An example of using a composite observer

CompositeObserver.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 CompositeUserAgentObserver {

    let observers: [UserAgentObserver]

    init(observers: [UserAgentObserver]) {
        self.observers = observers
    }
}

extension CompositeUserAgentObserver: UserAgentObserver {

    func userAgentDidFinishStarting(userAgent: UserAgent) {
        onEachObserver { $0.userAgentDidFinishStarting(userAgent) }
    }

    func userAgentDidFinishStopping(userAgent: UserAgent) {
        onEachObserver { $0.userAgentDidFinishStopping(userAgent) }
    }

    private func onEachObserver(function: UserAgentObserver -> Void) {
        for observer in observers { function(observer) }
    }
}

class UserAgentEventLogger: UserAgentObserver {

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

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

let eventLoggers: [UserAgentObserver] = [UserAgentEventLogger(), UserAgentEventLogger()]
let compositeObserver = CompositeUserAgentObserver(observers: eventLoggers)
let userAgent = UserAgent(observer: compositeObserver)
userAgent.start()
userAgent.stop()