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管理和显示加载状态

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。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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