Message control linking a Flow.Publisher and Flow.Subscriber. Subscribers receive items only when requested,
and may cancel at any time. The methods in this interface are
intended to be invoked only by their Subscribers; usages in
other contexts have undefined effects.
Adds the given number n of items to the current
unfulfilled demand for this subscription. If n is
less than or equal to zero, the Subscriber will receive an
onError signal with an IllegalArgumentException argument. Otherwise, the
Subscriber will receive up to n additional
onNext invocations (or fewer if terminated).
n - the increment of demand; a value of
Long.MAX_VALUE may be considered as effectively unbounded
Causes the Subscriber to (eventually) stop receiving
messages. Implementation is best-effort -- additional
messages may be received after invoking this method.
A cancelled subscription need not ever receive an
onComplete or onError signal.