Reference cycle example

reference_cycle.swift

//: Playground - noun: a place where people can play

import UIKit
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

class Child {
    func bar(completion: @escaping () -> Void) {
        DispatchQueue.main.async {
            completion()
        }
    }
}

class Parent {
    var child = Child()
    var x = 0

    func foo() {
        self.child.bar {
            /**
             By using `self`, we are capturing the parent instance,
             and passing it to the Child trhough the closure. 
             But the parent already contains a reference to child, neither child
             will be able to release parent, nor parent will release child. 
             */
            self.x = 42
            print(self.x) // 42
        }
        print(self.x) // 0
    }
}

let parent = Parent()
parent.foo()

reference_cycle_fixed.swift

//: Playground - noun: a place where people can play

import UIKit
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

class Child {
    func bar(completion: @escaping () -> Void) {
        DispatchQueue.main.async {
            completion()
        }
    }
}

class Parent {
    var child = Child()
    var x = 0

    func foo() {
        self.child.bar {
            unowned let unownedSelf = self
            unownedSelf.x = 42
            print(unownedSelf.x) // 0
        }
        print(self.x) // 0
    }
}

let parent = Parent()
parent.foo()