Delegation Patterns in Kotlin

// Class delegation
interface Base {
    fun print()
    val message: String
}

class BaseImpl(val x: Int) : Base {
    override fun print() = println(x)
    override val message = "BaseImpl: $x"
}

class Derived(b: Base) : Base by b {
    // Can override members
    override val message = "Derived message"
}

val base = BaseImpl(10)
val derived = Derived(base)
derived.print()  // Delegates to base

// Delegated properties
class User {
    // Lazy property
    val lazyValue: String by lazy {
        println("Computing lazy value")
        "Hello"
    }

    // Observable property
    var name: String by Delegates.observable("Initial") {
        prop, old, new ->
        println("$old -> $new")
    }

    // Vetoable property
    var age: Int by Delegates.vetoable(0) {
        prop, old, new ->
        new >= 0  // Reject negative values
    }
}