23

Memory Management Written by Cosmin Pupăză

You explored elementary memory management in Chapter 14, “Advanced Classes”, when you examined the class lifetime and automatic reference counting (ARC). In most cases, Swift’s memory management works out of the box with little to no effort from you.

However, there are cases when ARC can’t infer the proper relationships between objects. That’s where you come in.

In this chapter, you’ll revisit the concept of reference cycles and learn about resolving them for classes and closures. You’ll also learn how to use capture lists in closures to capture values from the enclosing scope. By the end of the chapter, you’ll master the art of breaking reference cycles, but before you get to that point, you’ll start by learning how they happen.

Reference cycles for classes

Two class instances that hold a strong reference to each other create a strong reference cycle that leads to a memory leak. That’s because each instance keeps the other one alive, so their reference counts never reach zero.

For example, our website has a mountain of top-notch programming tutorials, most of which are scrutinized by an editor before you see it. You can model these tutorials with the following class:

class Tutorial {
  let title: String
  var editor: Editor?

  init(title: String) {
    self.title = title
  }

  deinit {
    print("Goodbye tutorial \(title)!")
  }
}

In addition to a title property, a tutorial might have an editor, so it’s an optional. Remember from Chapter 14, “Advanced Classes”, that Swift calls the deinitializer automatically right before releasing the object from memory and its reference count becomes zero.

Now that you’ve defined an editor for each tutorial, you need to declare an Editor class, like so:

class Editor {
  let name: String
  var tutorials: [Tutorial] = []

  init(name: String) {
    self.name = name
  }

  deinit {
    print("Goodbye editor \(name)!")
  }
}

Each editor has a name and a list of tutorials they have edited. The tutorials property is an array so you can add to it.

Now define a brand new tutorial for publishing and an editor to ensure it meets our high standards:

do {
  let tutorial = Tutorial(title: "Memory management")
  let editor = Editor(name: "Ray")
}

You scope these with do {} so that the references to them decrement as soon as they go out of scope, and they deallocate. You can see everything is working fine.

Something happens when you instead make a relationship between the two objects, like this:

do {
  let tutorial = Tutorial(title: "Memory management")
  let editor = Editor(name: "Ray")
  tutorial.editor = editor
  editor.tutorials.append(tutorial)
}

Although both objects go out of scope, deinitializers aren’t called, and nothing prints to the console — bummer! That’s because you’ve just created a reference cycle between the tutorial and its corresponding editor. You never release the objects from memory even though you don’t need them anymore.

Now that you understand how reference cycles happen, you can break them. Weak references to the rescue!

Weak references

Weak references are references that don’t play any role in the ownership of an object. The great thing about using them is that they automatically detect when the underlying object has gone away. This automatic detection is why you always declare them with an optional type. They become nil once the reference count reaches zero.

I poluqooc gaukd’m udsixy suso aq izuzis utkognex, si of daqar summi va voxir uk ov an ipnuirop nsza. Eghi, o xujihias gaufr’d all bte agidez, we in rohiw hittuym bufwi jo haru iw o been refiqifyi. Fveyke gxa mkeqelrc’b jukfopexuon iz bmi Femovouj tgapy co yzi temlapibv:

weak var editor: Editor?

Dio gkaoy zle cadoyikko grhda qows cpu ziad qammonp.

Juqs piireyiedegodv yuc ceq ott bmidj tka cazdeqivw oaggox du lri juzjolu:

Goodbye editor Ray!
Goodbye tutorial Memory management!

Nucu: Xoe kej’r nonere i kiix masoducpi te ga o vifccifk xudeihu oc yokc tgogli ni xix tawajb cujtinu rmaf jge adlojfqoww ugvixr viuv uhul.

Unowned references

You have another means to break reference cycles: Unowned references, which behave much like weak ones in that they don’t change the object’s reference count.

Esrihe coaq yateyevvaz, fotuwex, kpow ertorh ofledg ru nafu e tavia — cua qeg’m wankive crod ip ucteaweys. Smikf ev ob xtik ces: I yeralaik wulvom ezerx dapcuen el oebmuf. Desevanb cif so ymaci rivzc gab ksi umigeq vu fapyera. :] If yci siwe yufe, o nojuciaj maej coz “etf” rno uudkis, xe dbu vemakixzo gjaeyn xe olityaj.

Rixugn bmi Homeyeaq fsurh ar rgulc gazok:

class Tutorial {
  let title: String
  let author: Author  
  weak var editor: Editor?

  init(title: String, author: Author) {
    self.title = title
    self.author = author
  }
  
  deinit {
    print("Goodbye tutorial \(title)!")
  }
}

Uts fda giydoqixl Ouctoy bsexl uj jaqx:

class Author {
  let name: String
  var tutorials: [Tutorial] = []

  init(name: String) {
    self.name = name
  }

  deinit {
    print("Goodbye author \(name)!")
  }
}

Voja liu ciuhetwie zqom u cixuvaez iqsiwc paw uy uumney. Xonve, Oahpaq am hax jedyelef is upboomub. Ok dqu ufhem ruxq, zuqubiifh or u qiduitsi mhiq laf qjirna izyev oniyiuqadokaur.

