加载并显示分页数据

Paging 库提供了强大的功能,用于从大型数据集中加载和显示分页数据。本指南演示了如何使用 Paging 库从网络数据源设置分页数据流,并将其显示在 RecyclerView 中。

定义数据源

第一步是定义 PagingSource 实现,以标识数据源。PagingSource API 类包含 load() 方法,您需要覆盖此方法以指示如何从相应数据源检索分页数据。

直接使用 PagingSource 类来使用 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。

选择键和值类型

PagingSource<Key, Value> 有两个类型参数:KeyValue。键定义了用于加载数据的标识符,而值是数据本身的类型。例如,如果您通过将 Int 页码传递给 Retrofit 从网络加载 User 对象的页面,请选择 Int 作为 Key 类型,User 作为 Value 类型。

定义 PagingSource

以下示例实现了 PagingSource,它按页码加载项目页面。Key 类型是 IntValue 类型是 User

Kotlin

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 for
      // expected errors (such as a network failure).
    }
  }

  override fun getRefreshKey(state: PagingState<Int, User>): Int? {
    // Try to find the page key of the closest page to anchorPosition from
    // either the prevKey or the nextKey; you need to handle nullability
    // here.
    //  * prevKey == null -> anchorPage is the first page.
    //  * nextKey == null -> anchorPage is the last page.
    //  * both prevKey and nextKey are null -> anchorPage is the
    //    initial page, so return null.
    return state.anchorPosition?.let { anchorPosition ->
      val anchorPage = state.closestPageToPosition(anchorPosition)
      anchorPage?.prevKey?.plus(1) ?: anchorPage?.nextKey?.minus(1)
    }
  }
}

Java

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 SearchUserResponse response) {
    return new LoadResult.Page<>(
      response.getUsers(),
      null, // Only paging forward.
      response.getNextPageNumber(),
      LoadResult.Page.COUNT_UNDEFINED,
      LoadResult.Page.COUNT_UNDEFINED);
  }

  @Nullable
  @Override
  public Integer getRefreshKey(@NotNull PagingState<Integer, User> state) {
    // Try to find the page key of the closest page to anchorPosition from
    // either the prevKey or the nextKey; you need to handle nullability
    // here.
    //  * prevKey == null -> anchorPage is the first page.
    //  * nextKey == null -> anchorPage is the last page.
    //  * both prevKey and nextKey are null -> anchorPage is the
    //    initial page, so return null.
    Integer anchorPosition = state.getAnchorPosition();
    if (anchorPosition == null) {
      return null;
    }

    LoadResult.Page<Integer, User> anchorPage = state.closestPageToPosition(anchorPosition);
    if (anchorPage == null) {
      return null;
    }

    Integer prevKey = anchorPage.getPrevKey();
    if (prevKey != null) {
      return prevKey + 1;
    }

    Integer nextKey = anchorPage.getNextKey();
    if (nextKey != null) {
      return nextKey - 1;
    }

    return null;
  }
}

Java

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 SearchUserResponse response) {
    return new LoadResult.Page<>(response.getUsers(),
    null, // Only paging forward.
    response.getNextPageNumber(),
    LoadResult.Page.COUNT_UNDEFINED,
    LoadResult.Page.COUNT_UNDEFINED);
  }

  @Nullable
  @Override
  public Integer getRefreshKey(@NotNull PagingState<Integer, User> state) {
    // Try to find the page key of the closest page to anchorPosition from
    // either the prevKey or the nextKey; you need to handle nullability
    // here.
    //  * prevKey == null -> anchorPage is the first page.
    //  * nextKey == null -> anchorPage is the last page.
    //  * both prevKey and nextKey are null -> anchorPage is the
    //    initial page, so return null.
    Integer anchorPosition = state.getAnchorPosition();
    if (anchorPosition == null) {
      return null;
    }

    LoadResult.Page<Integer, User> anchorPage = state.closestPageToPosition(anchorPosition);
    if (anchorPage == null) {
      return null;
    }

    Integer prevKey = anchorPage.getPrevKey();
    if (prevKey != null) {
      return prevKey + 1;
    }

    Integer nextKey = anchorPage.getNextKey();
    if (nextKey != null) {
      return nextKey - 1;
    }

    return null;
  }
}

典型的 PagingSource 实现将其构造函数中提供的参数传递给 load() 方法,以加载查询的适当数据。在上面的示例中,这些参数是:

