Is it possible to convert instance of a class to an instance of a subclass? - swift

Question

I've recently came across a case where it would be very convenient to convert an instance of a class to a subclass, while the instance has been created within the parent class. But I've never seen such thing. So is there a way to do something like:
class Foo {
var name: String
}
class Bar: Foo {
var friendName: String
}
let foo = Foo(name: "Alice")
foo.toBar(friendName: "Bob")
// foo now of type Bar, as if I'd done
// foo = Bar(name: "Alice", friendName: "Bob")
If that's not possible, is there some reasons this would be impossible from a design perspective?
===edit=== description of a use case where it could make sense
Let say there's two views representing what correspond to the same database record for a book, on is a just a preview of the book and another is a more complex view. Models could be:
protocol BookMetaDelegate {
func onReadStatusUpdate()
}
/// describe a book
class BookMeta {
var delegate: BookMetaDelegate?
private var _hasBeenRead: Bool
var hasBeenRead: Bool {
get {
return _hasBeenRead
}
set {
guard newValue != _hasBeenRead else { return }
_hasBeenRead = newValue
delegate?.onReadStatusUpdate()
}
}
var title: String
}
/// contains all the content of a book
class Book: BookMeta {
var content: BookContent
var lastPageRead: Int
/// some logic that only makes sense in a Book instance
func getLastPageRead() {
return content.getPage(lastPageRead)
}
}
and views could look like:
class BookPreview: UIView, BookMetaDelegate {
var book: BookMeta
init(book: BookMeta) {
book.delegate = self
}
func onReadStatusUpdate() {
print("read status has changed! UI should update")
}
}
class BookView: UIView {
var book: Book
init(book: Book) {
book.hasBeenRead = true
}
}
Then things could happen like
fetch(bookMetaWithId: 123).then { bookMeta in // bookMeta is of type BookMeta
let preview = BookPreview(book: bookMeta)
...
fetch(contentOf: bookMeta).then { content, lastPageRead in
bookMeta.asBook(content: content, lastPageRead: lastPageRead)
let bookView = BookView(book: bookMeta) // doing so will change the hasBeenRead flag and message the instance's delegate, ie the preview
...
}
}

Answer 1

Thinking more about it, it sounds like that if such thing was possible, it'd break things like:
class Foo {
var name: String
}
class Bar: Foo {
var friendName: String
}
class Bla: Foo {
var surname: String
}
func something(foo: Foo) {
foo.toBla(surname: "Will")
}
let bar = Bar(name: "Alice", friendName: "Bob")
something(foo: bar) // what does that do ??? is bar a Bla now ?
so that'd be a good reason for making such casting impossible.