An example of using the observer pattern instead of NSnotifications and KVO

ObserverPattern.swift

class UserAgent {

    let observer: UserAgentObserver

    init(observer: UserAgentObserver) {
        self.observer = observer
    }

    func start() {
        observer.userAgentDidFinishStarting(self)
    }

    func stop() {
        observer.userAgentDidFinishStopping(self)
    }
}

protocol UserAgentObserver {
    func userAgentDidFinishStarting(userAgent: UserAgent)
    func userAgentDidFinishStopping(userAgent: UserAgent)
}

class UserAgentEventLogger: UserAgentObserver {

    func userAgentDidFinishStarting(userAgent: UserAgent) {
        print("User agent started")
    }

    func userAgentDidFinishStopping(userAgent: UserAgent) {
        print("User agent stopped")
    }
}

let userAgent = UserAgent(observer: UserAgentEventLogger())
userAgent.start()
userAgent.stop()