SignalProducer+ReplayingShare.swift

extension SignalProducer {
    func replayingShare() -> SignalProducer<Value, Error> {
        let subCount = Atomic<UInt64>(0)
        var upstreamLifetime = Lifetime.empty
        var token = Lifetime.Token()
        var replayedProducer: SignalProducer<Value, Error> = .empty
        
        return SignalProducer { observer, lifetime in
            if upstreamLifetime.hasEnded {
                upstreamLifetime = Lifetime(token)
                replayedProducer = self
                    .take(during: upstreamLifetime)
                    .replayLazily(upTo: 1)
            }
            lifetime.observeEnded {
                var dispose = false
                subCount.modify {
                    $0 -= 1
                    if $0 < 1 {
                        dispose = true
                    }
                }
                if dispose {
                    // Overwrite the previous token with a new one.
                    // This causes the Lifetime associated with that Token to end.
                    token = Lifetime.Token()
                }
            }
            subCount.modify { $0 += 1 }
            lifetime += replayedProducer.start(observer)
        }
    }
}