swiftpropertiesdidset

Need some knowledge on Struct and specially property wrapper

struct XA {

static var xa = "Advanced"

var xb: String {
    didSet {
        XA.xa = oldValue
    }
}}

var objXA = XA(xb: "Turing")
print(XA.xa) // Advanced
objXA.xb = "Swift"
print(XA.xa) // Turing

let objXB = XA(xb: "Quiz")
print(XA.xa) // Turing

i need to understand how these output coming in little bit depth. On the last line why its printing Turing not Swift.

Solution

  • The reason, as Joakim says in their comment, is that initializers do not invoke didSet/willSet. That's by design.

    To Quote the Apple Swift eBook (emphasis added by me)

    The willSet and didSet observers of superclass properties are called when a property is set in a subclass initializer, after the superclass initializer has been called. They aren’t called while a class is setting its own properties, before the superclass initializer has been called. For more information about initializer delegation, see Initializer Delegation for Value Types and Initializer Delegation for Class Types.

    Excerpt From The Swift Programming Language (Swift 5.7) Apple Inc. https://books.apple.com/us/book/the-swift-programming-language-swift-5-7/id881256329 This material may be protected by copyright.