ItemKeyedDataSource

public <Key extends Object, Value extends Object> ItemKeyedDataSource()

public abstract @NonNull Key getKey(@NonNull 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 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)

public abstract void loadAfter(
    @NonNull ItemKeyedDataSource.LoadParams<@NonNull Key> params,
    @NonNull ItemKeyedDataSource.LoadCallback<@NonNull 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 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.

public abstract void loadBefore(
    @NonNull ItemKeyedDataSource.LoadParams<@NonNull Key> params,
    @NonNull ItemKeyedDataSource.LoadCallback<@NonNull 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 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.

public abstract void loadInitial(
    @NonNull ItemKeyedDataSource.LoadInitialParams<@NonNull Key> params,
    @NonNull ItemKeyedDataSource.LoadInitialCallback<@NonNull 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 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.

public final @NonNull ItemKeyedDataSource<@NonNull Key, @NonNull ToValue> <ToValue extends Object> map(
    @NonNull Function<@NonNull Value, @NonNull ToValue> function
)

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

Same as mapByPage, but operates on individual items.

public final @NonNull ItemKeyedDataSource<@NonNull Key, @NonNull ToValue> <ToValue extends Object> map(
    @NonNull Function1<@NonNull Value, @NonNull ToValue> function
)

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.

public final @NonNull ItemKeyedDataSource<@NonNull Key, @NonNull ToValue> <ToValue extends Object> mapByPage(
    @NonNull Function<@NonNull List<@NonNull Value>, @NonNull List<@NonNull ToValue>> function
)

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

Same as map, but allows for batch conversions.

public final @NonNull ItemKeyedDataSource<@NonNull Key, @NonNull ToValue> <ToValue extends Object> mapByPage(
    @NonNull Function1<@NonNull List<@NonNull Value>, @NonNull List<@NonNull ToValue>> function
)

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.