Paging 库会跟踪分页数据的加载请求状态，并通过 LoadState 类将其公开。应用可以向 PagingDataAdapter 注册一个监听器，用于接收有关当前状态的信息并相应地更新界面。这些状态在该适配器中提供，因为它们与界面保持同步。这意味着，如果已对界面应用页面加载，您的监听器就会收到更新。
每个 LoadType 和数据源类型（PagingSource 或 RemoteMediator）都会获得一个单独的 LoadState 信号。监听器提供的 CombinedLoadStates 对象则会提供来自所有这些信号的加载状态的信息。您可以利用此详细信息向用户显示相应的加载指示器。
加载状态
Paging 库通过 LoadState 对象公开要在界面中使用的加载状态。LoadState 对象根据当前的加载状态采用以下三种形式之一：
- 如果没有正在执行的加载操作且没有错误，那么
LoadState为LoadState.NotLoading对象。此子类还包含endOfPaginationReached属性，用于指示是否已到达分页结束处。 - 如果有正在执行的加载操作，那么
LoadState为LoadState.Loading对象。 - 如果出现错误，那么
LoadState为LoadState.Error对象。
可通过两种方法在界面中使用 LoadState：使用监听器，或使用特殊的列表适配器直接在 RecyclerView 列表中显示加载状态。
使用监听器获取加载状态
为了获取加载状态以用于界面中的一般用途，请使用 loadStateFlow 流或 PagingDataAdapter 提供的 addLoadStateListener() 方法。这些机制可用于获取 CombinedLoadStates 对象，该对象包含有关每个加载类型的 LoadState 行为的信息。
在以下示例中，PagingDataAdapter 根据刷新加载的当前状态显示不同的界面组件：
Kotlin
// Activities can use lifecycleScope directly, but Fragments should instead use // viewLifecycleOwner.lifecycleScope. lifecycleScope.launch { pagingAdapter.loadStateFlow.collectLatest { loadStates -> progressBar.isVisible = loadStates.refresh is LoadState.Loading retry.isVisible = loadState.refresh !is LoadState.Loading errorMsg.isVisible = loadState.refresh is LoadState.Error } }
Java
pagingAdapter.addLoadStateListener(loadStates -> { progressBar.setVisibility(loadStates.refresh instanceof LoadState.Loading ? View.VISIBLE : View.GONE); retry.setVisibility(loadStates.refresh instanceof LoadState.Loading ? View.GONE : View.VISIBLE); errorMsg.setVisibility(loadStates.refresh instanceof LoadState.Error ? View.VISIBLE : View.GONE); });
Java
pagingAdapter.addLoadStateListener(loadStates -> { progressBar.setVisibility(loadStates.refresh instanceof LoadState.Loading ? View.VISIBLE : View.GONE); retry.setVisibility(loadStates.refresh instanceof LoadState.Loading ? View.GONE : View.VISIBLE); errorMsg.setVisibility(loadStates.refresh instanceof LoadState.Error ? View.VISIBLE : View.GONE); });
如需详细了解 CombinedLoadStates，请参阅获取更多加载状态信息。
使用适配器显示加载状态
Paging 库提供了另一个名为 LoadStateAdapter 的列表适配器，用于直接在显示的分页数据列表中呈现加载状态。您可以通过此适配器获取列表的当前加载状态，将该状态传递给显示该信息的自定义 ViewHolder。
首先，创建一个 ViewHolder 类，用于保留对屏幕上的加载视图和错误视图的引用。创建一个接受 LoadState 作为参数的 bind() 函数。此函数应根据加载状态参数切换视图可见性：
Kotlin
class LoadStateViewHolder( parent: ViewGroup, retry: () -> Unit ) : RecyclerView.ViewHolder( LayoutInflater.from(parent.context) .inflate(R.layout.load_state_item, parent, false) ) { private val binding = LoadStateItemBinding.bind(itemView) private val progressBar: ProgressBar = binding.progressBar private val errorMsg: TextView = binding.errorMsg private val retry: Button = binding.retryButton .also { it.setOnClickListener { retry() } } fun bind(loadState: LoadState) { if (loadState is LoadState.Error) { errorMsg.text = loadState.error.localizedMessage } progressBar.isVisible = loadState is LoadState.Loading retry.isVisible = loadState is LoadState.Error errorMsg.isVisible = loadState is LoadState.Error } }
Java
