管理和呈现加载状态

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。您的应用可以使用 PagingDataAdapter 注册一个监听器，以接收有关当前状态的信息并相应地更新界面。这些状态由适配器提供，因为它们与界面同步。这意味着当页面加载已应用于界面时，您的监听器会收到更新。

针对每种 LoadType 和数据源类型（PagingSource 或 RemoteMediator）提供了单独的 LoadState 信号。监听器提供的 CombinedLoadStates 对象提供了所有这些信号的加载状态信息。您可以使用这些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 对象公开可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：

您可以通过两种方式在界面中使用 LoadState：使用监听器，或使用特殊的列表适配器直接在 RecyclerView 列表中呈现加载状态。

使用监听器访问加载状态

要在界面中一般性地获取加载状态，请使用您的 PagingDataAdapter 提供的 loadStateFlow 流或 addLoadStateListener() 方法。这些机制提供对 CombinedLoadStates 对象的访问，该对象包含每种加载类型的 LoadState 行为信息。

在以下示例中，PagingDataAdapter 会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);
});

如需了解有关 CombinedLoadStates 的更多信息，请参阅访问其他加载状态信息。

使用适配器呈现加载状态

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。此适配器提供对列表当前加载状态的访问，您可以将其传递给显示信息的自定义视图持有者。

首先，创建一个视图持有者类，该类保留屏幕上的加载视图和错误视图的引用。创建一个 bind() 函数，该函数接受一个 LoadState 作为参数。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);
  }
}

接下来，创建一个实现 LoadStateAdapter 的类，并定义 onCreateViewHolder() 和 onBindViewHolder() 方法。这些方法会创建您的自定义视图持有者的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);
  }
}

若要在页眉和页脚中显示加载进度，请从您的 PagingDataAdapter 对象中调用 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));

如果您希望 RecyclerView 列表仅在页眉或仅在页脚中显示加载状态，则可以改为调用 withLoadStateHeader() 或 withLoadStateFooter()。

访问其他加载状态信息

来自 PagingDataAdapter 的 CombinedLoadStates 对象提供了您的 PagingSource 实现以及您的 RemoteMediator 实现（如果存在）的加载状态信息。

为了方便起见，您可以使用 CombinedLoadStates 中的 refresh、append 和 prepend 属性来访问相应加载类型的 LoadState 对象。如果存在 RemoteMediator 实现，这些属性通常会引用其中的加载状态；否则，它们包含 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;
});

但请务必记住，只有 PagingSource 加载状态才能保证与界面更新同步。由于 refresh、append 和 prepend 属性可能会获取 PagingSource 或 RemoteMediator 中的加载状态，因此不能保证它们与界面更新同步。这可能会导致界面问题，即加载似乎在新数据添加到界面之前就已完成。

因此，便捷访问器非常适合在页眉或页脚中显示加载状态，但对于其他用例，您可能需要专门从 PagingSource 或 RemoteMediator 访问加载状态。CombinedLoadStates 为此提供了 source 和 mediator 属性。这些属性各自公开一个 LoadStates 对象，其中分别包含 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;
});

在 LoadState 上链式调用运算符

由于 CombinedLoadStates 对象提供了对加载状态所有变更的访问，因此根据特定事件过滤加载状态流非常重要。这可确保您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，假设您想要显示一个空视图，但前提是初始数据加载完成后。此用例要求您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。您必须滤除所有不需要的信号，只保留您需要的信号

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);
  }
});

此示例会等待刷新加载状态更新，但仅在状态为 NotLoading 时触发。这可确保在发生任何界面更新之前，远程刷新已完全完成。

流式 API 使此类操作成为可能。您的应用可以指定所需的加载事件，并在满足相应条件时处理新数据。