PageKeyedDataSource

public abstract class PageKeyedDataSource
extends DataSource

java.lang.Object
   ↳ android.arch.paging.DataSource
     ↳ android.arch.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

class PageKeyedDataSource.LoadCallback

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

class PageKeyedDataSource.LoadInitialCallback

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

class PageKeyedDataSource.LoadInitialParams

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

class PageKeyedDataSource.LoadParams

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

Public constructors

PageKeyedDataSource()

Public methods

abstract void loadAfter(LoadParams params, LoadCallback callback)

Append page with the key specified by LoadParams.key.

abstract void loadBefore(LoadParams params, LoadCallback callback)

Prepend page with the key specified by LoadParams.key.

abstract void loadInitial(LoadInitialParams params, LoadInitialCallback callback)

Load initial data.

final PageKeyedDataSource map(Function function)

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

final PageKeyedDataSource mapByPage(Function, List> function)

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

Inherited methods

Public constructors

PageKeyedDataSource

PageKeyedDataSource ()

Public methods

loadAfter

void loadAfter (LoadParams params, 
                LoadCallback 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 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

void loadBefore (LoadParams params, 
                LoadCallback 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 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

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

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

PageKeyedDataSource map (Function 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 A new DataSource, which transforms items using the given function.

mapByPage

PageKeyedDataSource mapByPage (Function, List> 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 A new DataSource, which transforms items using the given function.