带有LiveData + ViewModel的Android Room刷新问题

问题描述

我有一个小应用程序，正在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。

当前：打开应用->加载数据+存储在数据库中->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，其中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，如果您观察的是LiveData实例，则似乎无法刷新。我知道，但是我发现实际上进行网络调用并将其存储在数据库中的方式返回了LiveData的实例，我不确定如何最好地进行。

让我告诉你代码。

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()
    }
}

ViewModel

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() }

performGetOperation 这是我在网上看到的用于处理我想做的事情的课程。我认为问题出在这里，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)
    }
}

RemoteDataSource

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")
}

NewResult

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,message)
    }

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

很长的信息表示歉意，但是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Fragment中，因为如果再次调用它，它将返回一个新的LiveData。因此，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此处。 感谢任何帮助！ 谢谢

解决方法

像在GithubBrowserSample项目中here一样，在Transformations.switchMap上使用MutableLiveData来触发存储库调用。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
    }
}