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How to copy objects in Swift using copy()

Swift version: 5.2

Paul Hudson    @twostraws   

There are two main complex data types in Swift – objects and structs – and they do so many things similarly that you'd be forgiven for not being sure exactly where they differ. Well, one of the key areas is down to copying: two variables can point at the same object so that changing one changes them both, whereas if you tried that with structs you'd find that Swift creates a full copy so that changing the copy does not affect the original.

Having lots of objects point at the same data can be useful, but frequently you'll want to modify copies so that modifying one object doesn't have an effect on anything else. To make this work you need to do three things:

  • Make your class conform to NSCopying. This isn't strictly required, but it makes your intent clear.
  • Implement the method copy(with:), where the actual copying happens.
  • Call copy() on your object.

Here's an example of a Person class that conforms fully to the NSCopying protocol:

    class Person: NSObject, NSCopying {
    var firstName: String
    var lastName: String
    var age: Int

    init(firstName: String, lastName: String, age: Int) {
        self.firstName = firstName
        self.lastName = lastName
        self.age = age
    }

    func copy(with zone: NSZone? = nil) -> Any {
        let copy = Person(firstName: firstName, lastName: lastName, age: age)
        return copy
    }
}

Note that copy(with:) is implemented by creating a new Person object using the current person's information.

With that done, you can test out your copying like this:

let paul = Person(firstName: "Paul", lastName: "Hudson", age: 36)
let sophie = paul.copy() as! Person

sophie.firstName = "Sophie"
sophie.age = 6

print("\(paul.firstName) \(paul.lastName) is \(paul.age)")
print("\(sophie.firstName) \(sophie.lastName) is \(sophie.age)")
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