PubSub protocol

pubsub.swift

protocol PubSubMessageType {
    associatedtype Topic
    
    var topic: Topic { get }
}

protocol PubSubType {
    associatedtype Message = PubSubMessageType
    associatedtype Topic
    
    associatedtype Handler = Message -> Void
    associatedtype Unsubscribe = Void -> Void
    
    func send(message: Message)
    func subscribe(topic: Topic, handler: Handler) -> Unsubscribe
}

extension PubSubType {
    func subscribeOnce(fn: (Topic, Handler) -> Unsubscribe) -> (Topic, Handler) -> Void {
        return { topic, handler in
            var unsubscribe: Unsubscribe?
            let myHandler: Handler = { // cannot convert value of type '(_) -> ()' to specified type 'Self.Handler'
                unsubscribe?()
                handler($0)
            }
            unsubscribe = fn(topic, myHandler)
        }
    }
}

// Usage

let p = PubSub<Message, String>()
let _ = p.subscribe("topic") {
    print($0) // will print "topic" as many times as it's sent
}
p.subscribeOnce(p.subscribe)("topic") {
    print("Once:", $0) // will print "topic" once
}

p.send(Message(topic: "topic"))
p.send(Message(topic: "topic"))