willSet didSet playground

gistfile1.swift

// Playground - noun: a place where people can play

import UIKit

class Person {
    var name: String?
    
    var address: Address? {
        willSet(newValue) {
            if let newAddress = newValue {
                println("Will change address to a new address: \(newAddress.street) \(newAddress.buildingNumber)")
            }else{
                println("Will change address to a nil value")
            }
        }
        didSet (oldAddress) {
            if let newAddress = address {
                println("Did change address to a new address: \(newAddress.street) \(newAddress.buildingNumber)")
            }else{
                println("Did change address to a nil value")
            }
        }
    }
    
    init(name: String) {
        self.name = name
    }
}

class Address {
    
    var buildingNumber: String
    
    var street: String
    
    init(street: String, buildingNumber: String) {
        self.street = street
        self.buildingNumber = buildingNumber
    }
}

var john = Person(name: "John")
let someAddress = Address(street: "Acacia Road", buildingNumber: "29")

john.address = someAddress
john.address = nil