“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

It's important to know that the special variable newValue only works within willSet, while oldValue only works within didSet.

var vc: UIViewController? {
    willSet {
        // Here you can use vc as the old value since it's not changed yet
        print("New value is \(newValue)")
    }
    didSet {
        // Here you can use vc as the new value since it's already DID set
        print("Old value is \(oldValue)") 
    }
}