Challenge 2: Observable Property — Solution

import kotlin.properties.Delegates

class ShoppingCart {
    var itemCount: Int by Delegates.observable(0) { property, old, new ->
        println("${property.name} changed from $old to $new")
    }
}

fun main() {
    val cart = ShoppingCart()
    cart.itemCount = 1
    cart.itemCount = 3
    cart.itemCount = 2
}

/*
Output:
itemCount changed from 0 to 1
itemCount changed from 1 to 3
itemCount changed from 3 to 2

Notes:
- Delegates.observable(0) { ... } takes the initial value (0) as its
  first argument, then a callback that fires on every subsequent
  reassignment.
- The callback receives the KProperty itself (property.name gives
  "itemCount"), the old value, and the new value — all three are useful
  for building a generic logging/reactive delegate.
- Each of the three assignments in main() triggers one callback
  invocation, in order.
*/