class LoadStateViewHolder extends RecyclerView.ViewHolder { private ProgressBar mProgressBar; private TextView mErrorMsg; private Button mRetry; LoadStateViewHolder( @NonNull ViewGroup parent, @NonNull View.OnClickListener retryCallback) { super(LayoutInflater.from(parent.getContext()) .inflate(R.layout.load_state_item, parent, false)); LoadStateItemBinding binding = LoadStateItemBinding.bind(itemView); mProgressBar = binding.progressBar; mErrorMsg = binding.errorMsg; mRetry = binding.retryButton; } public void bind(LoadState loadState) { if (loadState instanceof LoadState.Error) { LoadState.Error loadStateError = (LoadState.Error) loadState; mErrorMsg.setText(loadStateError.getError().getLocalizedMessage()); } mProgressBar.setVisibility(loadState instanceof LoadState.Loading ? View.VISIBLE : View.GONE); mRetry.setVisibility(loadState instanceof LoadState.Error ? View.VISIBLE : View.GONE); mErrorMsg.setVisibility(loadState instanceof LoadState.Error ? View.VISIBLE : View.GONE); } }
Java
class LoadStateViewHolder extends RecyclerView.ViewHolder { private ProgressBar mProgressBar; private TextView mErrorMsg; private Button mRetry; LoadStateViewHolder( @NonNull ViewGroup parent, @NonNull View.OnClickListener retryCallback) { super(LayoutInflater.from(parent.getContext()) .inflate(R.layout.load_state_item, parent, false)); LoadStateItemBinding binding = LoadStateItemBinding.bind(itemView); mProgressBar = binding.progressBar; mErrorMsg = binding.errorMsg; mRetry = binding.retryButton; } public void bind(LoadState loadState) { if (loadState instanceof LoadState.Error) { LoadState.Error loadStateError = (LoadState.Error) loadState; mErrorMsg.setText(loadStateError.getError().getLocalizedMessage()); } mProgressBar.setVisibility(loadState instanceof LoadState.Loading ? View.VISIBLE : View.GONE); mRetry.setVisibility(loadState instanceof LoadState.Error ? View.VISIBLE : View.GONE); mErrorMsg.setVisibility(loadState instanceof LoadState.Error ? View.VISIBLE : View.GONE); } }
首先，创建一个实现 LoadStateAdapter 的类，并定义 onCreateViewHolder() 和 onBindViewHolder() 方法：这些方法会创建自定义 ViewHolder 的一个实例，并绑定关联的加载状态。
Kotlin
// Adapter that displays a loading spinner when // state is LoadState.Loading, and an error message and retry // button when state is LoadState.Error. class ExampleLoadStateAdapter( private val retry: () -> Unit ) : LoadStateAdapter<LoadStateViewHolder>() { override fun onCreateViewHolder( parent: ViewGroup, loadState: LoadState ) = LoadStateViewHolder(parent, retry) override fun onBindViewHolder( holder: LoadStateViewHolder, loadState: LoadState ) = holder.bind(loadState) }
Java
// Adapter that displays a loading spinner when // state is LoadState.Loading, and an error message and retry // button when state is LoadState.Error. class ExampleLoadStateAdapter extends LoadStateAdapter<LoadStateViewHolder> { private View.OnClickListener mRetryCallback; ExampleLoadStateAdapter(View.OnClickListener retryCallback) { mRetryCallback = retryCallback; } @NotNull @Override public LoadStateViewHolder onCreateViewHolder(@NotNull ViewGroup parent, @NotNull LoadState loadState) { return new LoadStateViewHolder(parent, mRetryCallback); } @Override public void onBindViewHolder(@NotNull LoadStateViewHolder holder, @NotNull LoadState loadState) { holder.bind(loadState); } }
Java
// Adapter that displays a loading spinner when // state is LoadState.Loading, and an error message and retry // button when state is LoadState.Error. class ExampleLoadStateAdapter extends LoadStateAdapter<LoadStateViewHolder> { private View.OnClickListener mRetryCallback; ExampleLoadStateAdapter(View.OnClickListener retryCallback) { mRetryCallback = retryCallback; } @NotNull @Override public LoadStateViewHolder onCreateViewHolder(@NotNull ViewGroup parent, @NotNull LoadState loadState) { return new LoadStateViewHolder(parent, mRetryCallback); } @Override public void onBindViewHolder(@NotNull LoadStateViewHolder holder, @NotNull LoadState loadState) { holder.bind(loadState); } }
