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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.Nullable;
import android.support.annotation.RestrictTo;
import android.support.annotation.WorkerThread;
import java.util.ArrayList;
import java.util.List;
/**
* Simplest data source form that provides all of its data through a single loadRange() method.
* <p>
* Requires that your data resides in positions <code>0</code> through <code>N</code>, where
* <code>N</code> is the value returned from {@link #countItems()}. You must return the exact number
* requested, so that the data as returned can be safely prepended/appended to what has already
* been loaded.
* <p>
* For more flexibility in how many items to load, or to avoid counting your data source, override
* {@link PositionalDataSource} directly.
*
* @param <Value> Value type returned by the data source.
*
* @hide
*/
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public abstract class BoundedDataSource<Value> extends PositionalDataSource<Value> {
/**
* Called to load items at from the specified position range.
*
* @param startPosition Index of first item to load.
* @param loadCount Exact number of items to load. Returning a different number will cause
* an exception to be thrown.
* @return List of loaded items. Null if the BoundedDataSource is no longer valid, and should
* not be queried again.
*/
@WorkerThread
@Nullable
public abstract List<Value> loadRange(int startPosition, int loadCount);
@WorkerThread
@Nullable
@Override
public List<Value> loadAfter(int startIndex, int pageSize) {
return loadRange(startIndex, pageSize);
}
@WorkerThread
@Nullable
@Override
public List<Value> loadBefore(int startIndex, int pageSize) {
if (startIndex < 0) {
return new ArrayList<>();
}
int loadSize = Math.min(pageSize, startIndex + 1);
startIndex = startIndex - loadSize + 1;
List<Value> result = loadRange(startIndex, loadSize);
if (result != null) {
if (result.size() != loadSize) {
throw new IllegalStateException("invalid number of items returned.");
}
}
return result;
}
}
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