ItemKeyedDataSource
abstract classItemKeyedDataSource<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 | |
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abstract |
Callback for ItemKeyedDataSource loadBefore and loadAfter to return data. |
abstract |
Callback for loadInitial to return data and, optionally, position/count information. |
open |
Holder object for inputs to loadInitial. |
open |
Holder object for inputs to loadBefore and loadAfter. |
Public constructors | |
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<init>() Incremental data loader for paging keyed content, where loaded content uses previously loaded items as input to future loads. |
Public methods | |
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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> |
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> |
Applies the given function to each value emitted by the DataSource. |
ItemKeyedDataSource<Key, ToValue> |
Applies the given function to each value emitted by the DataSource. |
Inherited functions | |
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Inherited properties | |
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Public constructors
<init>
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.
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 | |
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item: Value | Item to get the key from. |
Return | |
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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 | |
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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.
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 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. |