Question

我最近遇到过这样一种情况：将类的实例转换为子类非常方便，而实例已在父类中创建。但我从来没有见过这样的事情。那么有办法做一些事情：

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")

如果不可能，从设计的角度来看，是否存在某些原因？

=== edit ===描述一个有意义的用例

假设有两个视图表示与书籍的相同数据库记录相对应的内容，它只是书籍的预览，另一个是更复杂的视图。模型可能是：

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)
  }
}

和观点看起来像：

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
  }
}

然后就会发生像

这样的事情

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

更多地考虑它，听起来如果这样的事情可能发生，它会破坏像：

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 ?

所以这是使这种铸造无法进行的一个很好的理由。

是否可以将类的实例转换为子类的实例？

1 个答案: