public interface QueueSubscription<T>
extends QueueFuseable<T>, Subscription 

An interface extending SimpleQueue and Subscription and allows negotiating the fusion mode between subsequent operators of the Flowable base reactive type.

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

In synchronous fusion, all upstream values are either already available or is generated when SimpleQueue.poll() is called synchronously. When the SimpleQueue.poll() returns null, that is the indication if a terminated stream. Downstream should not call Subscription.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 SimpleQueue.poll() eventually. Upstream signals onError() and onComplete() as usual, however, onNext will be called with null instead of the actual value. Downstream should treat such onNext as indication that SimpleQueue.poll() can be called. In this mode, the downstream still has to call Subscription.request(long) to indicate it is prepared to receive more values.

The general rules for consuming the SimpleQueue interface:

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

Implementations should only allow calling the following methods and the rest of the SimpleQueue interface methods should throw UnsupportedOperationException:

• SimpleQueue.poll()
• SimpleQueue.isEmpty()
• SimpleQueue.clear()