ARC

首先，我們可以先從官方的範例 ARC 開始‧

class Person {
    let name: String
    init(name: String) { self.name = name }
    var apartment: Apartment?
    deinit { print("\(name) is being deinitialized") }
}

class Apartment {
    let unit: String
    init(unit: String) { self.unit = unit }
    var tenant: Person?
    deinit { print("Apartment \(unit) is being deinitialized") }
}

第一步，我們分別先建立了 john 以及 unit4A

var john: Person? = Person(name: "John Appleseed")
var unit4A: Apartment? = Apartment(unit: "4A")

此時區域變數便會對物件建立強引用(Strong Reference)

john -> Person(name: "John Appleseed")
unit4A -> Apartment(unit: "4A")

第二步，我們分別對 john 以及 unit4A 分別建立關聯

john?.apartment = unit4A
unit4A?.tennant = john

第三步，斷開區域變數對物件的強引用

john = nil
unit4A = nil

此時兩個物件並沒有被釋放，因為在前一個步驟時已經發生了 retain cycle

解法

解法其實也很簡單，只要將其中一方改為弱引用(weak reference)

class Apartment {
-  var tennant: Person?
+  weak var tennant: Person?