Maintain value semantics in Swift struct, which contains object reference - swift

Question

I have a Swift struct which contains an object for internal storage. How can I make sure the struct has value semantics?
public struct Times {
private let times = NSMutableIndexSet()
mutating func addTimeRange(openTime: Int, closeTime: Int) {
self.times.addIndexesInRange(NSRange(location: openTime, length: closeTime - openTime))
}
}

Answer 1

Swift 3 Update
Swift 3 includes value types for many types from the Foundation framework. There is now an IndexSet struct, which bridges to NSIndexSet. The internal implementation is similar to the Swift 2 solution below.
For more information on the new Foundation value types see: https://github.com/apple/swift-evolution/blob/master/proposals/0069-swift-mutability-for-foundation.md
Old approach in Swift 2
The copy-on-write approach is the right solution. However, it is not necessary to create a copy of the NSMutableIndexSet, if only one struct instance references it. Swift provides a global function called isUniquelyReferencedNonObjC() to determine if a pure Swift object is only referenced once.
Since we cannot use this function with Objective-C classes, we need to wrap NSMutableIndexSet in a Swift class.
public struct Times {
private final class MutableIndexSetWrapper {
private let mutableIndexSet: NSMutableIndexSet
init(indexSet: NSMutableIndexSet) {
self.mutableIndexSet = indexSet
}
init() {
self.mutableIndexSet = NSMutableIndexSet()
}
}
private let times = MutableIndexSetWrapper()
mutating func addTimeRange(openTime: Int, closeTime: Int) {
// Make sure our index set is only referenced by this struct instance
if !isUniquelyReferencedNonObjC(&self.times) {
self.times = MutableIndexSetWrapper(indexSet: NSMutableIndexSet(indexSet: self.times.mutableIndexSet))
}
let range = NSRange(location: openTime, length: closeTime - openTime)
self.times.mutableIndexSet.addIndexesInRange(range)
}
}

Answer 2

Store an NSIndexSet instead of an NSMutableIndexSet. That is exactly why the immutable superclass exists.
public struct Times {
private var times = NSIndexSet()
mutating func addTimeRange(openTime: Int, closeTime: Int) {
let t = NSMutableIndexSet(indexSet:self.times)
t.addIndexesInRange(NSRange(location: openTime, length: closeTime - openTime))
self.times = NSIndexSet(indexSet:t)
}
}
If this were a class instead of a struct, you could cause the last step to be performed automatically by declaring times as #NSCopying and then just using simple assignment:
public class Times {
#NSCopying private var times = NSIndexSet()
func addTimeRange(openTime: Int, closeTime: Int) {
let t = NSMutableIndexSet(indexSet:self.times)
t.addIndexesInRange(NSRange(location: openTime, length: closeTime - openTime))
self.times = t // ensure immutable copy
}
}

Answer 3

It might be an option to use Swift's native Set type which has value-semantics built in since it is a struct itself.
public struct Times {
private var times = Set<Int>()
mutating func addTimeRange(openTime: Int, closeTime: Int) {
(openTime ..< closeTime).map({ index -> Void in self.times.insert(index) })
}
}
let t1 = Times()
var t2 = t1
t2.addTimeRange(0, closeTime: 3)
println(t1.times) // []
println(t2.times) // [2, 0, 1]