在页眉或页脚中显示加载状态
如需在页眉和页脚中显示加载进度，请从 PagingDataAdapter 对象调用 withLoadStateHeaderAndFooter() 方法：
Kotlin
pagingAdapter .withLoadStateHeaderAndFooter( header = ExampleLoadStateAdapter(adapter::retry), footer = ExampleLoadStateAdapter(adapter::retry) )
Java
pagingAdapter .withLoadStateHeaderAndFooter( new ExampleLoadStateAdapter(pagingAdapter::retry), new ExampleLoadStateAdapter(pagingAdapter::retry));
Java
pagingAdapter .withLoadStateHeaderAndFooter( new ExampleLoadStateAdapter(pagingAdapter::retry), new ExampleLoadStateAdapter(pagingAdapter::retry));
如果您想让 RecyclerView 列表仅在页眉中或仅在页脚中显示加载状态，可改为调用 withLoadStateHeader() 或 withLoadStateFooter()。
获取更多加载状态信息
PagingDataAdapter 中的 CombinedLoadStates 对象会提供 PagingSource 实现的加载状态信息，如果存在 RemoteMediator 实现，也会提供其加载状态信息。
为方便起见，您可以使用 CombinedLoadStates 中的 refresh、append 和 prepend 属性访问相应加载类型的 LoadState 对象。这些属性通常取决于 RemoteMediator 实现（如有）中的加载状态；如没有该实现，这些属性将包含 PagingSource 实现中的相应加载状态。如需详细了解底层逻辑，请参阅 CombinedLoadStates 的参考文档。
Kotlin
lifecycleScope.launch { pagingAdapter.loadStateFlow.collectLatest { loadStates -> // Observe refresh load state from RemoteMediator if present, or // from PagingSource otherwise. refreshLoadState: LoadState = loadStates.refresh // Observe prepend load state from RemoteMediator if present, or // from PagingSource otherwise. prependLoadState: LoadState = loadStates.prepend // Observe append load state from RemoteMediator if present, or // from PagingSource otherwise. appendLoadState: LoadState = loadStates.append } }
Java
pagingAdapter.addLoadStateListener(loadStates -> { // Observe refresh load state from RemoteMediator if present, or // from PagingSource otherwise. LoadState refreshLoadState = loadStates.refresh; // Observe prepend load state from RemoteMediator if present, or // from PagingSource otherwise. LoadState prependLoadState = loadStates.prepend; // Observe append load state from RemoteMediator if present, or // from PagingSource otherwise. LoadState appendLoadState = loadStates.append; });
Java
pagingAdapter.addLoadStateListener(loadStates -> { // Observe refresh load state from RemoteMediator if present, or // from PagingSource otherwise. LoadState refreshLoadState = loadStates.refresh; // Observe prepend load state from RemoteMediator if present, or // from PagingSource otherwise. LoadState prependLoadState = loadStates.prepend; // Observe append load state from RemoteMediator if present, or // from PagingSource otherwise. LoadState appendLoadState = loadStates.append; });
不过必须注意，只有 PagingSource 加载状态一定会与界面更新保持同步。由于 refresh、append 和 prepend 属性可能会从 PagingSource 获取加载状态，也可能会从 RemoteMediator 获取加载状态，因此不能保证它们一定会与界面更新保持同步。这可能会导致以下界面问题：在任何新数据添加到界面中之前，加载似乎就已完成。
因此，便利访问函数适用于在页眉或页脚中显示加载状态，但在其他用例中，您可能需要从 PagingSource 或 RemoteMediator 中明确获取加载状态。为此，CombinedLoadStates 提供了 source 和 mediator 属性。这两个属性会各公开一个 LoadStates 对象，分别包含 PagingSource 或 RemoteMediator 的 LoadState 对象：
Kotlin
lifecycleScope.launch { pagingAdapter.loadStateFlow.collectLatest { loadStates -> // Directly access the RemoteMediator refresh load state. mediatorRefreshLoadState: LoadState? = loadStates.mediator.refresh // Directly access the RemoteMediator append load state. mediatorAppendLoadState: LoadState? = loadStates.mediator.append // Directly access the RemoteMediator prepend load state. mediatorPrependLoadState: LoadState? = loadStates.mediator.prepend // Directly access the PagingSource refresh load state. sourceRefreshLoadState: LoadState = loadStates.source.refresh // Directly access the PagingSource append load state. sourceAppendLoadState: LoadState = loadStates.source.append // Directly access the PagingSource prepend load state. sourcePrependLoadState: LoadState = loadStates.source.prepend } }
