Android Room with LiveData + ViewModel 刷新问题

Question

I have a small app I am using to try learn more about some of the newer Android components. I'm finding it difficult to find information and understand how best to do what I want.

Currently: Open app -> load data + stores in DB -> display data in list

I want to be able to query data again upon button press. I have 2 buttons, 1 to fetch data again, 1 to delete the list data from the DB.

Problem is that it seems you cannot refresh if you are observing on an instance of LiveData, which I am. I understand that however the way I found to actually do a Network call and store in the Database returns an instance of LiveData and I am not sure how best to proceed.

Let me show you the code.

Fragment

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()
}

Repository

    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 This is a class I saw online for handling what I want to do. I think the issue stems from here as it is returning LiveData, I'm not sure how best to fix it

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, data, message)
    }

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

Apologies for the very long message, but I guess I need to show all the little bits and bobs I'm using. I think the problem is in the Fragment where I do viewModel.quoteLiveDataList.observe , as it is returning a new LiveData if it is called again. So I'm not sure how I can do another server call and update the DB and return it here. Appreciate any help! Thanks

Answer 1

Use Transformations.switchMap on a MutableLiveData to trigger your repository call like it is done here in the GithubBrowserSample project. This will allow you to implement the refresh functionality -

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
    }
}