"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 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?

score 91 · Answer 1 · edited Jun 07 '18 at 00:36

91

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)")
    }
}

edited Jun 07 '18 at 00:36

pkamb

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answered Sep 18 '16 at 16:03

FranMowinckel

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What is the current Swift version of this? – Feb 11 '20 at 21:46

score 69 · Answer 2 · answered Aug 06 '18 at 03:23

69

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.

answered Aug 06 '18 at 03:23

Robin Stewart

3,147
20
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score 25 · Answer 3 · edited Sep 18 '16 at 15:37

25

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

edited Sep 18 '16 at 15:37

Nirav D

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answered Sep 18 '16 at 15:17

Dmytro Shvetsov

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1

Not entirely right, oldValue still seems to be unrecognized. Removing `viewController` parameter seems to make it work. – Adithya Sep 18 '16 at 15:24
Yes, sorry. I did not notice ur variable. – Dmytro Shvetsov Sep 18 '16 at 15:35
2

From your answer, I was able to deduce that `newValue` works within `willSet` and `oldValue` works for `didSet` block given there are no custom parameters. – Adithya Sep 18 '16 at 15:32
Incidentally, https://docs.swift.org/swift-book/LanguageGuide/Properties.html documents the default parameter names of `newValue` and `oldValue` for the `willSet` and `didSet` property observers, respectively. – Kurt Peek Feb 27 '19 at 22:02

score 16 · Answer 4 · answered Sep 16 '19 at 13:57

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)") 
    }
}

score 0 · Answer 5 · edited Jan 21 '18 at 12:53

0

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)")
   }
}

edited Jan 21 '18 at 12:53

pirho

11,565
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answered Jan 21 '18 at 12:30

gomozor

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score -7 · Answer 6 · answered Feb 11 '20 at 17:23

-7

That's why we use NSKeyValueObservation to monitor class object

answered Feb 11 '20 at 17:23

JackMao

1

"oldValue" and "newValue" default parameters names inside willSet / didSet unrecognized

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