WWDC.2019.Combine.2.swift

struct MyPublisher : Publisher {
    public typealias Failure = Error
    public typealias Output = Int
    
    @available(iOS 13.0, *)
    public func receive<S>(subscriber: S) where S : Subscriber, Failure == S.Failure, Output == S.Input {
        DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(2), execute: {
            _ = subscriber.receive(42)
            subscriber.receive(completion: .finished)
        })
    }
}