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 java.util.List;

abstract class ContiguousDataSource<Key, Value> extends DataSource<Key, Value> {
    @Override
    boolean isContiguous() {
        return true;
    }

    abstract void loadInitial(Key key, int initialLoadSize, boolean enablePlaceholders,
            @NonNull PageResult.Receiver<Key, Value> receiver);

    void loadAfter(int currentEndIndex, @NonNull Value currentEndItem, int pageSize,
            @NonNull PageResult.Receiver<Key, Value> receiver) {
        if (!isInvalid()) {
            List<Value> list = loadAfterImpl(currentEndIndex, currentEndItem, pageSize);

            if (list != null && !isInvalid()) {
                receiver.postOnPageResult(new PageResult<>(
                        PageResult.APPEND, new Page<Key, Value>(list), 0, 0, 0));
                return;
            }
        }
        receiver.postOnPageResult(new PageResult<Key, Value>(PageResult.APPEND));
    }

    void loadBefore(int currentBeginIndex, @NonNull Value currentBeginItem, int pageSize,
            @NonNull PageResult.Receiver<Key, Value> receiver) {
        if (!isInvalid()) {
            List<Value> list = loadBeforeImpl(currentBeginIndex, currentBeginItem, pageSize);

            if (list != null && !isInvalid()) {
                receiver.postOnPageResult(new PageResult<>(
                        PageResult.PREPEND, new Page<Key, Value>(list), 0, 0, 0));
                return;
            }
        }
        receiver.postOnPageResult(new PageResult<Key, Value>(PageResult.PREPEND));
    }

    /**
     * Get the key from either the position, or item, or null if position/item invalid.
     * <p>
     * Position may not match passed item's position - if trying to query the key from a position
     * that isn't yet loaded, a fallback item (last loaded item accessed) will be passed.
     */
    abstract Key getKey(int position, Value item);

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

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