vetoableData.kt

var vetoableData: Int by Delegates.vetoable(0) {
    property, oldValue, newValue ->
    println("${property.name}: $oldValue -> $newValue")
    newValue >= 0
}

fun main(args: Array<String>) {
    vetoableData = -1
    println("vetoableData = $vetoableData")
    vetoableData = 1
    println("vetoableData = $vetoableData")
}