加载并显示分页数据

Paging 库提供了强大的功能，用于从更大的数据集加载和显示分页数据。本指南演示了如何使用 Paging 库设置来自网络数据源的分页数据流并将其显示在 RecyclerView 中。

定义数据源

第一步是定义用于标识数据源的 PagingSource 实现。PagingSource API 类包含 load() 方法，您必须替换该方法，以指明如何从相应数据源检索分页数据。

直接使用 PagingSource 类即可通过 Kotlin 协程进行异步加载。Paging 库还提供了支持其他异步框架的类：

• 如需使用 RxJava，请改为实现 RxPagingSource。
• 如需使用 Guava 中的 ListenableFuture，请改为实现 ListenableFuturePagingSource。

选择键和值类型

PagingSource<Key, Value> 有两种类型参数：Key 和 Value。键定义了用于加载数据的标识符，值是数据本身的类型。例如，如果您通过将 Int 页码传递给 Retrofit 来从网络加载 User 对象的页面，则应选择 Int 作为 Key 类型，选择 User 作为 Value 类型。

定义 PagingSource

下面的示例实现了按页码加载项页面的 PagingSource。Key 类型为 Int，Value 类型为 User。

class ExamplePagingSource(
    val backend: ExampleBackendService,
    val query: String
) : PagingSource<Int, User>() {
  override suspend fun load(
    params: LoadParams<Int>
  ): LoadResult<Int, User> {
    try {
      // Start refresh at page 1 if undefined.
      val nextPageNumber = params.key ?: 1
      val response = backend.searchUsers(query, nextPageNumber)
      return LoadResult.Page(
        data = response.users,
        prevKey = null, // Only paging forward.
        nextKey = response.nextPageNumber
      )
    } catch (e: Exception) {
      // Handle errors in this block and return LoadResult.Error if it is an
      // expected error (such as a network failure).
    }
  }
}

class ExamplePagingSource extends RxPagingSource<Integer, User> {
  @NonNull
  private ExampleBackendService mBackend;
  @NonNull
  private String mQuery;

  ExamplePagingSource(@NonNull ExampleBackendService backend,
    @NonNull String query) {
    mBackend = backend;
    mQuery = query;
  }

  @NotNull
  @Override
  public Single<LoadResult<Integer, User>> loadSingle(
    @NotNull LoadParams<Integer> params) {
    // Start refresh at page 1 if undefined.
    Integer nextPageNumber = params.getKey();
    if (nextPageNumber == null) {
      nextPageNumber = 1;
    }

    return mBackend.searchUsers(mQuery, nextPageNumber)
      .subscribeOn(Schedulers.io())
      .map(this::toLoadResult)
      .onErrorReturn(LoadResult.Error::new);
  }

  private LoadResult<Integer, User> toLoadResult(
    @NonNull SearchUsersResponse response) {
    return new LoadResult.Page<>(
      response.getUsers(),
      null, // Only paging forward.
      response.getNextPageNumber(),
      LoadResult.Page.COUNT_UNDEFINED,
      LoadResult.Page.COUNT_UNDEFINED);
  }
}

class ExamplePagingSource extends ListenableFuturePagingSource<Integer, User> {
  @NonNull
  private ExampleBackendService mBackend;
  @NonNull
  private String mQuery;
  @NonNull
  private Executor mBgExecutor;

  ExamplePagingSource(
    @NonNull ExampleBackendService backend,
    @NonNull String query, @NonNull Executor bgExecutor) {
    mBackend = backend;
    mQuery = query;
    mBgExecutor = bgExecutor;
  }

  @NotNull
  @Override
  public ListenableFuture<LoadResult<Integer, User>> loadFuture(
    @NotNull LoadParams<Integer> params) {
    // Start refresh at page 1 if undefined.
    Integer nextPageNumber = params.getKey();
    if (nextPageNumber == null) {
      nextPageNumber = 1;
    }

    ListenableFuture<LoadResult<Integer, User>> pageFuture = Futures.transform(
      mBackend.searchUsers(mQuery, nextPageNumber),
      this::toLoadResult, mBgExecutor);

