Android Room with LiveData + ViewModel 刷新问题

Question

我正在使用一个小应用程序来尝试了解有关一些较新的 Android 组件的更多信息。我发现很难找到信息并了解如何最好地做我想做的事。

目前：打开应用->加载数据+数据库存储->在列表中显示数据

我希望能够在按下按钮时再次查询数据。 我有 2 个按钮，1 个用于再次获取数据，1 个用于从数据库中删除列表数据。

问题是，如果您正在观察 LiveData 的实例，我就是这样，您似乎无法刷新。我知道，但是我发现实际执行网络调用并存储在数据库中的方式会返回 LiveData 的实例，但我不确定如何最好地继续。

让我给你看代码。

片段

private val viewModel: quoteViewModel by viewModels()
private lateinit var binding: FragmentHomeBinding
private lateinit var adapter: QuoteAdapter

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {
    binding = FragmentHomeBinding.inflate(inflater, container, false)
    return binding.root
}

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)

    initRecyclerView()
    setupRetrieveQuotesObserver()
    setupDeleteDataListener()
    setupFetchNewDataListener()
    setupSwipeToRefresh()
}

private fun initRecyclerView() {
    adapter = QuoteAdapter()
    binding.recyclerView.layoutManager = LinearLayoutManager(requireContext())
    binding.recyclerView.adapter = adapter
}

private fun setupDeleteDataListener() {
    binding.removeQuotesButton.setOnClickListener {
        viewModel.removeAllQuotes()
    }
}

private fun setupFetchNewDataListener() {
    binding.getQuotesButton.setOnClickListener {
        viewModel.removeQuotes()
        viewModel.getQuotes()
    }
}

private fun setupRetrieveQuotesObserver() {
    viewModel.quoteLiveDataList.observe(viewLifecycleOwner, Observer { result ->
        when (result.status) {
            NewResult.Status.SUCCESS -> {
                result.data.let { adapter.setItems(ArrayList(result.data)) }
                binding.progressBar.visibility = View.GONE
                binding.swipeContainer.isRefreshing = false
            }

            NewResult.Status.ERROR -> {
                binding.progressBar.visibility = View.GONE
                Snackbar.make(binding.root, "Some error has occurred", Snackbar.LENGTH_SHORT)
                    .show()
            }

            NewResult.Status.LOADING -> {
                binding.progressBar.visibility = View.VISIBLE
            }
        }
    })
}

private fun setupSwipeToRefresh() {
    binding.swipeContainer.setOnRefreshListener {
        viewModel.getQuotes()
    }
}

视图模型

val quoteLiveDataList: LiveData<NewResult<List<Quote>>> = repository.quotes

fun getQuotes() = viewModelScope.launch {
    repository.quotes
}

fun removeAllQuotes() = viewModelScope.launch {
    repository.deleteAllQuotes()
}

存储库

    val quotes = performGetOperation(
    databaseQuery = { dao.getAllQuotes() },
    networkCall = { remoteSource.getAllQuotes() },
    saveCallResult = {
        val quotesList = ArrayList<Quote>()

        for (messageString in it.messages.non_personalized) {
            quotesList.add(
                Quote(
                    messageString,
                    FaceImageProvider().getRandomFacePicture(),
                    false
                )
            )
        }

        dao.insertQuotes(quotesList)
    }
)

@WorkerThread
suspend fun deleteAllQuotes() = withContext(Dispatchers.IO) { dao.deleteAllQuotes() }

执行GetOperation 这是我在网上看到的一个处理我想做的事情的课程。我认为问题源于这里，因为它正在返回 LiveData，我不确定如何最好地解决它

fun <T, A> performGetOperation(
databaseQuery: () -> LiveData<T>,
networkCall: suspend () -> NewResult<A>,
saveCallResult: suspend (A) -> Unit
): LiveData<NewResult<T>> =
liveData(Dispatchers.IO) {
    emit(NewResult.loading())
    val source = databaseQuery.invoke().map { NewResult.success(it) }
    emitSource(source)

    val responseStatus = networkCall.invoke()
    if (responseStatus.status == NewResult.Status.SUCCESS) {
        saveCallResult(responseStatus.data!!)

    } else if (responseStatus.status == NewResult.Status.ERROR) {
        emit(NewResult.error(responseStatus.message!!))
        emitSource(source)
    }
}

远程数据源

suspend fun getQuotes() = getResult { service.getQuotes() }

getResult

    protected suspend fun <T> getResult(call: suspend () -> Response<T>): NewResult<T> {
    try {
        val response = call.invoke()
        if (response.isSuccessful) {
            val body = response.body()
            if (body != null) {
                return NewResult.success(body)
            }
        }

        return error("${response.code()} ${response.message()}")
    } catch (e: Exception) {
        return error(e.message ?: e.toString())
    }
}

private fun <T> error(message: String): NewResult<T> {
    Log.d("BaseDataSource", message)
    return NewResult.error("Network called failed due to:  $message")
}

新结果

data class NewResult<out T>(val status: Status, val data: T?, val message: String?) {

enum class Status {
    SUCCESS,
    ERROR,
    LOADING,
}

companion object {
    fun <T> success(data: T): NewResult<T> {
        return NewResult(Status.SUCCESS, data, null)
    }

    fun <T> error(message: String, data: T? = null): NewResult<T> {
        return NewResult(Status.ERROR, data, message)
    }

    fun <T> loading(data: T? = null): NewResult<T> {
        return NewResult(Status.LOADING, data, null)
    }
}

对于很长的信息，我深表歉意，但我想我需要展示我正在使用的所有小细节。 我认为问题出在我执行 viewModel.quoteLiveDataList.observe 的片段中，因为如果再次调用它，它将返回一个新的 LiveData。所以我不确定如何进行另一个服务器调用并更新数据库并将其返回这里。 感谢任何帮助！ 谢谢

Answer 1

在MutableLiveData 上使用Transformations.switchMap 来触发您的存储库调用，就像在GithubBrowserSample 项目中完成here 一样。这将允许您实现刷新功能 -

private val _getQuotes = MutableLiveData<Boolean>()

val quotes: LiveData<NewResult<List<Quote>>> = _getQuotes.switchMap { getQuotes ->
    repository.quotes
}

fun getQuotes() {
    _getQuotes.value = true
}

fun refresh() {
    _getQuotes.value?.let {
        _getQuotes.value = it
    }
}