問題在Recyclerview上使用LiveData、Retrofit、Coroutine實作更新UI：配接器Recyclerview未更新-有解無憂

我是在我的開發應用程式 android 上使用 Kotlin 的新手，現在，我逐步學習在 Recyclerview 上使用 LiveData、Retrofit、Coroutine 實作更新 UI。我的應用程式：

MainActivity > MainFragment with 3 Tab fragment > HomeFragment, DashboardFragment, and SettingsFragment

我在onCreateView HomeFragment上呼叫函式從服務器獲取資料，并在加載時在我的 Recylerview 上顯示閃光資料來觀察這一點，成功時嘗試更新 RecyclerView，并在錯誤時顯示視圖加載失敗按鈕重繪。

問題是：

成功從服務器獲取資料時配接器 Recyclerview 不更新。配接器仍顯示微光資料
出現案例錯誤（沒有互聯網），我顯示視圖加載失敗，按鈕重繪。點擊重繪，我重新呼叫函式來獲取資料服務器，但函式不能正常作業。Recyclerview 顯示最后的資料，不再顯示加載失敗。

咆哮我的代碼

首頁片段


private var _binding: FragmentHomeBinding? = null
private val binding get() = _binding!!
private lateinit var adapterNews: NewsAdapter
private var shimmerNews: Boolean = false
private var itemsDataNews = ArrayList<NewsModel>()

override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View {
// Inflate the layout for this fragment
_binding = FragmentHomeBinding.inflate(inflater, container, false)
......

newsViewModel = ViewModelProvider(requireActivity()).get(NewsViewModel::class.java)
//news
binding.frameNews.rvNews.setHasFixedSize(true)
binding.frameNews.rvNews.layoutManager = llmh
adapterNews = NewsAdapter(itemsDataNews, shimmerNews)
binding.frameNews.rvNews.adapter = adapterNews
// Observe

//get News
newsViewModel.refresh()

newsViewModel.newsList.observe(
            viewLifecycleOwner,
            androidx.lifecycle.Observer { newsList ->
                newsList?.let {
                    binding.frameNews.rvNews.visibility = View.VISIBLE
                    binding.frameNews.rvNews.isNestedScrollingEnabled = true
                    binding.frameNews.itemNewsLayoutFailed.visibility = View.GONE
                    if (it.size == 0)
                        binding.frameNews.root.visibility = View.GONE
                    else
                        getDataNews(it)
                }
            })
        newsViewModel.loading.observe(viewLifecycleOwner) { isLoading ->
            isLoading?.let {
                binding.frameNews.rvNews.visibility = View.VISIBLE
                binding.frameNews.rvNews.isNestedScrollingEnabled = false
                binding.frameNews.itemNewsLayoutFailed.visibility = View.GONE
                getDataNewsShimmer()
            }
        }

        newsViewModel.loadError.observe(viewLifecycleOwner) { isError ->
            isError?.let {
                binding.frameNews.rvNews.visibility = View.INVISIBLE
                binding.frameNews.itemNewsLayoutFailed.visibility = View.VISIBLE
                binding.frameNews.btnNewsFailed.setOnClickListener {
                    newsViewModel.refresh()
                }

            }
        }
....

return binding.root
}

@SuppressLint("NotifyDataSetChanged")
    private fun getDataNewsShimmer() {
        shimmerNews = true
        itemsDataNews.clear()
        itemsDataNews.addAll(NewsData.itemsShimmer)
        adapterNews.notifyDataSetChanged()
    }

    @SuppressLint("NotifyDataSetChanged")
    private fun getDataNews(list: List<NewsModel>) {
        Toast.makeText(requireContext(), list.size.toString(), Toast.LENGTH_SHORT).show()
        shimmerNews = false
        itemsDataNews.clear()
        itemsDataNews.addAll(list)
        adapterNews.notifyDataSetChanged()
    }

override fun onDestroyView() {
        super.onDestroyView()
        _binding=null
    }

