【Swift記憶體管理】strong VS weak VS unowned

在ARC下，讓開發者對記憶體管理，省下很功夫，同時也減少不必要的bug，讓成APP閃退。但在某些情境，仍需開發者費心，把變數宣告成weak或unowned，避免retain cycle。本篇文章，帶大家認識strong、weak和unowned差別。

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strong VS weak VS unowned

• 在ARC下，變數預設是strong reference
• strong：對引用物件，reference count會累加
• weak和unowned：對引用物件，reference count不會累加

weak：當指向的物件不存在時，會被設成nil，所以宣告變數必須是可選型別(optional type)

unowned：一定要非可選型別(non-optional type)，才不會引發指向物件不存時，對unowned變數存取，導致runtime error

weak的變數不能用 let 宣告，因為它有可能變成nil，但常數不允許被修改

Memory Leak

如何造成memory leak

當兩個物件互相引用時，會造成retain cycle (strong reference cycle)，該被回收物件，無法從記憶體中釋放，導致memory leak發生。

程式碼範例

class Person {
    var name: String
    var device: Device?
    init(name: String) {
        self.name = name
        print("Person is being initialized.")
    }
    deinit {
        print("Person is being deinitialized.")
    }
}
class Device {
    var name: String
    var owner: Person?
    init(name: String) {
        self.name = name
        print("Device is being initialized.")
    }
    deinit {
        print("Device is being deinitialized.")
    }
}
var eric: Person? = Person(name: "Eric")
var iphoneXR: Device? = Device(name: "iPhone XR")
eric?.device = iphoneXR
iphoneXR?.owner = eric
eric = nil
iphoneXR = nil

Referece Counting 過程

執行結果

Person和Device的「deinit」沒有被執行

Person is being initialized.
Device is being initialized.

Person引用Device，而Device也引用Person，互相引用對方，無法從記憶體釋放，造成了memory leak (記憶體洩漏) 問題。

如何解決memory leak

用weak或unowned去修飾宣告變數，就不會對引用物件，累加reference counting

class Device {
    var name: String
    weak var owner: Person?
    init(name: String) {
        self.name = name
        print("Device is being initialized.")
    }
    deinit {
        print("Device is being deinitialized.")
    }
}

Reference Counting 過程

執行結果

Person和Device都被回收

Person is being initialized.
Device is being initialized.
Person is being deinitialized.
Device is being deinitialized.

Reference

Weak self and unowned self explained in Swift

Memory Management in Swift: Understanding Strong, Weak and Unowned References

3 most common memory management issues in iOS & ways to fix them

iOS ＠ Taipei