Demonstrates how a Swift value constant can mutate when using Copy-on-Write (CoW) and multi-threading.

LetTheCoWGo.swift

import Foundation

struct NuclearPowerStationOperator {
    
    class Storage {
        var turnOffCores: Bool = false
        func copy() -> Storage {
            let new = Storage()
            new.turnOffCores = turnOffCores
            return new
        }
    }
    
    private var storage: Storage = Storage()
    
    var turnOffCores: Bool {
        get {
            return storage.turnOffCores
        }
        set {
            if isKnownUniquelyReferenced(&storage) {
                Thread.sleep(forTimeInterval: 1.0) // Sleep to simulate race condition
                storage.turnOffCores = newValue
            } else {
                storage = storage.copy()
                storage.turnOffCores = newValue
            }
        }
    }
    
    var description: String {
        return "\(turnOffCores ? "We are in danger" : "We are safe")"
    }
}

// Create a mutable value
var crazyOperator = NuclearPowerStationOperator()

DispatchQueue.global(qos: .background).async {
    Thread.sleep(forTimeInterval: 0.5) // Sleep a little to give main thread time to start setting property
    let saneOperator = crazyOperator // Create a constant copy of the operator from main thread
    print(saneOperator.description) // Print our intial property value
    Thread.sleep(forTimeInterval: 2.0) // Simulate race by waiting for setter on main thread to finish
    print(saneOperator.description) // Test property (it will be different)
}

// Update the value. Note that the setter simulates a race condition by being very slow
crazyOperator.turnOffCores = true