F - the type of the stream of items.S - the type of the single item.public interface FlowableWithSingleSubscriber<F,S> extends WithSingleSubscriber<F,S>, FlowableSubscriber<F>
WithSingleSubscriber that is RxJava 2 only and weakens rules for
gaining performance. Most important is the weakening of rule ยง1.3: onNext/onSingle could be called concurrently until
onSubscribe returns.FlowableSubscriberonSingleonSubscribeonComplete, onError, onNext