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

replay_subject_send_completion.swift

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

	private let bufferSize: Int

	private var completion: Subscribers.Completion<Failure>? /// (1) We’ll need to hold onto any received completion event.

	private var isActive: Bool { /// (2) Providing a more readable shim over `completion`, we can lean on this
	/// computed property to bottom out any value or completion events sent after the first (since forwarding them
	/// through would likely cause undefined behavior in the subscription graph).
		completion == nil
	}

	private var subscriptions = [Subscription]() /// (3) Holding onto any existing subscriptions, so we can
	/// forward completions their way.

	init(bufferSize: Int) {
		self.bufferSize = bufferSize
	}

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

	func send(completion: Subscribers.Completion<Failure>) {
		guard isActive else { return } /// (4) Fence off any completions received after the first.

		self.completion = completion

		subscriptions.forEach { $0.forward(completion: completion) } /// (5) Forwarding completions. For now, let’s
		/// add a placeholder `ReplaySubject.Subscription.forward(completion:)` method over at `(6)`.
	}

	func send(_ value: Output) { /* … */ }

	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>) { /// (6) To be filled in, soon.
			/* … */
		}
	}
}