swift - 是否可以将类的实例转换为子类的实例？

Question

我最近遇到一个案例，将类的实例转换为子类非常方便，而实例已经在父类中创建。但我从未见过这样的事情。那么有没有办法做这样的事情:

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")

如果那不可能，从设计的角度来看是否有某些原因是不可能的？

===edit=== 有意义的用例描述

假设有两个 View 表示对应于一本书的同一数据库记录的内容，一个是该书的预览，另一个是更复杂的 View 。模型可以是:

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)
  }
}

View 可能看起来像:

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
  }
}

然后事情可能会发生这样的

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

仔细想想，听起来如果这样的事情是可能的，它会破坏像这样的东西:

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 ?

所以这就是使这种转换成为不可能的一个很好的理由。