Java
pagingAdapter.addLoadStateListener(loadStates -> { // Directly access the RemoteMediator refresh load state. LoadState mediatorRefreshLoadState = loadStates.mediator.refresh; // Directly access the RemoteMediator append load state. LoadState mediatorAppendLoadState = loadStates.mediator.append; // Directly access the RemoteMediator prepend load state. LoadState mediatorPrependLoadState = loadStates.mediator.prepend; // Directly access the PagingSource refresh load state. LoadState sourceRefreshLoadState = loadStates.source.refresh; // Directly access the PagingSource append load state. LoadState sourceAppendLoadState = loadStates.source.append; // Directly access the PagingSource prepend load state. LoadState sourcePrependLoadState = loadStates.source.prepend; });
Java
pagingAdapter.addLoadStateListener(loadStates -> { // Directly access the RemoteMediator refresh load state. LoadState mediatorRefreshLoadState = loadStates.mediator.refresh; // Directly access the RemoteMediator append load state. LoadState mediatorAppendLoadState = loadStates.mediator.append; // Directly access the RemoteMediator prepend load state. LoadState mediatorPrependLoadState = loadStates.mediator.prepend; // Directly access the PagingSource refresh load state. LoadState sourceRefreshLoadState = loadStates.source.refresh; // Directly access the PagingSource append load state. LoadState sourceAppendLoadState = loadStates.source.append; // Directly access the PagingSource prepend load state. LoadState sourcePrependLoadState = loadStates.source.prepend; });
LoadState 中的链操作符
由于 CombinedLoadStates 对象可以获取加载状态的所有更改，因此必须根据特定事件过滤加载状态流。这样可以确保在适当的时间更新界面，以避免卡顿和不必要的界面更新。
例如，假设您想显示一个空视图，但仅在初始数据加载完成后显示。此用例需要验证数据刷新加载是否已开始，然后等待 NotLoading 状态以确认刷新已完成。您必须过滤掉除所需信号外的所有信号：
Kotlin
lifecycleScope.launchWhenCreated { adapter.loadStateFlow // Only emit when REFRESH LoadState for RemoteMediator changes. .distinctUntilChangedBy { it.refresh } // Only react to cases where REFRESH completes, such as NotLoading. .filter { it.refresh is LoadState.NotLoading } // Scroll to top is synchronous with UI updates, even if remote load was // triggered. .collect { binding.list.scrollToPosition(0) } }
Java
PublishSubject<CombinedLoadStates> subject = PublishSubject.create(); Disposable disposable = subject.distinctUntilChanged(CombinedLoadStates::getRefresh) .filter( combinedLoadStates -> combinedLoadStates.getRefresh() instanceof LoadState.NotLoading) .subscribe(combinedLoadStates -> binding.list.scrollToPosition(0)); pagingAdapter.addLoadStateListener(loadStates -> { subject.onNext(loadStates); });
Java
LiveData<CombinedLoadStates> liveData = new MutableLiveData<>(); LiveData<LoadState> refreshLiveData = Transformations.map(liveData, CombinedLoadStates::getRefresh); LiveData<LoadState> distinctLiveData = Transformations.distinctUntilChanged(refreshLiveData); distinctLiveData.observeForever(loadState -> { if (loadState instanceof LoadState.NotLoading) { binding.list.scrollToPosition(0); } });
此示例将一直等到刷新加载状态更新，但仅在状态为 NotLoading 时触发。这样可以确保在发生任何界面更新之前，远程刷新已完全完成。
流 API 使此类操作成为可能。应用可以指定需要的加载事件，并在满足相应条件时处理新数据。
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