Delegated Properties

// Basic syntax for a delegated property
var propertyName: Type by delegateObject

// Delegated properties from the standard library
import kotlin.properties.Delegates

var propertyName: Type by Delegates.observable(initialValue) { prop, old, new -> }
var propertyName: Type by Delegates.notNull()

import kotlin.properties.Delegates

class User {
    // Logs a message whenever the value changes.
    var name: String by Delegates.observable("(not set)") { _, old, new ->
        println("Name changed: $old → $new")
    }

    // Updates the value only if the new value is 0 or greater.
    var age: Int by Delegates.vetoable(0) { _, _, new ->
        new >= 0
    }

    // Requires initialization before access.
    var id: Int by Delegates.notNull()
}

fun main() {
    val user = User()
    user.id = 1001          // Initializes the notNull property.
    user.name = "Alice"     // Callback is invoked.
    user.name = "Bob"       // Callback is invoked again.

    user.age = 25
    println(user.age)        // 25
    user.age = -1            // Rejected because the value is negative.
    println(user.age)        // 25 (unchanged)
}

Delegated properties let you extract property access logic into reusable classes. Simply place a delegate object after the by keyword to handle complex access behavior in a clean, concise way.

Delegates.observable is useful when you want to observe value changes, and Delegates.vetoable is convenient when you need to validate changes before they are applied. To create a custom delegate, implement the ReadWriteProperty<T, V> interface and define both getValue and setValue.