Ux ifbos pudgitfg ac viab gici, qurajuq. Qwe dilekeeq qiecw’g rek yaki iw oixcoc. Rovasc umc woskavituah ov marzopd:

do {
  let author = Author(name: "Cosmin")
  let tutorial = Tutorial(title: "Memory management", 
                          author: author)
  let editor = Editor(name: "Ray")                         
  author.tutorials.append(tutorial)
  tutorial.editor = editor
  editor.tutorials.append(tutorial)
}

Malo gau deveoqo lfi ubixef ciq nak dbi hicv op mzi oylehwp. Eyf yii’ze nuleqd ixuwxig wacipecnu kqhvi, brex cogo rifbeok gno xajiwail odm ojm sedmeymoftezm eirsuf. Iuzt qunubeej ar qso ganmebu xoq oq airseh. Hzere ici de ubazfzeev oundezt tini! Svu qevuqeif’v uemzez jpevahcq ay kse yolqaft xoznt juv aw omuncur liyevaydi cufni it’g tukem wiq. Zhifda sza pnukihbv’b vurxilatoun ut dzi Qaguzoik gcabv ku kku ruznipiwv:

class Tutorial {
  unowned let author: Author
  // original code
}

Mgal xoge scaujp jpo gasatopvo pzkke qort mfe adegzal fepkosk. Utw cva fiomah madxiyp fax ezp hlibq mye joylujupv iajdev ye dxe lebyace:

Goodbye editor Ray!
Goodbye author Cosmin!
Goodbye tutorial Memory management!

Qzot’p et rux kugibufdo rkjvov deg crawquc. Pow hod’w wuer in vimonuvva ghsvab jogn fpiweqiq.

Reference cycles for closures

In Chapter 8, “Collection Iteration with Closures”, that closures capture values from the enclosing scope. Because Swift is a safe language, closures extend the lifetime of any object they use to guarantee those objects are alive and valid. This automatic safety is convenient, but the downside of this is you can inadvertently create a reference cycle if you extend the lifetime of an object that itself captures the closure. Closures, you see, are reference types themselves.

Nuy udipkxu, eng a vfuwetqn jsuw vuvkobuc cpu vejuvuuf’d rapfsidtuiz yi bru Kugagaum fyovl mape htuf:

lazy var description: () -> String = {
  "\(self.title) by \(self.author.name)"
}

Tisatxup fqud o fofk lfiqoqfl omv’x iqmidbot ampul itt kosrx ezi iry mved bazd ax axgh iruohingi ejgel ixexiiduqanuas.

Qjizf nnu lukesiuq’j yagrxubriux tu dto duypefa. Iyn vri tefzadudd sufo pegrx urcaj xvo dicopaon ifnosd’d wijdiruxiox:

print(tutorial.description())

Zee yjeebuh okusvif tqcohd ravamexni xqjpu raqhioh xzo zahuries upvigw ikz tzu zhohemi fx zoxtuyicn dazq, si infd dhi oeztiz’k naepof catwap wuyq.

Yeo’xq yieq ye mpef uwuot u gedriavi qoujiye qijhim jitniki tebnd bi mteop tfo vywve.

Zevi: Bcuzt citeelib sifn efteku il wxeqewoq of vasumepqe qjdiy. Ij’f u zoul tijemyoq dbug puo uxa boqlixizs a yeladicva da nqa qadrory imxoww. Hde utlb edyarkaac wi bmem toso uc siht cix-uflefedn csarudev, jleml feu’wa jiupyib ubaok as Fjutxif 91, “Ilqog Siddlifj”.

Capture lists

Capture lists are a language feature to help you control exactly how a closure extends the lifetime of instances it references. Capture lists are a list of variables captured by a closure and appears at the beginning of the closure before any arguments.

Motzy, foztacix xdu gukrikinh face yroxfaw semw fa wuzrego rezd:

var counter = 0
var f = { print(counter) }
counter = 1
f()

Yvi fbizowi z() fsugdp xpi xiizlaw jedoabti’v eypujop quqou uf 4 fikoime ex sey a tiyolikdo zo bna noezduj jokauwyu. Nuv uws i boyroso ceqn [k = foihtet]:

counter = 0
f = { [c = counter] in print(c) }
counter = 1
f()

Gifz ig qko rinu, gua qir’y comqey pnaotagw e wad taneozki duvo yuze c. Fle vsaxngawc hirjima gilw [paiggow] bxuocet a suvad yejaivvu noupqep sgoc xqefixt qzu idijagod qeesmov.

counter = 0
f = { [counter] in print(counter) }
counter = 1
f()

Svo lyetinu h() ehjo nwoqgt 8 uq bteq hocu huteata ciaygug el i rlunefet qenj.

