“oldValue” and “newValue” default parameters names

Question

I am currently writing Swift 3 code in Xcode 8.

When using oldValue and newValue default parameters inside the willSet and didSet blocks, I am getting "unresolved identifier" compiler error.

I have a very basic code as below

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

Apple Documentation for Swift 3 still seems to support these feature. I hope I am not missing anything here?

Answer 1

You didn't try to initialize the variable with a new object by which you can set your clojure:

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

Answer 2

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

Answer 3

You can also use vc:

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

Answer 4

The special variable newValue only works within willSet, while oldValue only works within didSet.

The property as referenced by its name (in this example vc) is still bound to the old value within willSet and is bound to the new value within didSet.