PositionalDataSource

Added in 2.0.0
Deprecated in 3.0.0
kotlin.Any
   ↳ androidx.paging.DataSource
     ↳ androidx.paging.PositionalDataSource

Position-based data loader for a fixed-size, countable data set, supporting fixed-size loads at arbitrary page positions.

Extend PositionalDataSource if you can load pages of a requested size at arbitrary positions, and provide a fixed item count. If your data source can't support loading arbitrary requested page sizes (e.g. when network page size constraints are only known at runtime), either use PageKeyedDataSource or ItemKeyedDataSource, or pass the initial result with the two parameter LoadInitialCallback.onResult.

Room can generate a Factory of PositionalDataSources for you:

@Dao
interface UserDao {
    @Query("SELECT * FROM user ORDER BY age DESC")
    public abstract DataSource.Factory<Integer, User> loadUsersByAgeDesc();
}
Parameters
<T : Any>

Type of items being loaded by the PositionalDataSource.

Summary

Nested types

Callback for loadInitial to return data, position, and count.

Holder object for inputs to loadInitial.

Callback for PositionalDataSource loadRange to return data.

Holder object for inputs to loadRange.

Public companion functions
Int

Helper for computing an initial position in loadInitial when total data set size can be computed ahead of loading.
android
Int
computeInitialLoadSize(
    params: PositionalDataSource.LoadInitialParams,
    initialLoadPosition: Int,
    totalCount: Int
)

Helper for computing an initial load size in loadInitial when total data set size can be computed ahead of loading.
android

Public constructors
android

Public functions
abstract Unit

Load initial list data.
android
abstract Unit

Called to load a range of data from the DataSource.
android
final PositionalDataSource<V>
<V : Any> map(function: Function<T, V>)

Applies the given function to each value emitted by the DataSource.
android
final PositionalDataSource<V>
<V : Any> map(function: (T) -> V)

Applies the given function to each value emitted by the DataSource.
android
final PositionalDataSource<V>
<V : Any> mapByPage(function: Function<List<T>, List<V>>)

Applies the given function to each value emitted by the DataSource.
android
final PositionalDataSource<V>
<V : Any> mapByPage(function: (List<T>) -> List<V>)

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

Inherited functions
From androidx.paging.DataSource
open Unit

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

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

Remove a previously added invalidate callback.
android

Inherited properties
From androidx.paging.DataSource
open Boolean
android

Public companion functions

computeInitialLoadPosition

android
Added in 3.3.4
Deprecated in 3.3.4
fun computeInitialLoadPosition(
    params: PositionalDataSource.LoadInitialParams,
    totalCount: Int
): Int

Helper for computing an initial position in loadInitial when total data set size can be computed ahead of loading.

The value computed by this function will do bounds checking, page alignment, and positioning based on initial load size requested.

Example usage in a PositionalDataSource subclass:

class ItemDataSource extends PositionalDataSource<Item> {
    private int computeCount() {
        // actual count code here
    }

    private List<Item> loadRangeInternal(int startPosition, int loadCount) {
        // actual load code here
    }

    @Override
    public void loadInitial(@NonNull LoadInitialParams params,
        @NonNull LoadInitialCallback<Item> callback) {
        int totalCount = computeCount();
        int position = computeInitialLoadPosition(params, totalCount);
        int loadSize = computeInitialLoadSize(params, position, totalCount);
        callback.onResult(loadRangeInternal(position, loadSize), position, totalCount);
    }

    @Override
    public void loadRange(@NonNull LoadRangeParams params,
        @NonNull LoadRangeCallback<Item> callback) {
        callback.onResult(loadRangeInternal(params.startPosition, params.loadSize));
    }
}
Parameters
params: PositionalDataSource.LoadInitialParams

Params passed to loadInitial, including page size, and requested start / loadSize.
totalCount: Int

Total size of the data set.
Returns
Int

Position to start loading at.
See also
computeInitialLoadSize

computeInitialLoadSize

android
Added in 3.3.4
Deprecated in 3.3.4
fun computeInitialLoadSize(
    params: PositionalDataSource.LoadInitialParams,
    initialLoadPosition: Int,
    totalCount: Int
): Int

Helper for computing an initial load size in loadInitial when total data set size can be computed ahead of loading.

This function takes the requested load size, and bounds checks it against the value returned by computeInitialLoadPosition.