    ListenableFuture<LoadResult<Integer, User>> partialLoadResultFuture = Futures.catching(
      pageFuture, HttpException.class,
      LoadResult.Error::new, mBgExecutor);

    return Futures.catching(partialLoadResultFuture,
      IOException.class, LoadResult.Error::new, mBgExecutor);
  }

  private LoadResult<Integer, User> toLoadResult(
    @NonNull SearchUsersResponse response) {
    return new LoadResult.Page<>(
      response.getUsers(),
      null, // Only paging forward.
      response.getNextPageNumber(),
      LoadResult.Page.COUNT_UNDEFINED,
      LoadResult.Page.COUNT_UNDEFINED);
  }
}

典型的 PagingSource 实现会将其构造函数中提供的参数传递给 load() 方法，以便为查询加载适当的数据。在上面的示例中，这些参数包括：

• backend：提供数据的后端服务实例。
• query：要发送到 backend 指示的服务的搜索查询。

LoadParams 对象包含有关要执行的加载操作的信息，其中包括要加载的键和要加载的项数。

LoadResult 对象包含加载操作的结果。LoadResult 是一个密封的类，根据 load() 调用是否成功，采用如下两种形式之一：

• 如果加载成功，则返回 LoadResult.Page 对象。
• 如果加载失败，则返回 LoadResult.Error 对象。

图 1 说明了此示例中的 load() 函数如何接收每次加载的键并为后续加载提供键。

处理错误

数据加载请求可因多种原因失败，特别是在通过网络加载时。通过从 load() 方法返回 LoadResult.Error 对象，可报告在加载过程中遇到的错误。

例如，对于上一个示例，您可以通过向 load() 方法添加以下内容捕获和报告 ExamplePagingSource 中的加载错误：

catch (e: IOException) {
  // IOException for network failures.
  return LoadResult.Error(e)
} catch (e: HttpException) {
  // HttpException for any non-2xx HTTP status codes.
  return LoadResult.Error(e)
}

return backend.searchUsers(searchTerm, nextPageNumber)
  .subscribeOn(Schedulers.io())
  .map(this::toLoadResult)
  .onErrorReturn(LoadResult.Error::new);

ListenableFuture<LoadResult<Integer, User>> pageFuture = Futures.transform(
  backend.searchUsers(query, nextPageNumber), this::toLoadResult,
  bgExecutor);

ListenableFuture<LoadResult<Integer, User>> partialLoadResultFuture = Futures.catching(
  pageFuture, HttpException.class, LoadResult.Error::new,
  bgExecutor);

return Futures.catching(partialLoadResultFuture,
  IOException.class, LoadResult.Error::new, bgExecutor);

如需详细了解如何处理 Retrofit 错误，请参阅 PagingSource API 参考文档中的示例。

PagingSource 会收集 LoadResult.Error 对象并将其传递给界面，以便您对其执行操作。如需详细了解如何在界面中显示加载状态，请参阅显示加载状态。

设置 PagingData 流

接下来，您需要来自 PagingSource 实现的分页数据流。通常，您应在 ViewModel 中设置数据流。此 Pager 类提供的方法可显示来自 PagingSource 的 PagingData 对象的响应式流。Paging 库支持使用多种流类型，包括 Flow、LiveData 以及 RxJava 中的 Flowable 和 Observable 类型。

当您创建 Pager 实例来设置响应式流时，必须为实例提供 PagingConfig 配置对象和告知 Pager 如何获取 PagingSource 实现实例的函数：

val flow = Pager(
  // Configure how data is loaded by passing additional properties to
  // PagingConfig, such as prefetchDistance.
  PagingConfig(pageSize = 20)
) {
  ExamplePagingSource(backend, query)
}.flow
  .cachedIn(viewModelScope)

// CoroutineScope helper provided by the lifecycle-viewmodel-ktx artifact.
CoroutineScope viewModelScope = ViewModelKt.getViewModelScope(viewModel);
Pager<Integer, User> pager = Pager<>(
  new PagingConfig(/* pageSize = */ 20),
  () -> ExamplePagingSource(backend, query));

Flowable<PagingData<User>> flowable = PagingRx.getFlowable(pager);
PagingRx.cachedIn(flowable, viewModelScope);

// CoroutineScope helper provided by the lifecycle-viewmodel-ktx artifact.
CoroutineScope viewModelScope = ViewModelKt.getViewModelScope(viewModel);
Pager<Integer, User> pager = Pager<>(
  new PagingConfig(/* pageSize = */ 20),
  () -> ExamplePagingSource(backend, query));

