Kotlin events, with one and many listeners.

multiple.kt

private fun raiseMoveMade() = moveMade.forEach { it.invoke() }
val moveMade = mutableListOf<() -> Unit>()

// invoking
raiseMoveMade()

// assigning a listener (in the outside world)
moveMade += { uiSounds.playMove() }

single.kt

var moveMade: (() -> Unit)? = null

// invoking
moveMade?.invoke()

// assignment
moveMade = { uiSounds.playMove() }