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/*
* Copyright (C) 2017 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package android.arch.paging;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.annotation.RestrictTo;
import android.support.annotation.WorkerThread;
import java.util.List;
/**
* Incremental data loader for paging positional content, where content can be loaded based on its
* integer position.
* <p>
* Use PositionalDataSource if you only need position as input for item loading - if for example,
* you're asking the backend for items at positions 10 through 20, or using a limit/offset database
* query to load items at query position 10 through 20.
* <p>
* Implement a DataSource using PositionalDataSource if position is the only information you need to
* load items.
* <p>
* Note that {@link BoundedDataSource} provides a simpler API for positional loading, if your
* backend or data store doesn't require
* <p>
* @param <Value> Value type of items being loaded by the DataSource.
* @hide
*/
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public abstract class PositionalDataSource<Value> extends ContiguousDataSource<Integer, Value> {
@Nullable
@Override
List<Value> loadAfterImpl(int currentEndIndex, @NonNull Value currentEndItem, int pageSize) {
return loadAfter(currentEndIndex + 1, pageSize);
}
@Nullable
@Override
List<Value> loadBeforeImpl(int currentBeginIndex, @NonNull Value currentBeginItem,
int pageSize) {
return loadBefore(currentBeginIndex - 1, pageSize);
}
/**
* Load initial data, starting after the passed position.
*
* @param position Index just before the data to be loaded.
* @param initialLoadSize Suggested number of items to load.
* @return List of initial items, representing data starting at position. Null if the
* DataSource is no longer valid, and should not be queried again.
* @hide
*/
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
@WorkerThread
@Nullable
@Override
public NullPaddedList<Value> loadInitial(
Integer position, int initialLoadSize, boolean enablePlaceholders) {
final int convertPosition = position == null ? 0 : position;
final int loadPosition = Math.max(0, (convertPosition - initialLoadSize / 2));
int count = COUNT_UNDEFINED;
if (enablePlaceholders) {
count = countItems();
}
List<Value> data = loadAfter(loadPosition, initialLoadSize);
if (count == COUNT_UNDEFINED) {
return new NullPaddedList<>(loadPosition, data);
} else {
return new NullPaddedList<>(loadPosition, count, data);
}
}
/**
* Load data after currently loaded content, starting at the provided index.
* <p>
* It's valid to return a different list size than the page size, if it's easier for this data
* source. It is generally safer to increase the number loaded than reduce.
*
* @param startIndex Load items starting at this index.
* @param pageSize Suggested number of items to load.
* @return List of items, starting at position currentEndIndex + 1. Null if the data source is
* no longer valid, and should not be queried again.
*/
@WorkerThread
@Nullable
public abstract List<Value> loadAfter(int startIndex, int pageSize);
/**
* Load data before the currently loaded content, starting at the provided index.
* <p>
* It's valid to return a different list size than the page size, if it's easier for this data
* source. It is generally safer to increase the number loaded than reduce.
*
* @param startIndex Load items, starting at this index.
* @param pageSize Suggested number of items to load.
* @return List of items, in descending order, starting at position currentBeginIndex - 1. Null
* if the data source is no longer valid, and should not be queried again.
*/
@WorkerThread
@Nullable
public abstract List<Value> loadBefore(int startIndex, int pageSize);
@Override
Integer getKey(int position, Value item) {
return position;
}
}
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