EventEmitter.swift

class EventListener<T> {
    typealias Listener = T -> ()
    
    let listener: Listener
    var calls: Int
    
    init(calls: Int, listener: Listener) {
        self.calls = calls
        self.listener = listener
    }
    
    func call(event: T) {
        calls -= 1
        if calls == 0 { // -1 passes the test
            active = false
        }
        listener(event)
    }
    
    var active = true
    func stop() {
        active = false
    }
    
}

protocol EventEmitter: class {
    typealias T
    var listeners: [EventListener<T>] { get set }
    
    func listen(times times: Int, listener: T -> Void) -> EventListener<T>

    func send(event: T)
}

extension EventEmitter {
    func listen(times times: Int = -1, listener: EventListener<T>.Listener) -> EventListener<T> {
        
        let listener = EventListener<T>(calls: times, listener: listener)
        
        listeners.append(listener)
        return listener
    }
    
    func send(event: T) {
        listeners
            .filter { $0.active }
            .forEach {
                $0.call(event)
        }
    }
}

class Emitter: EventEmitter {
    var listeners = [EventListener<Int>]()
}


var emitter = Emitter()

let first = emitter.listen { event in
    print("forever:", event)
}
emitter.listen(times: 3) { event in
    print("three:", event)
}
emitter.listen(times: 1) { event in
    print("once:", event)
}

emitter.send(10) // first, second, third
emitter.send(15) // first, second
emitter.send(20) // first, second
emitter.send(25) // first
emitter.send(30) // first
first.stop()
emitter.send(35) // none


/*
prints:
forever: 10
three: 10
once: 10
forever: 15
three: 15
forever: 20
three: 20
forever: 25
forever: 30
*/