PagingLiveData.cachedIn(PagingLiveData.getLiveData(pager), viewModelScope);

cachedIn() 运算符使数据流可共享，并使用提供的 CoroutineScope 缓存加载的数据。此示例使用 Lifecycle lifecycle-viewmodel-ktx 工件提供的 viewModelScope。

Pager 对象从 PagingSource 对象调用 load() 方法，为其提供 LoadParams 对象，并接收 LoadResult 对象作为交换。

定义 RecyclerView Adapter

您还需要设置一个 Adapter 来将数据接收到 RecyclerView 列表中。为此，Paging 库提供了 PagingDataAdapter 类。

定义扩展 PagingDataAdapter 的类。在此示例中，UserAdapter 扩展了 PagingDataAdapter，用于为 User 类型的列表项提供 RecyclerView Adapter，并使用 UserViewHolder 作为 ViewHolder：

class UserAdapter(diffCallback: DiffUtil.ItemCallback<User>) :
  PagingDataAdapter<User, UserViewHolder>(diffCallback) {
  override fun onCreateViewHolder(
    parent: ViewGroup,
    viewType: Int
  ): UserViewHolder {
    return UserViewHolder(parent)
  }

  override fun onBindViewHolder(holder: UserViewHolder, position: Int) {
    val item = getItem(position)
    // Note that item may be null. ViewHolder must support binding a
    // null item as a placeholder.
    holder.bind(item)
  }
}

class UserAdapter extends PagingDataAdapter<User, UserViewHolder> {
  UserAdapter(@NotNull DiffUtil.ItemCallback<User> diffCallback) {
    super(diffCallback);
  }

  @NonNull
  @Override
  public UserViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
    return new UserViewHolder(parent);
  }

  @Override
  public void onBindViewHolder(@NonNull UserViewHolder holder, int position) {
    User item = getItem(position);
    // Note that item may be null. ViewHolder must support binding a
    // null item as a placeholder.
    holder.bind(item);
  }
}

class UserAdapter extends PagingDataAdapter<User, UserViewHolder> {
  UserAdapter(@NotNull DiffUtil.ItemCallback<User> diffCallback) {
    super(diffCallback);
  }

  @NonNull
  @Override
  public UserViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
    return new UserViewHolder(parent);
  }

  @Override
  public void onBindViewHolder(@NonNull UserViewHolder holder, int position) {
    User item = getItem(position);
    // Note that item may be null. ViewHolder must support binding a
    // null item as a placeholder.
    holder.bind(item);
  }
}

您的适配器还必须定义 onCreateViewHolder() 和 onBindViewHolder() 方法，并指定 DiffUtil.ItemCallback。这与定义 RecyclerView 列表 Adapter 时的通常做法相同：

object UserComparator : DiffUtil.ItemCallback<User>() {
  override fun areItemsTheSame(oldItem: User, newItem: User): Boolean {
    // Id is unique.
    return oldItem.id == newItem.id
  }

  override fun areContentsTheSame(oldItem: User, newItem: User): Boolean {
    return oldItem == newItem
  }
}

class UserComparator extends DiffUtil.ItemCallback<User> {
  @Override
  public boolean areItemsTheSame(@NonNull User oldItem,
    @NonNull User newItem) {
    // Id is unique.
    return oldItem.id.equals(newItem.id);
  }

  @Override
  public boolean areContentsTheSame(@NonNull User oldItem,
    @NonNull User newItem) {
    return oldItem.equals(newItem);
  }
}

class UserComparator extends DiffUtil.ItemCallback<User> {
  @Override
  public boolean areItemsTheSame(@NonNull User oldItem,
    @NonNull User newItem) {
    // Id is unique.
    return oldItem.id.equals(newItem.id);
  }

  @Override
  public boolean areContentsTheSame(@NonNull User oldItem,
    @NonNull User newItem) {
    return oldItem.equals(newItem);
  }
}

在界面中显示分页数据

现在您已经定义了 PagingSource，为您的应用创建了生成 PagingData 流的方法，还定义了 PagingDataAdapter，接下来便可以将这些元素连接起来，在您的 Activity 中显示分页数据。

