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/**
 * Copyright (c) 2016-present, RxJava Contributors.
 *
 * 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 io.reactivex.internal.fuseable;

import java.util.Queue;

import org.reactivestreams.Subscription;

/**
 * An interface extending Queue and Subscription and allows negotiating
 * the fusion mode between subsequent operators  of the {@code Flowable} base reactive type.
 * 

* The negotiation happens in subscription time when the upstream * calls the {@code onSubscribe} with an instance of this interface. The * downstream has then the obligation to call {@link #requestFusion(int)} * with the appropriate mode before calling {@code request()}. *

* In synchronous fusion, all upstream values are either already available or is generated * when {@link #poll()} is called synchronously. When the {@link #poll()} returns null, * that is the indication if a terminated stream. Downstream should not call {@link #request(long)} * in this mode. In this mode, the upstream won't call the onXXX methods. *

* In asynchronous fusion, upstream values may become available to {@link #poll()} eventually. * Upstream signals onError() and onComplete() as usual but onNext may not actually contain * the upstream value but have {@code null} instead. Downstream should treat such onNext as indication * that {@link #poll()} can be called. In this mode, the downstream still has to call {@link #request(long)} * to indicate it is prepared to receive more values. *

* The general rules for consuming the {@link Queue} interface: *

    *
  • {@link #poll()} has to be called sequentially (from within a serializing drain-loop).
  • *
  • In addition, callers of {@link #poll()} should be prepared to catch exceptions.
  • *
  • Due to how computation attaches to the {@link #poll()}, {@link #poll()} may return * {@code null} even if a preceding {@link #isEmpty()} returned false.
  • *
*

* Implementations should only allow calling the following methods and the rest of the * {@link Queue} interface methods should throw {@link UnsupportedOperationException}: *

    *
  • {@link #poll()}
  • *
  • {@link #isEmpty()}
  • *
  • {@link #clear()}
  • *
* @param the value type transmitted through the queue */ public interface QueueSubscription extends QueueFuseable, Subscription { }




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