observer-demo.swift

import Foundation

protocol Notifier: class {
    func notify(_ data: Any)
}

protocol Destroyable {
    func destroy()
}

class Event<T> {
    typealias EventHandler = (T) -> ()
    var eventHandlers = [Notifier]()

    func notifyObservers(_ data: T) {
        eventHandlers.forEach { (eventHandler) in
            DispatchQueue.global(qos: .userInitiated).async { 
                eventHandler.notify(data)
            }
        }
    }

    func removeObservers() {
        eventHandlers.removeAll()
    }

    func addHandler(_ handler: @escaping EventHandler) -> Destroyable {
        let subscription = Subscription(handler: handler, event: self)
        eventHandlers.append(subscription)
        return subscription
    }
}

class Subscription<T>: Notifier, Destroyable {
    let handler: (T) -> ()
    let event: Event<T>

    init(handler: @escaping (T) -> (),
         event: Event<T>) {
        self.event = event
        self.handler = handler
    }

    func notify(_ data: Any) {
        if let data = data as? T {
            handler(data)
        }
    }

    func destroy() {
        event.eventHandlers = event.eventHandlers.filter { $0 as AnyObject? !== self}
    }
}

class Mailman {
    func deliverMailTo(_ house: House) {
        house.receiveMail()
    }
}

class House {
    let mailArrived = Event<String>()
    var address = ""

    init(address: String) {
        self.address = address
    }

    func receiveMail() {
        let date = Date()
        let formatter = DateFormatter()
        formatter.dateFormat = "HH:mm:ss dd.MM.yyyy"
        let dateString = formatter.string(from: date)
        mailArrived.notifyObservers(dateString)
    }
}

func main() {
    let house = House(address: "123 Tardis Ave")
    let subscription = house.mailArrived.addHandler({ date in 
        print("Mail arrived in \(house.address) at \(date)")
    })

    let mailman = Mailman()

    mailman.deliverMailTo(house)

    subscription.destroy()

    mailman.deliverMailTo(house)
}

main()