TypeSafe copy()/mutableCopy() (NSCopying/NSMutableCopying) in Swift

SwiftCopying.swift

import Foundation

//: Swift type-safe protocol versions of (Mutable)Copying
protocol SwiftCopying {
  associatedtype NonMutableType = Self
  func clone() -> NonMutableType
}
extension SwiftCopying where Self: NSCopying {
  func clone() -> NonMutableType {
    return self.copy() as! NonMutableType
  }
}

protocol SwiftMutableCopying {
  associatedtype MutableType
  func mutableClone() -> MutableType
}
extension SwiftMutableCopying where Self: NSMutableCopying {
  func mutableClone() -> MutableType {
    return self.mutableCopy() as! MutableType
  }
}

//: Some convenience wrapper for the mutableClone/clone dance
extension SwiftMutableCopying where Self.MutableType: SwiftCopying {
  func mutate(using mutation: (Self.MutableType) -> Void) -> Self.MutableType.NonMutableType {
    let mutableInstance = self.mutableClone()
    mutation(mutableInstance)
    return mutableInstance.clone()
  }
}

//: Make existing types conform
// Some Foundation types (non-exhaustive)
extension NSString: SwiftCopying, SwiftMutableCopying { typealias MutableType = NSMutableString }
extension NSArray: SwiftCopying, SwiftMutableCopying { typealias MutableType = NSMutableArray }
extension NSDictionary: SwiftCopying, SwiftMutableCopying { typealias MutableType = NSMutableDictionary }
extension NSAttributedString: SwiftCopying, SwiftMutableCopying { typealias MutableType = NSMutableAttributedString }

// Can be useful for other frameworks' types too
import UserNotifications
extension UNNotificationContent: SwiftCopying, SwiftMutableCopying { typealias MutableType = UNMutableNotificationContent }


//: Usage Example
let s1: NSString = "Hello"

// Manual mutableClone/clone() dance
let s2: NSMutableString = s1.mutableClone()
s2.append(" World")
let s3: NSString = s2.clone()
print(s3) // "Hello World"

// Using convenience mutate(using:) method
let s4: NSString = s3.mutate { $0.append("!!!") }
print(s4) // "Hello World!!!"