PageKeyedDataSource.LoadInitialCallback

public abstract class PageKeyedDataSource.LoadInitialCallback<Key extends Object, Value extends Object>


Callback for loadInitial to return data and, optionally, position/count information.

A callback can be called only once, and will throw if called again.

If you can compute the number of items in the data set before and after the loaded range, call the five parameter onResult to pass that information. You can skip passing this information by calling the three parameter onResult, either if it's difficult to compute, or if LoadInitialParams.placeholdersEnabled is false, so the positioning information will be ignored.

It is always valid for a DataSource loading method that takes a callback to stash the callback and call it later. This enables DataSources to be fully asynchronous, and to handle temporary, recoverable error states (such as a network error that can be retried).

Parameters
<Key extends Object>

Type of data used to query pages.

<Value extends Object>

Type of items being loaded.

Summary

Public constructors

<Key extends Object, Value extends Object> LoadInitialCallback()

Public methods

abstract void
onResult(
    @NonNull List<@NonNull Value> data,
    int position,
    int totalCount,
    @Nullable Key previousPageKey,
    @Nullable Key nextPageKey
)

Called to pass initial load state from a DataSource.

abstract void
onResult(
    @NonNull List<@NonNull Value> data,
    @Nullable Key previousPageKey,
    @Nullable Key nextPageKey
)

Called to pass loaded data from a DataSource.

Public constructors

LoadInitialCallback

@NonNull
public final <Key extends Object, Value extends Object> LoadInitialCallback()
Parameters
<Key extends Object>

Type of data used to query pages.

<Value extends Object>

Type of items being loaded.

Public methods

onResult

@NonNull
public abstract void onResult(
    @NonNull List<@NonNull Value> data,
    int position,
    int totalCount,
    @Nullable Key previousPageKey,
    @Nullable Key nextPageKey
)

Called to pass initial load state from a DataSource.

Call this method from your DataSource's loadInitial function to return data, and inform how many placeholders should be shown before and after. If counting is cheap to compute (for example, if a network load returns the information regardless), it's recommended to pass data back through this method.

It is always valid to pass a different amount of data than what is requested. Pass an empty list if there is no more data to load.

Parameters
@NonNull List<@NonNull Value> data

List of items loaded from the DataSource. If this is empty, the DataSource is treated as empty, and no further loads will occur.

int position

Position of the item at the front of the list. If there are N items before the items in data that can be loaded from this DataSource, pass N.

int totalCount

Total number of items that may be returned from this DataSource. Includes the number in the initial data parameter as well as any items that can be loaded in front or behind of data.

onResult

@NonNull
public abstract void onResult(
    @NonNull List<@NonNull Value> data,
    @Nullable Key previousPageKey,
    @Nullable Key nextPageKey
)

Called to pass loaded data from a DataSource.

Call this from loadInitial to initialize without counting available data, or supporting placeholders.

It is always valid to pass a different amount of data than what is requested. Pass an empty list if there is no more data to load.

Parameters
@NonNull List<@NonNull Value> data

List of items loaded from the PageKeyedDataSource.

@Nullable Key previousPageKey

Key for page before the initial load result, or null if no more data can be loaded before.

@Nullable Key nextPageKey

Key for page after the initial load result, or null if no more data can be loaded after.