PagedList.BoundaryCallback

public static abstract class PagedList.BoundaryCallback
extends Object

Signals when a PagedList has reached the end of available data.

When local storage is a cache of network data, it's common to set up a streaming pipeline: Network data is paged into the database, database is paged into UI. Paging from the database to UI can be done with a LiveData<PagedList>, but it's still necessary to know when to trigger network loads.

BoundaryCallback does this signaling - when a DataSource runs out of data at the end of the list, onItemAtEndLoaded(Object) is called, and you can start an async network load that will write the result directly to the database. Because the database is being observed, the UI bound to the LiveData<PagedList> will update automatically to account for the new items.

Note that a BoundaryCallback instance shared across multiple PagedLists (e.g. when passed to setBoundaryCallback(PagedList.BoundaryCallback)), the callbacks may be issued multiple times. If for example onItemAtEndLoaded(Object) triggers a network load, it should avoid triggering it again while the load is ongoing.

BoundaryCallback only passes the item at front or end of the list. Number of items is not passed, since it may not be fully computed by the DataSource if placeholders are not supplied. Keys are not known because the BoundaryCallback is independent of the DataSource-specific keys, which may be different for local vs remote storage.

The database + network Repository in the PagingWithNetworkSample shows how to implement a network BoundaryCallback using Retrofit, while handling swipe-to-refresh, network errors, and retry.

Summary

Public constructors

PagedList.BoundaryCallback ()

Public methods

void onItemAtEndLoaded (T itemAtEnd)

void onItemAtFrontLoaded (T itemAtFront)

void onZeroItemsLoaded ()