PageKeyedDataSource

public abstract class PageKeyedDataSource
extends DataSource<Key, Value>

java.lang.Object
   ↳ androidx.paging.DataSource<Key, Value>
     ↳ androidx.paging.PageKeyedDataSource<Key, Value>


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

class PageKeyedDataSource.LoadCallback<Key, Value>

Callback for PageKeyedDataSource PageKeyedDataSource.loadBefore(LoadParams, LoadCallback) and PageKeyedDataSource.loadAfter(LoadParams, LoadCallback) to return data. 

class PageKeyedDataSource.LoadInitialCallback<Key, Value>

Callback for PageKeyedDataSource.loadInitial(LoadInitialParams, LoadInitialCallback) to return data and, optionally, position/count information. 

class PageKeyedDataSource.LoadInitialParams<Key>

Holder object for inputs to PageKeyedDataSource.loadInitial(LoadInitialParams, LoadInitialCallback)

class PageKeyedDataSource.LoadParams<Key>

Holder object for inputs to PageKeyedDataSource.loadBefore(LoadParams, LoadCallback) and PageKeyedDataSource.loadAfter(LoadParams, LoadCallback)

Public constructors

PageKeyedDataSource()

Public methods

abstract void loadAfter(LoadParams<Key> params, LoadCallback<Key, Value> callback)

Append page with the key specified by LoadParams.key.

abstract void loadBefore(LoadParams<Key> params, LoadCallback<Key, Value> callback)

Prepend page with the key specified by LoadParams.key.

abstract void loadInitial(LoadInitialParams<Key> params, LoadInitialCallback<Key, Value> callback)

Load initial data.

final <ToValue> PageKeyedDataSource<Key, ToValue> map(Function<Value, ToValue> function)

Applies the given function to each value emitted by the DataSource.

final <ToValue> PageKeyedDataSource<Key, ToValue> mapByPage(Function<List<Value>, List<ToValue>> function)

Applies the given function to each value emitted by the DataSource.

Inherited methods

Public constructors

PageKeyedDataSource

public PageKeyedDataSource ()

Public methods

loadAfter

public abstract void loadAfter (LoadParams<Key> params, 
                LoadCallback<Key, Value> callback)

Append 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 DataSource.invalidate() to invalidate the data source, and prevent further loading.

Parameters
params LoadParams: Parameters for the load, including the key for the new page, and requested load size.

callback LoadCallback: Callback that receives loaded data.

loadBefore

public abstract void loadBefore (LoadParams<Key> params, 
                LoadCallback<Key, Value> callback)

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 DataSource.invalidate() to invalidate the data source, and prevent further loading.

Parameters
params LoadParams: Parameters for the load, including the key for the new page, and requested load size.

callback LoadCallback: Callback that receives loaded data.

loadInitial

public abstract void loadInitial (LoadInitialParams<Key> params, 
                LoadInitialCallback<Key, Value> callback)

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 PageKeyedDataSource.LoadInitialCallback.onResult(List, int, int, Object, Object). This enables PagedLists presenting data from this source to display placeholders to represent unloaded items.

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

Parameters
params LoadInitialParams: Parameters for initial load, including requested load size.

callback LoadInitialCallback: Callback that receives initial load data.

map

public final PageKeyedDataSource<Key, ToValue> map (Function<Value, ToValue> function)

Applies the given function to each value emitted by the DataSource.

Same as mapByPage(Function), but operates on individual items.

Parameters
function Function: Function that runs on each loaded item, returning items of a potentially new type.

Returns
PageKeyedDataSource<Key, ToValue> A new DataSource, which transforms items using the given function.

mapByPage

public final PageKeyedDataSource<Key, ToValue> mapByPage (Function<List<Value>, List<ToValue>> function)

Applies the given function to each value emitted by the DataSource.

Same as map(Function), but allows for batch conversions.

Parameters
function Function: Function that runs on each loaded page, returning items of a potentially new type.

Returns
PageKeyedDataSource<Key, ToValue> A new DataSource, which transforms items using the given function.