Swift setter semantics with value types

example.swift

import Foundation

class Foo {
    var _numbers: [Int] = [42]
    var numbers: [Int] {
        get { _numbers }
        set(value) {
            print("Old: ", _numbers)
            print("New: ", value)
        }
    }
}

let foo = Foo()
foo.numbers.append(43)

// http://online.swiftplayground.run/
// Old:  [42]
// New:  [42, 43]