`ReplaySubject.Subscription` with `DemandBuffer`.

replay_subject_with_demand_buffer.swift

final class ReplaySubject<Output, Failure: Error>: Subject { /* … */ }

extension ReplaySubject {
	final class Subscription<Subscriber: Combine.Subscriber>: Combine.Subscription
		where Subscriber.Input == Output, Subscriber.Failure == Failure {

		private var buffer: DemandBuffer<Subscriber>?
			
		fileprivate let innerSubscriberIdentifier: CombineIdentifier

		init(subscriber: Subscriber) {
			buffer = DemandBuffer(subscriber: subscriber)
			innerSubscriberIdentifier = subscriber.combineIdentifier
		}

		func request(_ demand: Subscribers.Demand) {
			_ = buffer?.demand(demand)
		}

		func cancel() {
			buffer = nil
		}

		func forward(completion: Subscribers.Completion<Failure>) {
			buffer?.complete(completion: completion)
		}

		func forward(value: Output) {
			_ = buffer?.buffer(value: value)
		}
	}
}