问题在 Recyclerview 上使用 LiveData、Retrofit、Coroutine 实施更新 UI：适配器 Recyclerview 未更新

Question

我是新手，在我的开发应用程序中使用 Kotlin android，现在，我正在学习使用 LiveData、Retrofit、Coroutine 在 Recyclerview 上实现 Update UI。我的应用程序：

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

我在 onCreateView HomeFragment 上调用函数从服务器获取数据，并在加载时在我的 Recylerview 上显示微光数据观察这一点，成功时尝试更新 RecyclerView，并显示视图出错时加载按钮刷新失败。

问题是：

成功从服务器获取数据后，适配器 Recyclerview 未更新。适配器仍然显示微光数据
如果出现大小写错误（无互联网），我会显示加载失败视图，并带有刷新按钮。点击刷新，我重新调用函数来获取数据服务器，但函数无法正常工作。 Recyclerview 显示最后的数据，不再显示失败的加载。

下面是我的代码

HomeFragment


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
    }

NewsViewModel

class NewsViewModel: ViewModel() {

    val newsService = KopraMobileService().getNewsApi()
    var job: Job? = null
    val exceptionHandler = CoroutineExceptionHandler { _, throwable ->
        onError("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 {
                    onError("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()
    }

}

新闻适配器

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
    }

希望有人能帮我解决问题。谢谢，对不起我的英语。

Answer 1

你有 3 个不同的 LiveData，对吧？一个是新闻数据列表，一个是加载错误信息，一个是加载状态。

您为其中的每一个都设置了一个 Observer，并且只要 LiveData 的值更新，就会调用该观察函数 。这很重要，因为当您的请求成功并获得一些新数据时会发生以下情况：

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

表示newsList更新，然后 loadError更新，然后 loading更新。

因此，您的观察者按顺序为 LiveData 中的每个运行函数。但是你没有测试新值（空检查除外），所以每个中的代码总是运行s值更新。因此，当您的响应成功时，会发生这种情况：

newsList观察者运行s，显示成功，调用getDataNews

loadError 观察者运行s，值为 null 所以没有任何反应

loading观察者运行s，值为false但未检查，显示为正在加载 , 来电 getDataNewsShimmer

所以即使响应成功，你做的最后一件事就是显示加载状态

您需要先检查状态（如 loading），然后再尝试显示它。但是如果你检查 loading 是 true，你将在 fetchNews 中有一个错误 - 设置 loading = true，启动一个协程完成之后，然后立即设置loading = false.

我建议尝试创建一个 class 代表不同的状态，用一个 LiveData 代表当前状态。像这样：

// 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()} ") ) } }

然后你只需要观察 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 } }

您可能还想将该显示代码分解为单独的函数（如 showError()、showList(state.newsList) 等）并从 when 的分支中调用它们，如果这样的话它更具可读性

我希望这是有道理的！当您有一个表示状态的单个值时，使用它会容易得多 - 在事情发生变化时设置当前状态，并让您的观察者通过更新显示来处理每个可能的 UI 状态。正在加载时，使其看起来像 this。当出现错误时，让它看起来像 this

这应该有助于避免您为多个事物多次更新，试图协调所有事物的错误。我不确定为什么您在错误后重新加载时会看到该问题，但这样做可能有助于修复它（或更容易查看导致它的原因）

问题在 Recyclerview 上使用 LiveData、Retrofit、Coroutine 实施更新 UI：适配器 Recyclerview 未更新

Problem Implement Update UI with LiveData, Retrofit, Coroutine on Recyclerview : adapter Recyclerview not update

android

kotlin

android-recyclerview

mutablelivedata

kotlin-coroutines