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Core module for the gcloud-java.
There is a newer version: 0.2.8
/*
 * Copyright 2016 Google Inc. All Rights Reserved.
 *
 * 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 com.google.cloud;

import com.google.common.base.Throwables;
import com.google.common.util.concurrent.Futures;
import com.google.common.util.concurrent.Uninterruptibles;

import java.io.Serializable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;

/**
 * Base implementation for asynchronously consuming Google Cloud paginated results.
 *
 * @param  the value type that the page holds
 */
public class AsyncPageImpl extends PageImpl implements AsyncPage {

  private static final long serialVersionUID = -6009473188630364906L;

  private final NextPageFetcher asyncPageFetcher;

  /**
   * Interface for asynchronously fetching the next page of results from the service.
   *
   * @param  the value type that the page holds
   */
  public interface NextPageFetcher extends Serializable {
    Future> nextPage();
  }

  private static class SyncNextPageFetcher implements PageImpl.NextPageFetcher {

    private static final long serialVersionUID = -4124568632363525351L;

    private final NextPageFetcher asyncPageFetcher;

    private SyncNextPageFetcher(NextPageFetcher asyncPageFetcher) {
      this.asyncPageFetcher = asyncPageFetcher;
    }

    @Override
    public Page nextPage() {
      try {
        return asyncPageFetcher != null
            ? Uninterruptibles.getUninterruptibly(asyncPageFetcher.nextPage()) : null;
      } catch (ExecutionException ex) {
        throw Throwables.propagate(ex.getCause());
      }
    }
  }

  /**
   * Creates an {@code AsyncPageImpl} object.
   */
  public AsyncPageImpl(NextPageFetcher asyncPageFetcher, String cursor, Iterable results) {
    super(new SyncNextPageFetcher(asyncPageFetcher), cursor, results);
    this.asyncPageFetcher = asyncPageFetcher;
  }

  @Override
  public Future> nextPageAsync() {
    if (nextPageCursor() == null || asyncPageFetcher == null) {
      return Futures.immediateCheckedFuture(null);
    }
    return asyncPageFetcher.nextPage();
  }
}