第 3 页初始加载未显示

Question

我正在处理第 3 页，除了初始加载状态外，一切正常。我正在添加withLoadStateFooter，但它在第一次调用时从不显示加载状态

这是我的实现

负载状态适配器

class LoadStateAdapter (
    private val retry: () -> Unit
): LoadStateAdapter<LoadStateViewHolder>() {

    override fun onBindViewHolder(holder: LoadStateViewHolder, loadState: LoadState) {
        holder.bindTo(loadState)
    }

    override fun onCreateViewHolder(
        parent: ViewGroup,
        loadState: LoadState
    ): LoadStateViewHolder {
        return LoadStateViewHolder.create(parent, retry)
    }
}

加载状态视图支架

class LoadStateViewHolder(
    view : View,
    private val retryCallback: () -> Unit
) : RecyclerView.ViewHolder(view) {

    private val progressBar = view.findViewById<ProgressBar>(R.id.progress_bar)
    private val errorMsg = view.findViewById<TextView>(R.id.error_msg)
    private val btnRetry = view.findViewById<Button>(R.id.retry_button)
        .also {
            it.setOnClickListener { retryCallback() }
        }
    private var loadState : LoadState? = null

    companion object {
        fun create(parent: ViewGroup, retryCallback: () -> Unit): LoadStateViewHolder {
            val view = LayoutInflater.from(parent.context)
                .inflate(R.layout.network_state_item, parent, false)
            return LoadStateViewHolder(
                view,
                retryCallback
            )
        }
    }


    fun bindTo(loadState: LoadState) {
        this.loadState = loadState

        btnRetry.isVisible = loadState !is LoadState.Loading
        errorMsg.isVisible = loadState !is LoadState.Loading
        progressBar.isVisible = loadState is LoadState.Loading

        if (loadState is LoadState.Error){
            errorMsg.text = loadState.error.localizedMessage
        }
    }
}

寻呼源

override suspend fun load(params: LoadParams<Int>): LoadResult<Int, Model> {

    try {
        // Load page 1 if undefined.
        val currentPage = params.key ?: 0
        val offset = currentPage * 50

        val requestParams = hashMapOf<String, Any>()
        requestParams.put("limit", 50)
        requestParams.put("offset", offset)

        val response =  repository.getList(requestParams)
        val isFinish = response.paging != null && response.paging!!.next == null

        return LoadResult.Page(
            data = response.data ?: mutableListOf(),
            prevKey = null, // Only paging forward.
            nextKey = if (isFinish) null else currentPage + 1
        )
    } catch (e: Exception) {
        // Handle errors in this block
        return LoadResult.Error(e)
    }
}

查看模型

val listPagingFlow = Pager(PagingConfig(pageSize = 50)) {
    MyPagingSource(repository)
}.flow.cachedIn(viewModelScope)

活动

    val pagingAdapter = MyPagingAdapter()
    list.apply {
        setHasFixedSize(true)
        adapter = pagingAdapter.withLoadStateFooter(
            footer = LoadStateAdapter { pagingAdapter.retry() }
        )
    }

    lifecycleScope.launch(Dispatchers.IO) {
        viewModel.listPagingFlow.collectLatest { pagingData ->
            pagingAdapter.submitData(pagingData)
        }
    }

MyPagingAdapter 很简单 PagingDataAdapter

简而言之；加载状态工作正常，但在第一次请求时没有显示。有人可以帮忙吗？

当前版本3.0.0-alpha04

Answer 1

withLoadStateFooter 返回一个ConcatAdapter，它将原始PagingDataAdapter 的结果与一个侦听CombinedLoadState.append 事件的LoadStateAdapter 连接起来。因此，它不会在初始加载期间返回项目 (loadType == REFRESH)，并且它是这样设计的，因为在任何项目加载之前显示“页脚”实际上没有任何意义。

但是，要实现您想要的，您可以简单地创建自己的 ConcatAdapter，它非常接近地反映了 .withLoadStateFooter 的实现：

val originalAdapter = MyPagingDataAdapter(...)
val footerLoadStateAdapter = MyFooterLoadStateAdapter(...)

addLoadStateListener { loadStates ->
    // You need to implement some logic here to update LoadStateAdapter.loadState
    // based on however you want to prioritize between REFRESH / APPEND load states.
    // Something really basic might be:
    // footerLoadStateAdapter.loadState = when {
    //     loadStates.refresh is NotLoading -> loadStates.append
    //     else -> loadStates.refresh
    // }
    footerLoadStateAdapter.loadState = ...
}
return ConcatAdapter(originalAdapter, footerLoadStateAdapter)

Answer 2

使用页脚LoadStateAdapter 显示初始加载会导致问题。如 cmets 中所述，列表将自动滚动到底部。

解决这个问题的方法是使用两个LoadStateAdapters，一个显示初始加载，第二个显示加载更多项目时的加载。

fun <T : Any, V : RecyclerView.ViewHolder> PagingDataAdapter<T, V>.withLoadStateAdapters(
    header: LoadStateAdapter<*>,
    footer: LoadStateAdapter<*>
): ConcatAdapter {
    addLoadStateListener { loadStates ->
        header.loadState = loadStates.refresh
        footer.loadState = loadStates.append
    }

    return ConcatAdapter(header, this, footer)
}

Answer 3

我建议将初始加载和空状态从分页适配器中取出，并这样做：

adapter.loadStateFlow.collect {
    // to determine if we are done with the loading state, 
    // you should have already  shown your loading view elsewhere when the entering your fragment
    if (it.prepend is LoadState.NotLoading && it.prepend.endOfPaginationReached) {
        loading.visibility = View.GONE
    }
    if (it.append is LoadState.NotLoading && it.append.endOfPaginationReached) {
        emptyState.isVisible = adapter.itemCount < 1
    }
}

逻辑是

• 如果追加已完成（it.append 为 LoadState.NotLoading && it.append.endOfPaginationReached == true），并且我们的适配器项目计数为零（adapter.itemCount )，表示没有什么可显示的，所以我们显示空状态
• 如果 prepend 已经完成（it.prepend 是 LoadState.NotLoading && it.prepend .endOfPaginationReached == true），那么我们已经完成了初始加载视图，我们应该隐藏它

Answer 4

作为@dlam 的回答，您可以在适配器中使用自定义 ConcatAdapter 而不是 withLoadStateFooter

fun withMySpecificFooter(
    footer: LoadStateAdapter<*>
): ConcatAdapter {
    addLoadStateListener { loadStates ->
        footer.loadState = when (loadStates.refresh) {
            is LoadState.NotLoading -> loadStates.append
            else -> loadStates.refresh
        }
    }
    return ConcatAdapter(this, footer)
}