Use property observers

Swift

Use Swift's property observers to observe and respond to changes in a property’s value. Both `willSet` and `didSet` takes a default parameter to get new and old values

var language: String = "ObjC" {
    willSet(newValue) {
        print("willSet \(language) to \(newValue)")
    }
    didSet(oldValue) {
        print("didSet \(oldValue) to \(language)")
    }
}

language = "Swift"

/* prints:
willSet ObjC to Swift
didSet ObjC to Swift
*/