Example usage in a PositionalDataSource subclass:

class ItemDataSource extends PositionalDataSource<Item> {
    private int computeCount() {
        // actual count code here
    }

    private List<Item> loadRangeInternal(int startPosition, int loadCount) {
        // actual load code here
    }

    @Override
    public void loadInitial(@NonNull LoadInitialParams params,
        @NonNull LoadInitialCallback<Item> callback) {
        int totalCount = computeCount();
        int position = computeInitialLoadPosition(params, totalCount);
        int loadSize = computeInitialLoadSize(params, position, totalCount);
        callback.onResult(loadRangeInternal(position, loadSize), position, totalCount);
    }

    @Override
    public void loadRange(@NonNull LoadRangeParams params,
        @NonNull LoadRangeCallback<Item> callback) {
        callback.onResult(loadRangeInternal(params.startPosition, params.loadSize));
    }
}
Parameters
params: PositionalDataSource.LoadInitialParams

Params passed to loadInitial, including page size, and requested start / loadSize.
initialLoadPosition: Int

Value returned by computeInitialLoadPosition
totalCount: Int

Total size of the data set.
Returns
Int

Number of items to load.
See also
computeInitialLoadPosition

Public constructors

PositionalDataSource

android 
<T : Any> PositionalDataSource()
Parameters
<T : Any>

Type of items being loaded by the PositionalDataSource.

Public functions

loadInitial

android
Added in 2.0.0
Deprecated in 3.0.0
@WorkerThread
abstract fun loadInitial(
    params: PositionalDataSource.LoadInitialParams,
    callback: PositionalDataSource.LoadInitialCallback<T>
): Unit

Load initial list data.

This method is called to load the initial page(s) from the DataSource.

LoadResult list must be a multiple of pageSize to enable efficient tiling.

Parameters
params: PositionalDataSource.LoadInitialParams

Parameters for initial load, including requested start position, load size, and page size.
callback: PositionalDataSource.LoadInitialCallback<T>

Callback that receives initial load data, including position and total data set size.

loadRange

android
Added in 2.0.0
Deprecated in 3.0.0
@WorkerThread
abstract fun loadRange(
    params: PositionalDataSource.LoadRangeParams,
    callback: PositionalDataSource.LoadRangeCallback<T>
): Unit

Called to load a range of data from the DataSource.

This method is called to load additional pages from the DataSource after the LoadInitialCallback passed to dispatchLoadInitial has initialized a PagedList.

Unlike loadInitial, this method must return the number of items requested, at the position requested.

Parameters
params: PositionalDataSource.LoadRangeParams

Parameters for load, including start position and load size.
callback: PositionalDataSource.LoadRangeCallback<T>

Callback that receives loaded data.

map

android
Added in 2.0.0
Deprecated in 3.0.0
final fun <V : Any> map(function: Function<T, V>): PositionalDataSource<V>

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

Same as mapByPage, but operates on individual items.

Parameters
function: Function<T, V>

Function that runs on each loaded item, returning items of a potentially new type.
Returns
PositionalDataSource<V>

A new DataSource, which transforms items using the given function.
See also
mapByPage
map
mapByPage

map

android
Added in 3.0.0
Deprecated in 3.0.0
final fun <V : Any> map(function: (T) -> V): PositionalDataSource<V>

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

An overload of map that accepts a kotlin function type.

Same as mapByPage, but operates on individual items.

Parameters
function: (T) -> V

Function that runs on each loaded item, returning items of a potentially new type.
Returns
PositionalDataSource<V>

A new DataSource, which transforms items using the given function.
See also
mapByPage
map

mapByPage

android
Added in 2.0.0
Deprecated in 3.0.0
final fun <V : Any> mapByPage(function: Function<List<T>, List<V>>): PositionalDataSource<V>

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

Same as map, but allows for batch conversions.

Parameters
function: Function<List<T>, List<V>>

Function that runs on each loaded page, returning items of a potentially new type.
Returns
PositionalDataSource<V>

A new DataSource, which transforms items using the given function.
See also
map
map
mapByPage

mapByPage

android 
final fun <V : Any> mapByPage(function: (List<T>) -> List<V>): PositionalDataSource<V>

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

An overload of mapByPage that accepts a kotlin function type.

Same as map, but allows for batch conversions.

Parameters
function: (List<T>) -> List<V>

Function that runs on each loaded page, returning items of a potentially new type.
Returns
PositionalDataSource<V>

A new DataSource, which transforms items using the given function.
See also
map
map
mapByPage