Kotlin Observer Pattern

observer.kt

import kotlin.properties.Delegates

fun main(args: Array<String>) {

    val kumbu = Wallet(0)
    kumbu.listener = AccountChangedListener()
    println("Saldo geral: ${kumbu.balance}")
    kumbu.premiumAccount = true
    println(kumbu.premiumAccount)
    kumbu.premiumAccount = false

}

interface AccountTypeChangedListener {
    fun onChange(account: Boolean, wallet: Wallet)
}

class AccountChangedListener: AccountTypeChangedListener {
    override fun onChange(account: Boolean, wallet: Wallet) {
         when(account) {
            true -> wallet.balance += 100 false -> wallet.balance = wallet.balance - 100
         }
    }
}

class Wallet(var balance: Int = 0) {
    var listener : AccountTypeChangedListener? = null
    var premiumAccount: Boolean by Delegates.observable(false) {
        _, _, new ->
        listener?.onChange(new, this)
        println("Saldo geral: ${this.balance}")
    }