Implementing copy() in Swift

Question

I want to be able to copy a custom class in Swift. So far, so good. In Objective-C I just had to implement the NSCopying protocol, which means implementing copyWithZone.

As an example, I have a basic class called Value which stores a NSDecimalNumber.

func copyWithZone(zone: NSZone) -> AnyObject! {     return Value(value: value.copy() as NSDecimalNumber) } 

In Objective-C I, could easily just call copy to copy my object. In Swift, there seems to be no way to call copy. Do I really need to call copyWithZone even if no zone is needed? And which zone do I need to pass as a parameter?

Answer 1

The copy method is defined in NSObject. If your custom class does not inherit from NSObject, copy won't be available.

You can define copy for any object in the following way:

class MyRootClass {     //create a copy if the object implements NSCopying, crash otherwise      func copy() -> Any {         guard let asCopying = ((self as AnyObject) as? NSCopying) else {             fatalError("This class doesn't implement NSCopying")         }          return asCopying.copy(with: nil)     } }  class A : MyRootClass {  }  class B : MyRootClass, NSCopying {      func copy(with zone: NSZone? = nil) -> Any {         return B()     } }   var b = B() var a = A()  b.copy()  //will create a copy a.copy()  //will fail 

I guess that copy isn't really a pure Swift way of copying objects. In Swift it is probably a more common way to create a copy constructor (an initializer that takes an object of the same type).

Answer 2

Well, there is a really easy solution for this and you do not have to create root class.

protocol Copyable {     init(instance: Self) }  extension Copyable {     func copy() -> Self {         return Self.init(instance: self)     } } 

Now, if you want to make your custom class be able to copy, you have to conform it to Copyable protocol and provide init(instance: Self) implementation.

class A: Copyable {     var field = 0      init() {     }      required init(instance: A) {         self.field = instance.field     }  } 

Finally, you can use func copy() -> Self on any instance of A class to create a copy of it.

let a = A() a.field = 1 let b = a.copy()