Drux Drux - 2 months ago 50
Swift Question

How to conform to NSCopying and implement copyWithZone in Swift 2?

I would like to implement a simple

GKGameModel
in Swift 2. Apple's example is expressed in Objective-C and includes this method declaration (as required by protocol
NSCopying
from which
GKGameModel
inherits):

- (id)copyWithZone:(NSZone *)zone {
AAPLBoard *copy = [[[self class] allocWithZone:zone] init];
[copy setGameModel:self];
return copy;
}


How does this translate into Swift 2? Is the following appropriate in terms of efficiency and ignoring zone?

func copyWithZone(zone: NSZone) -> AnyObject {
let copy = GameModel()
// ... copy properties
return copy
}

Answer

NSZone is no longer used in Objective-C for a long time. And passed zone argument is ignored. Quote from the allocWithZone... docs:

This method exists for historical reasons; memory zones are no longer used by Objective-C.

You're safe to ignore it as well.

Here's an example how to conform to NSCopying protocol.

class GameModel: NSObject, NSCopying {

  var someProperty: Int = 0

  required override init() {
    // This initializer must be required, because another
    // initializer `init(_ model: GameModel)` is required
    // too and we would like to instantiate `GameModel`
    // with simple `GameModel()` as well.
  }

  required init(_ model: GameModel) {
    // This initializer must be required unless `GameModel`
    // class is `final`
    someProperty = model.someProperty
  }

  func copyWithZone(zone: NSZone) -> AnyObject {
    // This is the reason why `init(_ model: GameModel)`
    // must be required, because `GameModel` is not `final`.
    return self.dynamicType.init(self)
  }

}

let model = GameModel()
model.someProperty = 10

let modelCopy = GameModel(model)
modelCopy.someProperty = 20

let anotherModelCopy = modelCopy.copy() as! GameModel
anotherModelCopy.someProperty = 30

print(model.someProperty)             // 10
print(modelCopy.someProperty)         // 20
print(anotherModelCopy.someProperty)  // 30

P.S. This example is for Xcode Version 7.0 beta 5 (7A176x). Especially the dynamicType.init(self).

Edit for Swift 3

Below is the copyWithZone method implementation for Swift 3 as dynamicType has been deprecated:

func copy(with zone: NSZone? = nil) -> Any
{
    return type(of:self).init(self)
}
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