EntityModule.kt

interface Entity

class Player(val name: String) : Entity
class NPC(val type: String) : Entity

enum class EventKey {
    INIT,
    PROCESS,
    FINISH
}

typealias Event<T> = (T) -> Unit

class EntityModule<T : Entity>(private val entity: T) {

    private val events = EnumMap<EventKey, MutableList<Event<T>>>(EventKey::class.java)

    fun process(key: EventKey) {
        val handlers = events[key] ?: return
        handlers.forEach { it.invoke(entity) }
    }

    fun register(key: EventKey, event: Event<T>) {
        val handlers = events[key] ?: mutableListOf()

        handlers.add(event)

        events[key] = handlers
    }

    fun deregister(key: EventKey, event: Event<T>) {
        val handlers = events[key] ?: return

        handlers.remove(event)

        if (handlers.isEmpty()) {
            events.remove(key)
        }
    }

}

object Main {

    @JvmStatic
    fun main(args: Array<String>) {

        //player
        val player = Player("Hc747")

        //module
        val module = EntityModule(player)

        //events
        val initialising: Event<Player> = { println("Initialising player with name: ${it.name}") }

        val processing: Event<Player> = { println("Processing player with name: ${it.name}") }

        val finishing: Event<Player> = { println("Finishing player with name: ${it.name}") }


        with(module) {
            register(EventKey.INIT, initialising)
            register(EventKey.PROCESS, processing)
            register(EventKey.FINISH, finishing)
            register(EventKey.FINISH) { println("Some other event with the player when they finish, name: ${it.name}") }
        }

        module.process(EventKey.INIT)

        module.process(EventKey.PROCESS)

        module.process(EventKey.FINISH)

    }

}