ItemKeyedDataSource

Kotlin |Java

abstract class ItemKeyedDataSource<Key : Any, Value : Any> : DataSource<Key, Value>

kotlin.Any
↳	androidx.paging.DataSource<Key, Value>
	↳	androidx.paging.ItemKeyedDataSource

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 uniquely 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
abstract	`LoadCallback` Callback for ItemKeyedDataSource loadBefore and loadAfter to return data.
abstract	`LoadInitialCallback` Callback for loadInitial to return data and, optionally, position/count information.
open	`LoadInitialParams` Holder object for inputs to loadInitial.
open	`LoadParams` Holder object for inputs to loadBefore and loadAfter.

Public constructors
`<init>()` Incremental data loader for paging keyed content, where loaded content uses previously loaded items as input to future loads.

Public methods
abstract Key	`getKey(item: Value)` Return a key associated with the given item.
abstract Unit	`loadAfter(params: ItemKeyedDataSource.LoadParams<Key>, callback: ItemKeyedDataSource.LoadCallback<Value>)` Load list data after the key specified in LoadParams.key.
abstract Unit	`loadBefore(params: ItemKeyedDataSource.LoadParams<Key>, callback: ItemKeyedDataSource.LoadCallback<Value>)` Load list data before the key specified in LoadParams.key.
abstract Unit	`loadInitial(params: ItemKeyedDataSource.LoadInitialParams<Key>, callback: ItemKeyedDataSource.LoadInitialCallback<Value>)` Load initial data.
ItemKeyedDataSource<Key, ToValue>	`map(function: Function<Value, ToValue>)` Applies the given function to each value emitted by the DataSource.
ItemKeyedDataSource<Key, ToValue>	`map(function: (Value) -> ToValue)` Applies the given function to each value emitted by the DataSource.
ItemKeyedDataSource<Key, ToValue>	`mapByPage(function: Function<List<Value>, List<ToValue>>)` Applies the given function to each value emitted by the DataSource.
ItemKeyedDataSource<Key, ToValue>	`mapByPage(function: (List<Value>) -> List<ToValue>)` Applies the given function to each value emitted by the DataSource.

Inherited functions

From class DataSource

`Unit`	`<no name provided>()` Invalidation callback for DataSource. Used to signal when a DataSource a data source has become invalid, and that a new data source is needed to continue loading data.
`Unit`	`addInvalidatedCallback(onInvalidatedCallback: DataSource.InvalidatedCallback)` 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.
`Unit`	`invalidate()` Signal the data source to stop loading, and notify its callback. If invalidate has already been called, this method does nothing.
`Unit`	`removeInvalidatedCallback(onInvalidatedCallback: DataSource.InvalidatedCallback)` Remove a previously added invalidate callback.

Inherited properties

From class DataSource

Boolean

isInvalid

Public constructors

<init>

ItemKeyedDataSource()

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

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

Parameters
`Key`	Type of data used to query Value types out of the DataSource.
`Value`	Type of items being loaded by the DataSource.

Public methods

getKey

abstract fun getKey(item: Value): Key

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 or loadAfter 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.

Return
Key associated with given item.

loadAfter

abstract fun loadAfter(
    params: ItemKeyedDataSource.LoadParams<Key>, 
    callback: ItemKeyedDataSource.LoadCallback<Value>
): Unit

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 preferred 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 invalidate to invalidate the data source, and prevent further loading.

Parameters
params: ItemKeyedDataSource.LoadParams<Key>	Parameters for the load, including the key to load after, and requested size.
callback: ItemKeyedDataSource.LoadCallback<Value>	Callback that receives loaded data.

loadBefore

abstract fun loadBefore(
    params: ItemKeyedDataSource.LoadParams<Key>, 
    callback: ItemKeyedDataSource.LoadCallback<Value>
): Unit

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 preferred 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.

Parameters
params: ItemKeyedDataSource.LoadParams<Key>	Parameters for the load, including the key to load before, and requested size.
callback: ItemKeyedDataSource.LoadCallback<Value>	Callback that receives loaded data.

loadInitial

abstract fun loadInitial(
    params: ItemKeyedDataSource.LoadInitialParams<Key>, 
    callback: ItemKeyedDataSource.LoadInitialCallback<Value>
): Unit

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 LoadInitialCallback.onResult. This enables PagedLists presenting data from this source to display placeholders to represent unloaded items.

LoadInitialParams.requestedInitialKey and 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 DataSource 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: ItemKeyedDataSource.LoadInitialParams<Key>	Parameters for initial load, including initial key and requested size.
callback: ItemKeyedDataSource.LoadInitialCallback<Value>	Callback that receives initial load data.