ItemKeyedDataSource

public abstract class ItemKeyedDataSource
extends DataSource<Key, Value>

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


Incremental data loader for paging keyed content, where loaded content uses previously loaded items as input to future loads.

Implement a DataSource using ItemKeyedDataSource if you need to use data from item N - 1 to load item N. This is common, for example, in sorted database queries where attributes of the item such just before the next query define how to execute it.

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

Summary

Nested classes

class ItemKeyedDataSource.LoadCallback<Value>

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

class ItemKeyedDataSource.LoadInitialCallback<Value>

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

class ItemKeyedDataSource.LoadInitialParams<Key>

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

class ItemKeyedDataSource.LoadParams<Key>

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

Public constructors

ItemKeyedDataSource()

Public methods

abstract Key getKey(Value item)

Return a key associated with the given item.

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

Load list data after the key specified in LoadParams.key.

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

Load list data before the key specified in LoadParams.key.

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

Load initial data.

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

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

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

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

Inherited methods

Public constructors

ItemKeyedDataSource

public ItemKeyedDataSource ()

Public methods

getKey

public abstract Key getKey (Value item)

Return a key associated with the given item.

If your ItemKeyedDataSource is loading from a source that is sorted and loaded by a unique integer ID, you would return item.getID() here. This key can then be passed to loadBefore(LoadParams, LoadCallback) or loadAfter(LoadParams, LoadCallback) to load additional items adjacent to the item passed to this function.

If your key is more complex, such as when you're sorting by name, then resolving collisions with integer ID, you'll need to return both. In such a case you would use a wrapper class, such as Pair<String, Integer> or, in Kotlin, data class Key(val name: String, val id: Int)

Parameters
item Value: Item to get the key from.

Returns
Key Key associated with given item.

loadAfter

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

Load list data after the key specified in 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 loadAfter 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 to load after, and requested size.

callback LoadCallback: Callback that receives loaded data.

loadBefore

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

Load list data before the key specified in 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.

Note: Data returned will be prepended just before the key passed, so if you vary size, ensure that the last item is adjacent to the passed key.

Data may be passed synchronously during the loadBefore method, or deferred and called at a later time. Further loads going up 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 to load before, and requested size.

callback LoadCallback: Callback that receives loaded data.

loadInitial

public abstract void loadInitial (LoadInitialParams<Key> params, 
                LoadInitialCallback<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 ItemKeyedDataSource.LoadInitialCallback.onResult(List, int, int). This enables PagedLists presenting data from this source to display placeholders to represent unloaded items.

ItemKeyedDataSource.LoadInitialParams.requestedInitialKey and ItemKeyedDataSource.LoadInitialParams.requestedLoadSize are hints, not requirements, so they may be altered or ignored. Note that ignoring the requestedInitialKey can prevent subsequent PagedList/DataSource pairs from initializing at the same location. If your data source never invalidates (for example, loading from the network without the network ever signalling that old data must be reloaded), it's fine to ignore the initialLoadKey and always start from the beginning of the data set.

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

callback LoadInitialCallback: Callback that receives initial load data.

map

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

mapByPage

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