`ReplaySubject` with `.send(_:)`.

replay_subject_send_value.swift

final class ReplaySubject<Output, Failure: Error>: Subject {
	typealias Output = Output
	typealias Failure = Failure

	private let bufferSize: Int

	private var completion: Subscribers.Completion<Failure>?

	private var isActive: Bool {
		completion == nil
	}

	private var buffer = [Output]() /// (1) We could swap `Array` for a type better suited for head removals and tail
	//// insertions—à la [Entwine’s `ReplaySubjectValueBuffer`](https://github.com/tcldr/Entwine/blob/b839c9fcc7466878d6a823677ce608da998b95b9/Sources/Entwine/Operators/ReplaySubject.swift#L162-L177)
	/// —but, for now, this can do.

	private var subscriptions = [Subscription]()

	init(bufferSize: Int) {
		self.bufferSize = bufferSize
		buffer.reserveCapacity(bufferSize) /// (2) Priming `buffer`’s capacity to avoid intermediary resizings.
	}

	func send(subscription: Combine.Subscription) {
		subscription.request(.unlimited)
	}

	func send(completion: Subscribers.Completion<Failure>) {
		guard isActive else { return }

		self.completion = completion

		subscriptions.forEach { $0.forward(completion: completion) }
	}

	func send(_ value: Output) {
		guard isActive else { return }

		buffer.append(value)

		if buffer.count > bufferSize { /// (3) Dequeue the first buffered value, if we’ve hit the max size.
			buffer.removeFirst()
		}

		subscriptions.forEach { $0.forward(value: value) }
	}

	func receive<Subscriber: Combine.Subscriber>(
		subscriber: Subscriber
	) where Failure == Subscriber.Failure, Output == Subscriber.Input { /* … */ }
}

extension ReplaySubject {
	final class Subscription: Combine.Subscription {
		func request(_ demand: Subscribers.Demand) { /* … */ }

		func cancel() { /* … */ }

		func forward(completion: Subscribers.Completion<Failure>) { /* … */ }

		func forward(value: Output) {  /// (4) To be filled in, soon.
			/* … */
		}
	}
}