Delegates and observable properties

// User.kt
import kotlin.properties.Delegates

class User {
    var name: String by Delegates.observable("<no name>") {
        prop, old, new ->
        println("$old -> $new")
    }
}

fun main() {
    val user = User()
    user.name = "first"
    user.name = "second"
}

// # output:
// <no name> -> first
// first -> second

Docs: Observable properties

Source: 7 Features in Kotlin's Standard Library that You Might Have Overlooked