新聞視圖模型

class NewsViewModel: ViewModel() {

    val newsService = KopraMobileService().getNewsApi()
    var job: Job? = null
    val exceptionHandler = CoroutineExceptionHandler { _, throwable ->
        one rror("Exception handled: ${throwable.localizedMessage}")
    }

    val newsList = MutableLiveData<List<NewsModel>>()
    val loadError = MutableLiveData<String?>()
    val loading = MutableLiveData<Boolean>()

    fun refresh() {
        fetchNews()
    }

    private fun fetchNews() {
        loading.postValue(true)
        job = CoroutineScope(Dispatchers.IO   exceptionHandler).launch {
            val response = newsService.getNewsList()
            withContext(Dispatchers.Main) {
                if (response.isSuccessful) {
                    newsList.postValue(response.body()?.data)
                    loadError.postValue(null)
                    loading.postValue(false)
                } else {
                    one rror("Error : ${response.message()} ")
                }
            }
        }
        loadError.postValue("")
        loading.postValue( false)
    }

    private fun onError(message: String) {
        loadError.postValue(message)
        loading.postValue( false)
    }

    override fun onCleared() {
        super.onCleared()
        job?.cancel()
    }

}

新聞配接器

NewsAdapter(
var itemsCells: List<NewsModel?>  = emptyList(),
    var shimmer: Boolean ,
) :
    RecyclerView.Adapter<ViewHolder>() {

    private lateinit var context: Context

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): NewsHomeViewHolder {
        context = parent.context
        return NewsHomeViewHolder(
                NewsItemHomeBinding.inflate(LayoutInflater.from(parent.context), parent, false)
            )
        

    }


    override fun onBindViewHolder(holder: NewsHomeViewHolder, position: Int) {
     
            holder.bind(itemsCells[position]!!)
        
    }

    inner class NewsHomeViewHolder(private val binding: NewsItemHomeBinding) :
        ViewHolder(binding.root) {
        fun bind(newsItem: NewsModel) {
          
            binding.newsItemFlat.newsTitle.text = newsItem.name
            binding.newsItemFlatShimmer.newsTitle.text = newsItem.name

            binding.newsItemFlat.newsSummary.text = newsItem.name
            binding.newsItemFlatShimmer.newsSummary.text = newsItem.name

            if (shimmer) {
                binding.layoutNewsItemFlat.visibility = View.INVISIBLE
                binding.layoutNewsItemFlatShimmer.visibility = View.VISIBLE
                binding.layoutNewsItemFlatShimmer.startShimmer()
            } else {

                binding.layoutNewsItemFlat.visibility = View.VISIBLE
                binding.layoutNewsItemFlatShimmer.visibility = View.INVISIBLE
            }
        }
    }

    override fun getItemCount(): Int {
        return itemsCells.size
    }

我希望有人可以幫助我解決問題。謝謝，對不起我的英語。

uj5u.com熱心網友回復：

你有3個不同LiveData的s，對吧？一個帶有新聞資料串列，一個帶有加載錯誤訊息，一個帶有加載狀態。

您為其中的每一個設定了一個，并且每當' 的值更新Observer時都會呼叫該觀察者函式。這很重要，因為當您的請求成功時會發生以下情況，并且您會獲得一些新資料：LiveData

if (response.isSuccessful) {
    newsList.postValue(response.body()?.data)
    loadError.postValue(null)
    loading.postValue(false)
}

這意味著newsList更新，然后 loadError更新，然后 loading更新。

因此，您的每個LiveDatas 的觀察者函式按該順序運行。但是您沒有測驗新值（空檢查除外），因此每個值中的代碼總是在值更新時運行。因此，當您的回應成功時，會發生這種情況：

newsList觀察者運行，顯示為成功，呼叫getDataNews
loadError觀察者運行，值為空，所以什么也沒有發生
loading觀察者運行，值為false但未檢查，顯示為 loading，呼叫getDataNewsShimmer

