DataSource

public abstract class DataSource
extends Object

java.lang.Object
   ↳ android.arch.paging.DataSource<Key, Value>

Base class for loading pages of snapshot data into a PagedList.

DataSource is queried to load pages of content into a PagedList. A PagedList can grow as it loads more data, but the data loaded cannot be updated. If the underlying data set is modified, a new PagedList / DataSource pair must be created to represent the new data.

Loading Pages

PagedList queries data from its DataSource in response to loading hints. PagedListAdapter calls loadAround(int) to load content as the user scrolls in a RecyclerView.

To control how and when a PagedList queries data from its DataSource, see PagedList.Config. The Config object defines things like load sizes and prefetch distance.

Updating Paged Data

A PagedList / DataSource pair are a snapshot of the data set. A new pair of PagedList / DataSource must be created if an update occurs, such as a reorder, insert, delete, or content update occurs. A DataSource 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 PagedList / DataSource pair would be created to load data from the new state of the Database query.

To page in data that doesn't update, you can create a single DataSource, and pass it to a single PagedList. For example, loading from network when the network's paging API doesn't provide updates.

To page in data from a source that does provide updates, you can create a DataSource.Factory, where each DataSource created is invalidated when an update to the data set occurs that makes the current snapshot invalid. For example, when paging a query from the Database, and the table being queried inserts or removes items. You can also use a DataSource.Factory to provide multiple versions of network-paged lists. If reloading all content (e.g. in response to an action like swipe-to-refresh) is required to get a new version of data, you can connect an explicit refresh signal to call invalidate() on the current DataSource.

If you have more granular update signals, such as a network API signaling an update to a single item in the list, it's recommended to load data from network into memory. Then present that data to the PagedList via a DataSource that wraps an in-memory snapshot. Each time the in-memory copy changes, invalidate the previous DataSource, and a new one wrapping the new state of the snapshot can be created.

Implementing a DataSource

To implement, extend one of the subclasses: PageKeyedDataSource, ItemKeyedDataSource, or PositionalDataSource.

Use PageKeyedDataSource if pages you load embed keys for loading adjacent pages. For example a network response that returns some items, and a next/previous page links.

Use ItemKeyedDataSource if you need to use data from item N-1 to load item N. For example, if requesting the backend for the next comments in the list requires the ID or timestamp of the most recent loaded comment, or if querying the next users from a name-sorted database query requires the name and unique ID of the previous.

Use PositionalDataSource if you can load pages of a requested size at arbitrary positions, and provide a fixed item count. PositionalDataSource supports querying pages at arbitrary positions, so can provide data to PagedLists in arbitrary order. Note that PositionalDataSource is required to respect page size for efficient tiling. If you want to override page size (e.g. when network page size constraints are only known at runtime), use one of the other DataSource classes.

Because a null item indicates a placeholder in PagedList, DataSource may not return null items in lists that it loads. This is so that users of the PagedList can differentiate unloaded placeholder items from content that has been paged in.

Summary

Nested classes

class DataSource.Factory<Key, Value>

Factory for DataSources. 

interface DataSource.InvalidatedCallback

Invalidation callback for DataSource. 

Public methods

void addInvalidatedCallback(DataSource.InvalidatedCallback onInvalidatedCallback)

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

void invalidate()

Signal the data source to stop loading, and notify its callback.

boolean isInvalid()

Returns true if the data source is invalid, and can no longer be queried for data.

abstract <ToValue> DataSource<Key, ToValue> map(Function<Value, ToValue> function)

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

abstract <ToValue> DataSource<Key, ToValue> mapByPage(Function<List<Value>, List<ToValue>> function)

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

void removeInvalidatedCallback(DataSource.InvalidatedCallback onInvalidatedCallback)

Remove a previously added invalidate callback.

Inherited methods

Public methods

addInvalidatedCallback

void addInvalidatedCallback (DataSource.InvalidatedCallback onInvalidatedCallback)

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.

Parameters
onInvalidatedCallback DataSource.InvalidatedCallback: The callback, will be invoked on thread that invalidate() is called on.

invalidate

void invalidate ()

Signal the data source to stop loading, and notify its callback.

If invalidate has already been called, this method does nothing.

isInvalid

boolean isInvalid ()

Returns true if the data source is invalid, and can no longer be queried for data.

Returns
boolean True if the data source is invalid, and can no longer return data.

map

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

mapByPage

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

removeInvalidatedCallback

void removeInvalidatedCallback (DataSource.InvalidatedCallback onInvalidatedCallback)

Remove a previously added invalidate callback.

Parameters
onInvalidatedCallback DataSource.InvalidatedCallback: The previously added callback.