Learning Kotlin: The vetoable delegate

The vetoable delegate is usable with variables only (i.e. you need to be able to change the value) and it takes a lambda which returns a boolean. If the return value is true, the value is set; otherwise, the value is not set.

As an example, we will have a User and ensure the age can never be lowered (since time only goes forward)

import kotlin.properties.Delegates

class User() {
    var age : Int by Delegates.vetoable(0)  { property, oldValue, newValue ->
        newValue > oldValue
    }
}

fun main(args:Array<String>) {
    val user = User()
    user.age = 10;
    println(user.age)

    user.age = 15
    println(user.age)

    user.age = 10
    println(user.age)
}

This produces:

10
15
15