所以即使回應成功，你做的最后一件事就是顯示加載狀態

loading在嘗試顯示狀態之前，您需要檢查狀態（如）。但是如果你檢查這loading是真的，你會有一個錯誤fetchNews——設定loading = true，啟動一個稍后完成的協程，然后立即設定loading = false。

我建議嘗試創建一個代表不同狀態的類，其中一個LiveData代表當前狀態。像這樣的東西：

// a class representing the different states, and any data they need
sealed class State {
    object Loading : State()
    data class Success(val newsList: List<NewsModel>?) : State()
    data class Error(val message: String) : State()
}

// this is just a way to keep the mutable LiveData private, so it can't be updated
private val _state = MutableLiveData<State>()
val state: LiveData<State> get() = _state

private fun fetchNews() {
    // initial state is Loading, until we get a response
    _state.value = State.Loading
    job = CoroutineScope(Dispatchers.IO   exceptionHandler).launch {
        val response = newsService.getNewsList()

        // if you're using postValue I don't think you need to switch to Dispatchers.Main?
        _state.postValue(
            // when you get a response, the state is now either Success or Error
            if (response.isSuccessful) State.Success(response.body()?.data)
            else State.Error("Error : ${response.message()} ")
        )
    }
}

然后你只需要觀察state：

// you don't need to create an Observer object, you can use a lambda!
newsViewModel.state.observe(viewLifecycleOwner) { state ->
    // Handle the different possible states, and display the current one

    // this lets us avoid repeating 'binding.frameNews' before everything
    with(binding.frameNews) {
        // You could use a when block, and describe each state explicitly,
        // like your current setup:
        when(state) {
            // just checking equality because Loading is a -singleton object instance-
            State.Loading -> {
                rvNews.visibility = View.VISIBLE
                rvNews.isNestedScrollingEnabled = false
                itemNewsLayoutFailed.visibility = View.GONE
                getDataNewsShimmer()
            }
            // Error and Success are both -classes- so we need to check their type with 'is'
            is State.Error -> {
                rvNews.visibility = View.INVISIBLE
                itemNewsLayoutFailed.visibility = View.VISIBLE
                btnNewsFailed.setOnClickListener {
                    newsViewModel.refresh()
                }
            }
            is State.Success -> {
                rvNews.visibility = View.VISIBLE
                rvNews.isNestedScrollingEnabled = true
                itemNewsLayoutFailed.visibility = View.GONE

                // Because we know state is a Success, we can access newsList on it
                // newsList can be null - I don't know how you want to handle that,
                // I'm just treating it as defaulting to size == 0
                // (make sure you make this visible when necessary too)
                if (state.newsList?.size ?: 0 == 0) root.visibility = View.GONE
                else getDataNews(state.newsList)
            }
        }

        // or, if you like, you could do this kind of thing instead:
        itemNewsLayoutFailed.visibility = if (state is Error) VISIBLE else GONE
    }
}

您可能還想將該顯示代碼分解為單獨的函式（如showError()等showList(state.newsList)）并從的分支呼叫它們when，如果這使其更具可讀性

我希望這是有道理的！當您有一個表示狀態的值時，使用起來會容易得多 - 隨著事物的變化設定當前狀態，并通過更新顯示讓您的觀察者處理每個可能的 UI 狀態。加載時，讓它看起來像這樣。當出現錯誤時，讓它看起來像這樣

這應該有助于避免您為多個事物多次更新，試圖協調一切的錯誤。我不確定為什么在發生錯誤后重新加載時會看到該問題，但這樣做可能有助于修復它（或者更容易查看導致它的原因）

轉載請註明出處，本文鏈接：https://www.uj5u.com/net/479634.html

標籤：安卓科特林 android-recyclerview kotlin 协程可变实时数据

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