WWDC.2019.Combine.interop.swift

extension Observable : Publisher {
    @available(iOS 13.0, *)
    public func receive<S>(subscriber: S) where S : Subscriber, Observable.Failure == S.Failure, Observable.Output == S.Input {
        _ = self.subscribe(onNext: { (next) in
            let demand = subscriber.receive(next)
        }, onError: { (err) in
            subscriber.receive(completion: .failure(err))
        }, onCompleted: {
            subscriber.receive(completion: .finished)
        })
    }

    public typealias Failure = Error
    public typealias Output = Element
}