“oldValue” and “newValue” default parameters names inside willSet / didSet unrecognized

Question

I am currently writing Swift 3 code in Xcode 8.

When using oldValue and newValue default parameters inside the willSet and

Answer 1

var vc:UIViewController? {
    willSet {
        print("New value is \(newValue)")
    }
    didSet {
        print("Old value is \(oldValue)")
    }
}