C#-style events in Kotlin

events.kt

fun main(args: Array<String>) {
    val duck = Duck()
    duck.talked += { said -> println("Duck: $said") }

    duck.poke()
}

class Event<T>(val eventHandler: EventHandler<T>) {
    operator fun plusAssign(observer: (T) -> Unit) { eventHandler.observers.add(observer) }
    operator fun minusAssign(observer: (T) -> Unit) { eventHandler.observers.remove(observer) }
}

class EventHandler<T> {
    val observers = arrayListOf<((T) -> Unit)>()
    operator fun invoke(value: T) { for (observer in observers) { observer(value) } }
}

class Duck {
    private val onTalk = EventHandler<String>()
    val talked = Event<String>(onTalk)

    fun poke() {
        onTalk("Quaak!")
    }
}