Google is committed to advancing racial equity for Black communities. See how.

Migrate to Paging 3

Paging 3 is significantly different from earlier versions of the Paging library. This version provides enhanced functionality and addresses common difficulties with using Paging 2. If your app already uses an earlier version of the Paging library, read this page to learn more about migrating to Paging 3.

If Paging 3 is the first version of the Paging library that you are using in your app, see Load and display paged data for basic usage information.

Benefits of migrating to Paging 3

Paging 3 includes the following features that were not present in earlier versions of the library:

  • First-class support for Kotlin coroutines and Flow.
  • Support for async loading using RxJava Single or Guava ListenableFuture primitives.
  • Built-in load state and error signals for responsive UI design, including retry and refresh functionality.
  • Improvements to the repository layer, including cancellation support and a simplified data source interface.
  • Improvements to the presentation layer, list separators, custom page transforms, and loading state headers and footers.

Migrate your app to Paging 3

To fully migrate to Paging 3, you must migrate all three of the major components from Paging 2:

  • DataSource classes
  • PagedList
  • PagedListAdapter

However, some Paging 3 components are backwards-compatible with previous versions of Paging. In particular, the PagingSource API from Paging 3 can be a data source for LivePagedListBuilder and RxPagedListBuilder from older versions. Similarly, the Pager API can use older DataSource objects with the toPagingSourceFactory() method. This means that you have the following migration options:

  • You can migrate your DataSource to PagingSource but leave the rest of your Paging implementation unchanged.
  • You can migrate your PagedList and PagedListAdapter but still use the older DataSource API.
  • You can migrate the entire Paging implementation to fully migrate your app to Paging 3.

The sections on this page explain how to migrate Paging components on each layer of your app.

DataSource classes

This section describes all of the necessary changes to migrate an older Paging implementation to use PagingSource.

The PageKeyedDataSource, PositionalDataSource, and ItemKeyedDataSource from Paging 2 are all combined into the PagingSource API in Paging 3. The loading methods from all of the old API classes are combined into a single load() method in PagingSource. This reduces code duplication because much of the logic across the loading methods in implementations of the old API classes is often identical.

All loading method parameters are replaced in Paging 3 with a LoadParams sealed class, which includes subclasses for each load type. If you need to differentiate among load types in your load() method, check which subclass of LoadParams was passed in: LoadParams.Refresh, LoadParams.Prepend, or LoadParams.Append.

To learn more about implementing PagingSource, see Define a data source.

Refresh keys

If you currently use PositionalDataSource or ItemKeyedDataSource, you should continue supporting refreshes and resuming from state in the middle of the loaded paged data. Do this by implementing getRefreshKey() to map the correct initial key using state.anchorPosition as the most recently accessed index.

Kotlin

// Replaces ItemKeyedDataSource.
override fun getRefreshKey(state: PagingState): String? {
  return state.anchorPosition?.let { anchorPosition ->
    state.getClosestItemToPosition(anchorPosition)?.id
  }
}

// Replacing PositionalDataSource.
override fun getRefreshKey(state: PagingState): Int? {
  return state.anchorPosition
}

Java

// Replaces ItemKeyedDataSource.
@Nullable
@Override
String getRefreshKey(state: PagingState) {
  Integer anchorPosition = state.anchorPosition;
  if (anchorPosition == null) {
    return null;
  }

  return state.getClosestItemToPosition(anchorPosition);
}

// Replaces PositionalDataSource.
@Nullable
@Override
Integer getRefreshKey(state: PagingState) {
  return state.anchorPosition;
}

Java

// Replacing ItemKeyedDataSource.
@Nullable
@Override
String getRefreshKey(state: PagingState) {
  Integer anchorPosition = state.anchorPosition;
  if (anchorPosition == null) {
    return null;
  }

  return state.getClosestItemToPosition(anchorPosition);
}

// Replacing PositionalDataSource.
@Nullable
@Override
Integer getRefreshKey(state: PagingState) {
  return state.anchorPosition;
}

List transformations

In older versions of the Paging library, transformation of the paged data relies on the following methods:

  • DataSource.map()
  • DataSource.mapByPage()
  • DataSource.Factory.map()
  • DataSource.Factory.mapByPage()

In Paging 3, all transformations are applied as operators on PagingData. If you use any of the methods in the preceding list to transform your paged list, you must move your transformation logic from the DataSource to the PagingData when constructing the Pager using your new PagingSource.

To learn more about applying transformations to paged data using Paging 3, see Transform data streams.

PagedList

This section describes all of the necessary changes to migrate an older Paging implementation to use Pager and PagingData in Paging 3.

PagedListBuilder classes

PagingData replaces the existing PagedList from Paging 2. To migrate to PagingData, you must update the following:

  • Paging configuration has moved from PagedList.Config to PagingConfig.
  • LivePagedListBuilder and RxPagedListBuilder have been combined into a single Pager class.
  • Pager exposes an observable Flow<PagingData> with its .flow property. RxJava and LiveData variants are also available as extension properties, which are callable from Java via static methods and are provided from the paging-rxjava* and paging-runtime modules respectively.

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);

To learn more about setting up a reactive stream of PagingData objects using Paging 3, see Set up a stream of PagingData.

BoundaryCallback for layered sources

In Paging 3, RemoteMediator replaces PagedList.BoundaryCallback as a handler for paging from network and database.

To learn more about using RemoteMediator to page from network and database in Paging 3, see the Android Paging codelab.

PagedListAdapter

This section describes all of the necessary changes to migrate an older Paging implementation to use the PagingDataAdapter or AsyncPagingDataDiffer classes from Paging 3.

Paging 3 provides PagingDataAdapter to handle the new PagingData reactive streams. Otherwise, PagedListAdapter and PagingDataAdapter have the same interface. To migrate from PagedListAdapter to PagingDataAdapter, change your implementation of PagedListAdapter to extend PagingDataAdapter instead.

To learn more about PagingDataAdapter, see Define a RecyclerView adapter.

AsyncPagedListDiffer

If you currently use a custom RecyclerView.Adapter implementation with AsyncPagedListDiffer, migrate your implementation to use the AsyncPagingDataDiffer provided in Paging 3 instead:

Kotlin

AsyncPagingDataDiffer(diffCallback, listUpdateCallback)

Java

new AsyncPagingDataDiffer(diffCallback, listUpdateCallback);

Java

new AsyncPagingDataDiffer(diffCallback, listUpdateCallback);

Additional resources

To learn more about the Paging library, see the following additional resources:

Codelabs

Samples