EntityModule.kt

interface Entity

class Player(val name: String) : Entity {

    fun say(message: String) = println("$name: $message")

}

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>) {

        val player = Player("Hc747")
        val module = EntityModule(player)

        with(module) {
            register(EventKey.INIT, Greeter::greet)
            register(EventKey.INIT, AppearanceInitialiser)

            register(EventKey.PROCESS) { it.say("I am currently processing!") }

            register(EventKey.FINISH, Greeter::farewell)
            register(EventKey.FINISH, DatabaseSerialiser())
        }

        module.process(EventKey.INIT)

        for (i in 1 .. 3) {
            module.process(EventKey.PROCESS)
        }

        module.process(EventKey.FINISH)

    }

}

object Greeter {

    fun greet(player: Player) {
        player.say("Welcome to RuneScape.")
    }

    fun farewell(player: Player) {
        player.say("Goodbye!")
    }

}



object AppearanceInitialiser : Event<Player> {

    override fun invoke(entity: Player) {
        //business logic here
        entity.say("Appearance initialised.")
    }

}

class DatabaseSerialiser : Event<Player> {

    override fun invoke(player: Player) {
        player.say("Saved to database!")
    }

}