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

LoadParams 对象包含要执行的加载操作的相关信息。这包括要加载的键和要加载的项目数量。

LoadResult 对象包含加载操作的结果。LoadResult 是一个密封类,根据 load() 调用是否成功,它有两种形式:

  • 如果加载成功,则返回 LoadResult.Page 对象。
  • 如果加载不成功,则返回 LoadResult.Error 对象。

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

On each load() call, the ExamplePagingSource takes in the current key
    and returns the next key to load.
图 1. 显示 load() 如何使用和更新键的图示。

PagingSource 实现还必须实现 getRefreshKey() 方法,该方法将 PagingState 对象作为参数。它返回在初始加载后数据刷新或失效时要传递到 load() 方法的键。Paging 库会在后续数据刷新时自动调用此方法。

处理错误

加载数据的请求可能会因多种原因而失败,尤其是在通过网络加载时。通过从 load() 方法返回 LoadResult.Error 对象来报告加载过程中遇到的错误。

例如,您可以通过将以下内容添加到 load() 方法中,捕获并报告之前示例中 ExamplePagingSource 的加载错误:

Kotlin

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)
}

Java

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

Java

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);

有关处理 Retrofit 错误的更多信息,请参阅 PagingSource API 参考中的示例。

PagingSource 会收集 LoadResult.Error 对象并将其传递给 UI,以便您可以对其采取行动。有关在 UI 中公开加载状态的更多信息,请参阅管理和显示加载状态

设置 PagingData 流

接下来,您需要从 PagingSource 实现获取分页数据流。在您的 ViewModel 中设置数据流。Pager 类提供了一些方法,可以从 PagingSource 中公开 PagingData 对象的响应式流。Paging 库支持使用多种流类型,包括 FlowLiveData 以及 RxJava 的 FlowableObservable 类型。

当您创建 Pager 实例以设置您的响应式流时,您必须为该实例提供一个 PagingConfig 配置对象和一个函数,该函数告诉 Pager 如何获取您的 PagingSource 实现的实例。

Kotlin

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)

Java

// 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);

Java

// 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,并将加载的数据缓存到提供的 CoroutineScope 中。此示例使用生命周期 lifecycle-viewmodel-ktx 工件提供的 viewModelScope

Pager 对象会调用 PagingSource 对象的 load() 方法,向其提供 LoadParams 对象,并接收返回的 LoadResult 对象。

定义 RecyclerView 适配器

您还需要设置一个适配器,以便将数据接收到您的 RecyclerView 列表中。Paging 库为此目的提供了 PagingDataAdapter 类。

定义一个扩展 PagingDataAdapter 的类。在该示例中,UserAdapter 扩展了 PagingDataAdapter,以提供一个 RecyclerView 适配器,用于类型为 User 的列表项,并使用 UserViewHolder 作为 视图持有者

Kotlin

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 can be null. ViewHolder must support binding a
    // null item as a placeholder.
    holder.bind(item)
  }
}

Java

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 can be null. ViewHolder must support binding a
    // null item as a placeholder.
    holder.bind(item);
  }
}

Java

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 can 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()onBindViewHolder() 方法,并指定 DiffUtil.ItemCallback。这与通常定义 RecyclerView 列表适配器的方式相同。

Kotlin

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
  }
}

Java

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);
  }
}

Java

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 UI 中显示分页数据

现在,您已经定义了 PagingSource,创建了让您的应用生成 PagingData 流的方法,并定义了 PagingDataAdapter,您就可以将这些元素连接起来,并在您的 Activity 中显示分页数据了。

在您的 Activity 的 onCreate 方法或 Fragment 的 onViewCreated 方法中执行以下步骤:

  1. 创建您的 PagingDataAdapter 类的实例。
  2. PagingDataAdapter 实例传递给您要显示分页数据的 RecyclerView 列表。
  3. 观察 PagingData 流,并将每个生成的值传递给适配器的 submitData() 方法。

Kotlin

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; fragments use
// viewLifecycleOwner.lifecycleScope.
lifecycleScope.launch {
  viewModel.flow.collectLatest { pagingData ->
    pagingAdapter.submitData(pagingData)
  }
}

Java

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));

Java

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; fragments 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,并在必要时自动加载下一页。

其他资源

要了解有关 Paging 库的更多信息,请参阅以下其他资源:

Codelabs