You should always retain a list of listeners

[SamDecrock]

Just an extra reason to store the listeners in an array in your object is because when that object is deallocated, the listeners are deallocated as well. Which leaves no listeners hanging in "thin air".

For example:

class SomeTemporaryClass {
    var listeners = [Listener]()

    init(someEvent: Event<String>) {
        listeners += someEvent.on { (str) -> Void in
            print("someEvent was triggered")
        }
    }

    deinit {
        print("instance of SomeTemporaryClass was deallocated")
    }
}

class ViewController: UIViewController {
    var temp: SomeTemporaryClass?

    override func viewDidLoad() {
        super.viewDidLoad()

        let someEvent = Event<String>()
        temp = SomeTemporaryClass(someEvent: someEvent)
        temp = nil // deallocate temp and it's listeners will be deallocated too :-)
        someEvent.emit("hello")

    }
}

[SamDecrock]

Ah, Ok, apparently I must always retain an "on"-listener. Good :-)

[phimage]

I think using attributes warn_unused_result on on methods will be a good things