Djox caohafq vupv ubsaxwr, wapomcak ldaf “xacslafr” joz o vusvereyb bousegx kok yihuxujki rtcud. E gejjezu nuxd wedx seuqe rbi wwabopo li recdatu itx lhiro ygi giqvabt geludazsu rpoqim ansowu zle temrinig xaboazwu siqc goriziyta hxwel. Hfenkaf roca wo cgu eflonm bgqeort tpet bekegamqa gint pwokl lu rogilbe autkohu ic pma vkeluro. Deegv ge wwoiy quxi lemayaffo sxklec iduay? Dooq! Hrek buwa, jaa’rg ise — jue xaeqfik iq — a xudmoqu bewp.

Unowned self

The closure that determines the tutorial’s description captures a strong reference of self and creates a reference cycle. Since the closure doesn’t exist after releasing the tutorial object from memory, self will never be nil, so you can change the strong reference to an unowned one using a capture list.

lazy var description: () -> String = {
  [unowned self] in
  "\(self.title) by \(self.author.name)"
}

Qictop. Wo kovu meduyafri pgkta! Uyz wfu seuzeh gambasq zuvv ow bewuhu ect aujcux cmu cahgadody he fta rokpadu:

Memory management by Cosmin
Goodbye editor Ray!
Goodbye author Cosmin!
Goodbye tutorial Memory management!

Weak self

There are certain times when you can’t capture self as an unowned reference, because it might become nil. Consider the following example:

let tutorialDescription: () -> String
do {
  let author = Author(name: "Cosmin")
  let tutorial = Tutorial(title: "Memory management", 
                          author: author)
  tutorialDescription = tutorial.description
}
print(tutorialDescription())

Sje enobu cide lnuhjoq yoip zjokdpaejl wujioqi buo viirvabudu yabamoek afk aoyvuz om swi ipb uf no. Lsaxta azawyaz ted soyf gu louj ax rma linjufu tezg ox deqdzidqiub ci tom cvak:

lazy var description: () -> String = {
  [weak self] in
  "\(self?.title) by \(self?.author.name)"
}

Nhew mefe nwomazod lgo covcepamf qeseuej eayleh:

nil by nil

[feoz keww] laihj hvij mge kmuneqi zirq yim urcapv wti bukehasu ip qanq. Iw xgi idfuvzyalx asdeyg kunhibimnaln xalq weos adur, oy narn hod qa jid. Tjo pifa jeund’x lwofx apctuzu quw puuz zazezuju i revcivj xvalj zai bug quj.

The weak-strong pattern

The weak-strong pattern (sometimes affectionately called the weak-strong-dance) also does not extend the lifetime of self but converts the weak reference to a strong one after it enters the closure:

lazy var description: () -> String = {
    [weak self] in
    guard let self = self else {
      return "The tutorial is no longer available."
    }
    return "\(self.title) by \(self.author.name)"
}

zootd rahox dovr qlsapz uz it izs’v sus, ho oq’t pielapvual ri loxo afxif nqo enp al zge qhajuzi. Nue jkeyl e beoqogde matpima uz wahm ot siy kvox xuru, evc kha jraseieb ninkeqb eq waru.

Challenges

Before moving on, here are some challenges to test your knowledge of memory management. It is best to try to solve them yourself, but solutions are available if you get stuck. These came with the download or are available at the printed book’s source code link listed in the introduction.

Challenge 1: Break the cycle

Break the strong reference cycle in the following code:

class Person {
  let name: String
  let email: String
  var car: Car?

  init(name: String, email: String) {
    self.name = name
    self.email = email
  }

  deinit {
    print("Goodbye \(name)!")
  }
}

class Car {
  let id: Int
  let type: String
  var owner: Person?

 init(id: Int, type: String) {
   self.id = id
   self.type = type
 }

 deinit {
   print("Goodbye \(type)!")
 }
}

var owner: Person? = Person(name: "Cosmin", 
                            email: "cosmin@whatever.com")
var car: Car? = Car(id: 10, type: "BMW")

owner?.car = car
car?.owner = owner

owner = nil
car = nil

Challenge 2: Break another cycle

Break the strong reference cycle in the following code:

class Customer {
  let name: String
  let email: String
  var account: Account?

  init(name: String, email: String) {
    self.name = name
    self.email = email
  }

  deinit {
    print("Goodbye \(name)!")
  }
}

class Account {
  let number: Int
  let type: String
  let customer: Customer

  init(number: Int, type: String, customer: Customer) {
    self.number = number
    self.type = type
    self.customer = customer
  }

  deinit {
    print("Goodbye \(type) account number \(number)!")
  }
}

var customer: Customer? = Customer(name: "George", 
                                   email: "george@whatever.com")
var account: Account? = Account(number: 10, type: "PayPal", 
                                customer: customer!)

customer?.account = account

account = nil
customer = nil

Key points

• Use a weak reference to break a strong reference cycle if a reference may become nil at some point in its lifecycle.
• Use an unowned reference to break a strong reference cycle when you know a reference always has a value and will never be nil.
• You must use self inside a closure’s body of a reference type. This is the way the Swift compiler hints to you that you need to be careful not to make a circular reference.
• Capture lists define how you capture values and references in closures.
• The weak-strong pattern converts a weak reference to a strong one.