ReplaySubject.swift

final class ReplaySubscriber<Input, Failure: Error>: Subscription {
    private var subscriber: AnySubscriber<Input, Failure>?

    init(subscriber: AnySubscriber<Input, Failure>) {
        self.subscriber = subscriber
    }

    func request(_ demand: Subscribers.Demand) {
        // We do nothing here as we only want to send events when they occur.
        // See, for more info: https://developer.apple.com/documentation/combine/subscribers/demand
    }

    func receive(_ value: Input) {
        subscriber?.receive(value)
    }

    func receive(_ completion: Subscribers.Completion<Failure>) {
        subscriber?.receive(completion: completion)
    }

    func cancel() {
        subscriber = nil
    }
}

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

    private var subscribers: [ReplaySubscriber<Output, Failure>] = []
    private var values: [Output] = []

    func receive<S>(subscriber: S) where S : Subscriber, S.Failure == Failure, S.Input == Output {
        let subscription = ReplaySubscriber(subscriber: AnySubscriber(subscriber))
        subscribers.append(subscription)
        subscriber.receive(subscription: subscription)
        values.forEach { subscription.receive($0) }
    }

    public func send(_ value: Output) {
        values.append(value)
        subscribers.forEach { $0.receive(value) }
    }

    public func send(completion: Subscribers.Completion<Failure>) {
        subscribers.forEach { $0.receive(completion) }
    }
}