Deligation

import kotlin.properties.Delegates  
class User { 
    var name: String by Delegates.observable("Welcome to Tutorialspoint.com") { 
        prop, old, new -> 
        println("$old -> $new") 
    } 
}  
fun main(args: Array<String>) { 
    val user = User() 
    user.name = "first" 
    user.name = "second" 
}