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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 {
}