在 Activity 的 onCreate 或 Fragment 的 onViewCreated 方法中执行以下步骤：

1. 创建 PagingDataAdapter 类的实例。
2. 将 PagingDataAdapter 实例传递给您要显示分页数据的 RecyclerView 列表。
3. 观察 PagingData 流，并将生成的每个值传递给 Adapter 的 submitData() 方法。

val viewModel by viewModels<ExampleViewModel>()

val pagingAdapter = UserAdapter(UserComparator)
val recyclerView = findViewById<RecyclerView>(R.id.recycler_view)
recyclerView.adapter = pagingAdapter

// Activities can use lifecycleScope directly, but Fragments should instead use
// viewLifecycleOwner.lifecycleScope.
lifecycleScope.launch {
  viewModel.flow.collectLatest { pagingData ->
    pagingAdapter.submitData(pagingData)
  }
}

ExampleViewModel viewModel = new ViewModelProvider(this)
  .get(ExampleViewModel.class);

UserAdapter pagingAdapter = new UserAdapter(new UserComparator());
RecyclerView recyclerView = findViewById<RecyclerView>(
  R.id.recycler_view);
recyclerView.adapter = pagingAdapter

viewModel.flowable
  // Using AutoDispose to handle subscription lifecycle.
  // See: https://github.com/uber/AutoDispose
  .to(autoDisposable(AndroidLifecycleScopeProvider.from(this)))
  .subscribe(pagingData -> pagingAdapter.submitData(lifecycle, pagingData));

ExampleViewModel viewModel = new ViewModelProvider(this)
  .get(ExampleViewModel.class);

UserAdapter pagingAdapter = new UserAdapter(new UserComparator());
RecyclerView recyclerView = findViewById<RecyclerView>(
  R.id.recycler_view);
recyclerView.adapter = pagingAdapter

// Activities can use getLifecycle() directly, but Fragments should instead use
// getViewLifecycleOwner().getLifecycle().
viewModel.liveData.observe(this, pagingData ->
  pagingAdapter.submitData(getLifecycle(), pagingData));

RecyclerView 列表现在会显示来自数据源的分页数据，并会在必要时自动加载另一个页面。

显示加载状态

Paging 库通过 LoadState 对象显示可在界面中使用的加载状态。LoadState 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，则 LoadState 为 LoadState.NotLoading 对象。
• 如果有正在执行的加载操作，则 LoadState 为 LoadState.Loading 对象。
• 如果出现错误，则 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，以及使用特殊的列表适配器在 RecyclerView 中直接显示加载状态。

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，PagingDataAdapter 包含了 addLoadStateListener() 方法。

// 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
  }
}

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);
});

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);
});

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。

首先，创建一个实现 LoadStateAdapter 的类，并定义 onCreateViewHolder() 和 onBindViewHolder() 方法：

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
  }
}

// Adapter that displays a loading spinner when
// state = 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)
}

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);
  }
}

// Adapter that displays a loading spinner when
// state instanceOf LoadState.Loading, and an error message and
// retry button when state instanceof 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);
  }
}

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);
  }
}

// Adapter that displays a loading spinner when
// state instanceOf LoadState.Loading, and an error message and
// retry button when state instanceof 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);
  }
}

然后，从 PagingDataAdapter 对象调用 withLoadStateHeaderAndFooter() 方法：

pagingAdapter
  .withLoadStateHeaderAndFooter(
    header = ExampleLoadStateAdapter(adapter::retry),
    footer = ExampleLoadStateAdapter(adapter::retry)
  )

pagingAdapter
  .withLoadStateHeaderAndFooter(
    new ExampleLoadStateAdapter(pagingAdapter::retry),
    new ExampleLoadStateAdapter(pagingAdapter::retry));

pagingAdapter
  .withLoadStateHeaderAndFooter(
    new ExampleLoadStateAdapter(pagingAdapter::retry),
    new ExampleLoadStateAdapter(pagingAdapter::retry));

如果您只想让 RecyclerView 在页眉或页脚中显示加载状态，则可以调用 withLoadStateHeader() 或 withLoadStateFooter()。

其他资源

如需详细了解 Paging 库，请参阅下列其他资源：

Codelab

• Android Paging Codelab

示例

• Android 架构组件示例：Paging
• Android 架构组件示例：将 Paging 与数据库和网络搭配使用