("Executed Send Email Repository")
        emit(Status.loading())
        if (hasInternet()) {
            db.collection("emails").add(email).await()
            emit(Status.success(Unit))
        } else {
            emit(Status.failed<Unit>("No Internet connection"))
        }
    }.catch {
        emit(Status.failed(it.message.toString()))
    }.flowOn(Dispatchers.Main)

    // Sends status and object to viewModel
    override suspend fun getCalibratePrice(): Flow<Status<CalibrateRepairPricing?>> = flow {
        emit(Status.loading())
        val entity = db.collection("emailprice").document("Kalibrieren").get().await().toObject<CalibrateRepairPricing>()
        emit(Status.success(entity))
    }.catch {
        Timber.d("Error on getCalibrate Price")
        emit(Status.failed(it.message.toString()))
    }

    // Sends status and object to viewModel
    override suspend fun getRepairPrice(): Flow<Status<CalibrateRepairPricing?>> = flow {
        emit(Status.loading())
        val entity = db.collection("emailprice").document("Kalibrieren").get().await().toObject<CalibrateRepairPricing>()
        emit(Status.success(entity))
    }.catch {
        Timber.d("Error on getRepairPrice")
        emit(Status.failed(it.message.toString()))
    }
}

View 模型

private lateinit var calibrateRepairPrice: CalibrateRepairPricing

private val _calirateRepairPriceErrorState = MutableLiveData<Status<Unit>>()
val calibrateRepairPriceErrorState: LiveData<Status<Unit>> get() = _calirateRepairPriceErrorState

init {
        viewModelScope.launch {
            when(_subject.value.toString()) {
                "Toast" -> emailRepository.getCalibratePrice().collect {
                    when(it) {
                        is Status.Success -> {
                            calibrateRepairPrice = it.data!!
                            _calirateRepairPriceErrorState.postValue(Status.success(Unit))
                        }
                        is Status.Loading -> _calirateRepairPriceErrorState.postValue(Status.loading())
                        is Status.Failure -> _calirateRepairPriceErrorState.postValue(Status.failed(it.message))
                    }
                }
                else -> emailRepository.getRepairPrice().collect {
                    when(it) {
                        is Status.Success -> {
                            calibrateRepairPrice = it.data!!
                            _calirateRepairPriceErrorState.postValue(Status.success(Unit))
                        }
                        is Status.Loading -> _calirateRepairPriceErrorState.postValue(Status.loading())
                        is Status.Failure -> _calirateRepairPriceErrorState.postValue(Status.failed(it.message))
                    }
                }
            }
            price.postValue(calibrateRepairPrice.head!!.basePrice)
        }
    }

您现在可以观察其中一个 fragment 的状态(但您不需要!)
分段

viewModel.calibrateRepairPriceErrorState.observe(viewLifecycleOwner) { status ->
            when(status) {
                is Status.Success -> requireContext().toast("Price successfully loaded")
                is Status.Loading -> requireContext().toast("Price is loading")
                is Status.Failure -> requireContext().toast("Error, Price could not be loaded")
            }
        }

这是我的 toast 扩展功能:

fun Context.toast(text: String, duration: Int = Toast.LENGTH_SHORT) {
    Toast.makeText(this, text, duration).show()
}