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PagingSource

abstract class PagingSource<Key : Any, Value : Any>
kotlin.Any
   ↳ androidx.paging.PagingSource

Base class for an abstraction of pageable static data from some source, where loading pages of data is typically an expensive operation. Some examples of common PagingSources might be from network or from a database.

An instance of a PagingSource is used to load pages of data for an instance of PagingData.

A PagingData can grow as it loads more data, but the data loaded cannot be updated. If the underlying data set is modified, a new PagingSource / PagingData pair must be created to represent an updated snapshot of the data.

Loading Pages

PagingData queries data from its PagingSource in response to loading hints generated as the user scrolls in a RecyclerView.

To control how and when a PagingData queries data from its PagingSource, see PagingConfig, which defines behavior such as PagingConfig.pageSize and PagingConfig.prefetchDistance.

Updating Data

A PagingSource / PagingData pair is a snapshot of the data set. A new PagingData / PagingData must be created if an update occurs, such as a reorder, insert, delete, or content update occurs. A PagingSource must detect that it cannot continue loading its snapshot (for instance, when Database query notices a table being invalidated), and call invalidate. Then a new PagingSource / PagingData pair would be created to represent data from the new state of the database query.

Presenting Data to UI

To present data loaded by a PagingSource to a RecyclerView, create an instance of Pager, which provides a stream of PagingData that you may collect from and submit to a PagingDataAdapter.


/**
 * Sample Page-Keyed PagingSource, which uses String tokens to load pages.
 *
 * Loads Items from network requests via Retrofit to a backend service.
 */
class MyPagingSource(
    val myBackend: MyBackendService
) : PagingSource<String, Item>() {
    override suspend fun load(params: LoadParams<String>): LoadResult<String, Item> {
        // Retrofit calls that return the body type throw either IOException for network
        // failures, or HttpException for any non-2xx HTTP status codes. This code reports all
        // errors to the UI, but you can inspect/wrap the exceptions to provide more context.
        return try {
            // Suspending network load via Retrofit. This doesn't need to be wrapped in a
            // withContext(Dispatcher.IO) { ... } block since Retrofit's Coroutine
            // CallAdapter dispatches on a worker thread.
            val response = myBackend.searchItems(params.key)
            LoadResult.Page(
                data = response.items,
                prevKey = response.prev,
                nextKey = response.next
            )
        } catch (e: IOException) {
            LoadResult.Error(e)
        } catch (e: HttpException) {
            LoadResult.Error(e)
        }
    }

    override fun getRefreshKey(state: PagingState<String, Item>): String? {
        return state.anchorPosition?.let { state.closestItemToPosition(it)?.id }
    }
}

/**
 * Sample Page-Indexed PagingSource, which uses Int page number to load pages.
 *
 * Loads Items from network requests via Retrofit to a backend service.
 *
 * Note that the key type is Int, since we're using page number to load a page.
 */
class MyPagingSource(
    val myBackend: MyBackendService
) : PagingSource<Int, Item>() {
    override suspend fun load(params: LoadParams<Int>): LoadResult<Int, Item> {

        // Retrofit calls that return the body type throw either IOException for network
        // failures, or HttpException for any non-2xx HTTP status codes. This code reports all
        // errors to the UI, but you can inspect/wrap the exceptions to provide more context.
        return try {
            // Key may be null during a refresh, if no explicit key is passed into Pager
            // construction. Use 0 as default, because our API is indexed started at index 0
            val pageNumber = params.key ?: 0

            // Suspending network load via Retrofit. This doesn't need to be wrapped in a
            // withContext(Dispatcher.IO) { ... } block since Retrofit's Coroutine
            // CallAdapter dispatches on a worker thread.
            val response = myBackend.searchItems(pageNumber)

            // Since 0 is the lowest page number, return null to signify no more pages should
            // be loaded before it.
            val prevKey = if (pageNumber > 0) pageNumber - 1 else null

            // This API defines that it's out of data when a page returns empty. When out of
            // data, we return `null` to signify no more pages should be loaded
            val nextKey = if (response.items.isNotEmpty()) pageNumber + 1 else null
            LoadResult.Page(
                data = response.items,
                prevKey = prevKey,
                nextKey = nextKey
            )
        } catch (e: IOException) {
            LoadResult.Error(e)
        } catch (e: HttpException) {
            LoadResult.Error(e)
        }
    }

    override fun getRefreshKey(state: PagingState<Int, Item>): Int? {
        return state.anchorPosition?.let {
            state.closestPageToPosition(it)?.prevKey?.plus(1)
                ?: state.closestPageToPosition(it)?.nextKey?.minus(1)
        }
    }
}

Summary

Nested classes
sealed

Params for a load request on a PagingSource from PagingSource.load.

sealed

Result of a load request from PagingSource.load.

Public constructors

Base class for an abstraction of pageable static data from some source, where loading pages of data is typically an expensive operation.

Public methods
abstract Key?
getRefreshKey(state: PagingState<Key, Value>)

Provide a Key used for the initial load for the next PagingSource due to invalidation of this PagingSource.

Unit

Signal the PagingSource to stop loading.

abstract suspend PagingSource.LoadResult<Key, Value>

Loading API for PagingSource.

Unit
registerInvalidatedCallback(onInvalidatedCallback: () -> Unit)

Add a callback to invoke when the PagingSource is first invalidated.

Unit
unregisterInvalidatedCallback(onInvalidatedCallback: () -> Unit)

Remove a previously added invalidate callback.

Properties
Boolean

Whether this PagingSource has been invalidated, which should happen when the data this PagingSource represents changes since it was first instantiated.

open Boolean

true if this PagingSource supports jumping, false otherwise.

open Boolean

true if this PagingSource expects to re-use keys to load distinct pages without a call to invalidate, false otherwise.

Public constructors

<init>

PagingSource()

Base class for an abstraction of pageable static data from some source, where loading pages of data is typically an expensive operation. Some examples of common PagingSources might be from network or from a database.

An instance of a PagingSource is used to load pages of data for an instance of PagingData.

A PagingData can grow as it loads more data, but the data loaded cannot be updated. If the underlying data set is modified, a new PagingSource / PagingData pair must be created to represent an updated snapshot of the data.

Loading Pages

PagingData queries data from its PagingSource in response to loading hints generated as the user scrolls in a RecyclerView.

To control how and when a PagingData queries data from its PagingSource, see PagingConfig, which defines behavior such as PagingConfig.pageSize and PagingConfig.prefetchDistance.

Updating Data

A PagingSource / PagingData pair is a snapshot of the data set. A new PagingData / PagingData must be created if an update occurs, such as a reorder, insert, delete, or content update occurs. A PagingSource must detect that it cannot continue loading its snapshot (for instance, when Database query notices a table being invalidated), and call invalidate. Then a new PagingSource /