Reference counting demo

referenceCounting.swift

import Foundation

// Retain count of object
let object = NSObject()

CFGetRetainCount(object)

let nextObject = object

CFGetRetainCount(object)

func retainCount(of object: NSObject) {
  CFGetRetainCount(object)
}

retainCount(of: object)

// Retain count of object in Array

let array = [NSObject()]

CFGetRetainCount(array[0])

var newArray = array

CFGetRetainCount(array[0])

newArray.append(NSObject())

CFGetRetainCount(array[0])

func retainCount(of objects: [NSObject]) {
  CFGetRetainCount(objects[0])
}

retainCount(of: array)

// copy on write

// Custom copy-on-write type

class _MagixBox<T> {
  var value: T
  init(value: T) {
    self.value = value
  }
}

struct MagicBox<T> {
  var box: _MagixBox<T>
  init(value: T) {
    self.box = _MagixBox(value: value)
  }

  var value: T {
    get {
      return box.value
    }
    set {
      if isKnownUniquelyReferenced(&box) {
        box.value = newValue
      } else {
        box = _MagixBox(value: newValue)
      }
    }
  }
}

var b = MagicBox(value: 0)
b.value

var b2 = b
b.value

b2.value = 10
b.value = 1
b2.value