/*
 * Copyright 2015 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.datastore;

import java.util.Iterator;

/**
 * The result of a Google Cloud Datastore query submission.
 * When the result is not typed it is possible to cast it to its appropriate type according to
 * the {@link #resultClass} value.
 * Results are loaded lazily in batches, where batch size is set by Cloud Datastore. As a result, it
 * is possible to get a {@code DatastoreException} upon {@link Iterator#hasNext hasNext} or
 * {@link Iterator#next next} calls.
 *
 * @param <V> the type of the results value.
 */
public interface QueryResults<V> extends Iterator<V> {

  /**
   * Returns the actual class of the result's values.
   */
  Class<?> resultClass();

  /**
   * Returns the Cursor for the point after the value returned in the last {@link #next} call. This
   * cursor can be used to issue subsequent queries (with the same constraints) that may return
   * additional results.
   *
   * <p>A simple use case:
   * <pre> {@code
   * Query<Entity> query = Query.entityQueryBuilder()
   *     .kind("Person")
   *     .filter(PropertyFilter.eq("favoriteFood", "pizza"))
   *     .build();
   * QueryResults<Entity> results = datastore.run(query);
   * // Consume some results (using results.next()) and do any other actions as necessary.
   * query = query.toBuilder().startCursor(results.cursorAfter()).build();
   * results = datastore.run(query); // now we will iterate over all entities not yet consumed
   * }</pre>
   */
  Cursor cursorAfter();
}
