无法从ViewModel向用户显示协程错误消息

Posted 2021-04-19

当Retrofit API调用出现问题时，我正在尝试向用户显示错误消息。我正在使用Kotlin协程，Kodein和MVVM模式。鉴于在片段中未真正观察到MutableLiveData exceptionMessage，因此我无法在烤面包消息中显示错误消息（我猜想这与异步函数的性质有关，该异步函数通过API获取数据（暂停乐趣） ））。

ViewModel：

class BarcodeViewModel(
    private val barcodeRepository: BarcodeRepository,
    private val productRepository: ProductRepository
) : ViewModel() {

    var exceptionMessage: MutableLiveData<String> = MutableLiveData()

    private val handler = CoroutineExceptionHandler { _, exception ->
        exceptionMessage.value = exception.localizedMessage
    }

    fun getBarcodeData(barcode: String) {
        CoroutineScope(Dispatchers.Main).launch(handler) {
            val currentArticle = barcodeRepository.getProductData(barcode)

            for (article in currentArticle.products) {
                val articleToAdd =
                    Product(...)

                val articleDb = productRepository.getProduct(barcode)
                if (articleDb.isEmpty()) {
                    productRepository.addProduct(articleToAdd)
                    exceptionMessage.value = ""
                } else {
                    exceptionMessage.value = "Product already exists"
                }
            }
        }
    }
}

片段：

class ArticleAddFragment : Fragment(), LifecycleOwner, KodeinAware {
    override val kodein: Kodein by kodein()
    private val factory: BarcodeViewModelFactory by instance()
    private lateinit var viewModel: BarcodeViewModel

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {
    val view = inflater.inflate(R.layout.fragment_article_add, container, false)

    viewModel = ViewModelProviders.of(this, factory).get(BarcodeViewModel::class.java)

    ...

    return view
}

private fun processResult(firebaseVisionBarcodes: List<FirebaseVisionBarcode>) {
    if (firebaseVisionBarcodes.isNotEmpty()) {
        for (item in firebaseVisionBarcodes) {
            when (item.valueType) {
                FirebaseVisionBarcode.TYPE_PRODUCT -> {
                    viewModel.getBarcodeData(item.rawValue!!)
                    viewModel.exceptionMessage.observe(viewLifecycleOwner, Observer {
                        it?.let {
                            if (!it.isBlank()) {
                                Toast.makeText(context, it, Toast.LENGTH_SHORT).show()
                            }
                        }
                    })
                    ...
                }
            }
        }
    }
}

在吐司中显示错误消息的最明智的方法是什么？

答案

我建议使用密封类或枚举来更新和观察状态。

State.kt

sealed class State<T> {
    class Loading<T> : State<T>()

    data class Success<T>(val data: T) : State<T>()

    data class Error<T>(val message: String) : State<T>()

    companion object {

        /**
         * Returns [State.Loading] instance.
         */
        fun <T> loading() = Loading<T>()

        /**
         * Returns [State.Success] instance.
         * @param data Data to emit with status.
         */
        fun <T> success(data: T) = Success(data)

        /**
         * Returns [State.Error] instance.
         * @param message Description of failure.
         */
        fun <T> error(message: String) = Error<T>(message)
    }

}

这些类是通用的，因此您可以根据成功和失败来相应地放置数据。