android/arch/paging/ContiguousDataSource.java

/*
 * 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;

/** @hide */
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public abstract class ContiguousDataSource<Key, Value> extends DataSource<Key, Value> {
    /**
     * Number of items that this DataSource can provide in total, or COUNT_UNDEFINED.
     *
     * @return number of items that this DataSource can provide in total, or COUNT_UNDEFINED
     * if difficult or undesired to compute.
     */
    public int countItems() {
        return COUNT_UNDEFINED;
    }

    @Override
    boolean isContiguous() {
        return true;
    }

    /** @hide */
    @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
    @WorkerThread
    @Nullable
    public abstract NullPaddedList<Value> loadInitial(
            Key key, int initialLoadSize, boolean enablePlaceholders);

    /**
     * Load data after the given position / item.
     * <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 number loaded than reduce.
     *
     * @param currentEndIndex Load items after this index, starting with currentEndIndex + 1.
     * @param currentEndItem  Load items after this item, can be used for precise querying based on
     *                        item contents.
     * @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     * no longer valid, and should not be queried again.
     *
     * @hide
     */
    @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
    @WorkerThread
    @Nullable
    public final List<Value> loadAfter(int currentEndIndex,
            @NonNull Value currentEndItem, int pageSize) {
        if (isInvalid()) {
            return null;
        }
        List<Value> list = loadAfterImpl(currentEndIndex, currentEndItem, pageSize);
        if (isInvalid()) {
            return null;
        }
        return list;
    }

    @Nullable
    abstract List<Value> loadAfterImpl(int currentEndIndex,
            @NonNull Value currentEndItem, int pageSize);

    /**
     * Load data before the given position / item.
     * <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 number loaded than reduce.
     *
     * @param currentBeginIndex Load items before this index, starting with currentBeginIndex - 1.
     * @param currentBeginItem  Load items after this item, can be used for precise querying based
     *                          on item contents.
     * @param pageSize          Suggested number of items to load.
     * @return List of items, in descending order, starting at position currentBeginIndex - 1.
     *
     * @hide
     */
    @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
    @WorkerThread
    @Nullable
    public final List<Value> loadBefore(int currentBeginIndex,
            @NonNull Value currentBeginItem, int pageSize) {
        if (isInvalid()) {
            return null;
        }
        List<Value> list = loadBeforeImpl(currentBeginIndex, currentBeginItem, pageSize);
        if (isInvalid()) {
            return null;
        }
        return list;

    }

    @Nullable
    abstract List<Value> loadBeforeImpl(int currentBeginIndex,
            @NonNull Value currentBeginItem, int pageSize);

    /**
     * Get the key from either the position, or item. Position may not match passed item's position,
     * if trying to query the key from a position that isn't yet loaded, so a fallback item must be
     * used.
     */
    abstract Key getKey(int position, Value item);
}