扩展名不能包含存储的属性时，@ NSManaged属性如何在扩展名中？为什么它们与存储的属性不同？

核心数据文件的一种流行格式是核心数据类，然后是包含核心数据属性的扩展名。

//User+CoreDataClass
@objc(User)
public class User: NSManagedObject {
  var fullName: String {
    return "\(firstName) \(lastName)" 
  }
}

//User+CoreDataProperties
extension User {
  // Why can these @NSManaged properties live in an extension?
  @NSManaged public var id: Int64
  @NSManaged public var firstName: String
  @NSManaged public var lastName: String

  var storedProp: String // Error: Extensions must not contain stored properties
}