PageKeyedDataSource

abstract class PageKeyedDataSource<Key : Any!, Value : Any!> : DataSource<Key, Value>

kotlin.Any
↳	androidx.paging.DataSource<Key, Value>
	↳	androidx.paging.PageKeyedDataSource

Incremental data loader for page-keyed content, where requests return keys for next/previous pages.

Implement a DataSource using PageKeyedDataSource if you need to use data from page N - 1 to load page N. This is common, for example, in network APIs that include a next/previous link or key with each page load.

The InMemoryByPageRepository in the PagingWithNetworkSample shows how to implement a network PageKeyedDataSource using Retrofit, while handling swipe-to-refresh, network errors, and retry.

Summary

Nested classes
abstract	`LoadCallback` Callback for PageKeyedDataSource `#loadBefore(LoadParams, LoadCallback)` and `#loadAfter(LoadParams, LoadCallback)` to return data.
abstract	`LoadInitialCallback` Callback for `#loadInitial(LoadInitialParams, LoadInitialCallback)` to return data and, optionally, position/count information.
open	`LoadInitialParams` Holder object for inputs to `#loadInitial(LoadInitialParams, LoadInitialCallback)`.
open	`LoadParams` Holder object for inputs to `#loadBefore(LoadParams, LoadCallback)` and `#loadAfter(LoadParams, LoadCallback)`.

Public methods
abstract Unit	`loadBefore(params: PageKeyedDataSource.LoadParams<Key>, callback: PageKeyedDataSource.LoadCallback<Key, Value>)` Prepend page with the key specified by `LoadParams.key`.
open Boolean	`isContiguous()`
abstract Unit	`loadInitial(params: PageKeyedDataSource.LoadInitialParams<Key>, callback: PageKeyedDataSource.LoadInitialCallback<Key, Value>)` Load initial data.
abstract Unit	`loadAfter(params: PageKeyedDataSource.LoadParams<Key>, callback: PageKeyedDataSource.LoadCallback<Key, Value>)` Append page with the key specified by `LoadParams.key`.
PageKeyedDataSource<Key, ToValue>	`map(function: Function)`
PageKeyedDataSource<Key, ToValue>	`mapByPage(function: Function)`

Inherited functions

From class DataSource

`Unit`	`addInvalidatedCallback(onInvalidatedCallback: DataSource.InvalidatedCallback)` Add a callback to invoke when the DataSource is first invalidated. Once invalidated, a data source will not become valid again. A data source will only invoke its callbacks once - the first time `#invalidate()` is called, on that thread.
`Unit`	`invalidate()` Signal the data source to stop loading, and notify its callback. If invalidate has already been called, this method does nothing.
`Boolean`	`isInvalid()` Returns true if the data source is invalid, and can no longer be queried for data.
`Unit`	`removeInvalidatedCallback(onInvalidatedCallback: DataSource.InvalidatedCallback)` Remove a previously added invalidate callback.

Public methods

loadBefore

abstract fun loadBefore(params: PageKeyedDataSource.LoadParams<Key>, callback: PageKeyedDataSource.LoadCallback<Key, Value>): Unit

Prepend page with the key specified by LoadParams.key.

It's valid to return a different list size than the page size if it's easier, e.g. if your backend defines page sizes. It is generally safer to increase the number loaded than reduce.

Data may be passed synchronously during the load method, or deferred and called at a later time. Further loads going down will be blocked until the callback is called.

If data cannot be loaded (for example, if the request is invalid, or the data would be stale and inconsistent, it is valid to call #invalidate() to invalidate the data source, and prevent further loading.

Parameters
`params`	PageKeyedDataSource.LoadParams<Key>: Parameters for the load, including the key for the new page, and requested load size.
`callback`	PageKeyedDataSource.LoadParams<Key>: Callback that receives loaded data.

isContiguous

open fun isContiguous(): Boolean

loadInitial

abstract fun loadInitial(params: PageKeyedDataSource.LoadInitialParams<Key>, callback: PageKeyedDataSource.LoadInitialCallback<Key, Value>): Unit

Load initial data.

This method is called first to initialize a PagedList with data. If it's possible to count the items that can be loaded by the DataSource, it's recommended to pass the loaded data to the callback via the three-parameter LoadInitialCallback#onResult(List, int, int, Object, Object). This enables PagedLists presenting data from this source to display placeholders to represent unloaded items.

LoadInitialParams#requestedLoadSize is a hint, not a requirement, so it may be may be altered or ignored.

Parameters
`params`	PageKeyedDataSource.LoadInitialParams<Key>: Parameters for initial load, including requested load size.
`callback`	PageKeyedDataSource.LoadInitialParams<Key>: Callback that receives initial load data.

loadAfter

abstract fun loadAfter(params: PageKeyedDataSource.LoadParams<Key>, callback: PageKeyedDataSource.LoadCallback<Key, Value>): Unit

Append page with the key specified by LoadParams.key.