`SingleSubscriberSubject` is a publisher that only supports a single subscriber at a time

SingleSubscriberSubject.swift

// Copyright (c) 2023 Alex Schäfer
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

import Combine

/// When a new subscriber is added, the previous subscriber is cancelled and the last value is set to nil.
/// When sending a new value, only the current subscriber will receive the value.

final class SingleSubscriberSubject<Output, Failure>: Publisher where Failure: Error {
    typealias Output = Output
    typealias Failure = Failure

    private var lastValue: Output?
    private var lastSubscriber: AnySubscriber<Output, Failure>?

    func receive<S>(subscriber: S) where S: Subscriber, Failure == S.Failure, Output == S.Input {
        if lastSubscriber != nil {
            lastValue = nil
        }
        lastSubscriber = AnySubscriber(subscriber)

        let subscription = Subscription(
            subscriber: subscriber,
            lastValue: lastValue
        ) { [weak self] in
            self?.lastSubscriber = nil
            self?.lastValue = nil
        }

        subscriber.receive(subscription: subscription)
    }

    func send(_ value: Output?) {
        lastValue = value

        guard
            let lastSubscriber = lastSubscriber,
            let value = value
        else {
            return
        }

        _ = lastSubscriber.receive(value)
    }
}

private extension SingleSubscriberSubject {
    final class Subscription<S: Subscriber> {
        private var subscriber: S?
        private var lastValue: S.Input?
        private let onCancel: () -> Void

        init(subscriber: S, lastValue: S.Input?, onCancel: @escaping () -> Void) {
            self.subscriber = subscriber
            self.lastValue = lastValue
            self.onCancel = onCancel
        }
    }
}

extension SingleSubscriberSubject.Subscription: Cancellable {
    func cancel() {
        subscriber = nil
        onCancel()
    }
}

extension SingleSubscriberSubject.Subscription: Subscription {
    func request(_ demand: Subscribers.Demand) {
        guard let subscriber = subscriber, demand > 0 else { return }

        if let lastValue = lastValue {
            _ = subscriber.receive(lastValue)
            self.lastValue = nil
        }
    }
}