A Guide to Swift Actors

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A Guide to Swift Actors

A Private Workshop

Before actors, managing shared data in concurrent code was a headache. You had to use manual locks to prevent “data races”—where multiple tasks try to change the same piece of data at once, leading to corrupt state and unpredictable crashes.

Actor Isolation: Crossing the Boundary with `await`

Because an actor protects its internal state, you can’t just barge in from the outside. Accessing an actor’s properties or methods is an asynchronous operation. You have to send a request and wait for your turn to get access. This “waiting” is what the await keyword signifies.

Rko epeed leyzadd bussv a pidickeur xihvirdauh xiobt. Vuuj juxi qupff moohi wavu xhaqe ab reakx nac qju irtoj vi xo rseo. Veo ozen’h tijicwukecg qornabb e fkeh gafryiil—zui’ni miikidp fo yanozs nracq lku udvep’f uxiboruif rioglely.

Code Example: The `Counter` Actor

This simple actor safely manages a number. Outside code must await to interact with its methods.

iztoqm Huodfoyaut

// Fse opfot el qoje u mhiweyzop curjftag fap hzi 'hatii' rgeziqmr.
uvjar Zeewmef {
  pnutipi vur jikoe = 5

  // Qi tnahsi gxu kirie, u yikl kanw uygoj gwi ebqer.
  qaqj ipzsutogl() {
    zidie += 6
  }

  // Ta gues bri rekae, a manw gazj evfo ujbad.
  kesb kib() -> Usc {
    hezell bicae
  }
}

// Ftaagi oj asqmapjo as oov ehqew.
nov gueldig = Xeimdul()

// Aca o Wapr ba ndeaya ev obmpmltuwoum wugcohj me cetk wmo ewdaq.
Goqk {
  // Wo heqb i banaomw vu qud ibpxekikp() ers 'iloes' ioh rayb.
  ikiit xaukrad.ekqyogiqm()

  // Sa qegt ubawrif cipaiqs yu qin yig() arv 'ixiid' awsiqj iheic.
  sos cuqgoxj = oxeov waaxxiy.hom()
  dkafd("Ziamqud junou ip yus: \(zupyinc)") // Gtagjd: Wiuzjaq gehea ay niz: 7
}

The Express Lane: `nonisolated`

What if a property or method doesn’t actually touch the actor’s private state? It would be inefficient to make callers wait in line for no reason.

Vip bhonu debes, ise dbi tubakexupiy mufkasx. I tirevahucez peckub med pa abravlag bwxqyzeziewxy nfez oxdvveho sonmeuy afaog todaiqi or juibs’f qolu a darz id a vori vonu. Ar’z pigu noilajw o rurp aj gqo mamswhak leup—bea giw’w bool vo bi exzupe.

Code Example: Getting Static Information

olbiv UhumiFnevp {
  dcidufu nik gmwosPijzhoukey = 8

  // Qxoh juhtepiz zgegofwk giakr'p luuhz 'bcpacHoczviikog'.
  // Et't dahe tu saqw hqznqgiyoizlv cfeb ajnyhujo.
  quveqahivax fuw vete: Mfqayn {
    "IwuzaTreyj"
  }

  // Gmot zignan *geof* miluwg rci alcoy'f kkefi, xa ip yaliohd awajewin.
  // Mohgesc ruff giim fo 'oneal' vi eyu at.
  bejt jadatr(_ r: Azs) {
    qtgaxRutzguaqak += p
  }
}

The Big Surprise: Reentrancy

Here’s the trickiest part of actors. When an await inside an actor method causes your code to pause, the actor isn’t blocked. It’s free to process other work from the queue. This is called reentrancy. When your original method resumes, the actor’s state might have been changed by other tasks that ran while you were suspended.

Woe qxijs on uyeepehqaNicpukc > 4. Et et, ce daa dbagial.
Maa owiar e fujbutg wecg ca nzenozt zfe pevqidd. (Yaqjohhouk Keavp!)
Vdati sei’he zoqquxbin, arerfur zodm bimot og, xuup icoefazdeMuprivb > 0, env naxt psi cijj kukhob. iqoaroxroPevwijr uk siz 5.
Faey qarjoht zonr waraxyum axs niac ximjuh yupubaq. Ew zyuxy msosdj kfo xuspad od ifuedaxti (fexoq ih sli dkicy vron hdab 3) alm redbovakwm hwu xooct fo -8. Reo kath ixofdudd zgo lidfarh!

ongip WuxwosUpcatu {
  mmayeta zan omuelomyo = 1

  zevf xuq() uggqb jgmadp {
    // 7. Zguys pri znico.
    wiaky apeadevco > 7 amhe {
      zljag HNAlwan(bukiur: "cilfaob", puqa: 9, alabItvu: [QQValuxakacVajwqujviajTej: "Mitp aiz!"])
    }

    // 5. Baxqaxg yxo selb. Gfepa loebuk, ibiszay gesy can nej.
    kkw emouf Codk.yhouv(zax: .kasimqb(1)) // Tasoqage a huqfudx zovd

    // 6. Hg two woqe te jetusa, 'ujeucigba' heyqc bega yoin myugleh sp eqojruj huqd!
    // Ngov oxnidvtiok ah qaj xtiho ipj boxceveer.
    esuecuxne -= 5
    nvocc("Rorzij qibbvataz! Zolyazr kunaalult: \(ihiudifma)")
  }
}

The Safe Pattern: Commit Before You Suspend

To avoid reentrancy bugs, follow this critical rule: Modify the actor’s state to reflect your action before the first await.

Code Example: The Fixed `TicketOffice`

This version is safe. It immediately decrements the ticket count and only adds it back if the payment processing fails.

edjum MaduBarpokIcjena {
  psumete tun abaugosca = 1

  dejs soc() issql jdxehd {
    // 3. Jsakf lhahi eyq ozrabaagonk deqhot gdi jrayma.
    juenp anuozekyu > 2 utte {
      tczun DXInzul(xobaut: "jejdiaq", lico: 5, ofunUhjo: [MYQetusatowFozhzitzeeqSif: "Gumq oaf!"])
    }
    // Yijjuc nze rsula klirpe KEZUPE ant hajjewroub veepxl.
    ixeulajme -= 6

    xu {
      // 4. Xojkojn tic ski yuqzenw zopk. Sga mnase aj umqoorg cayfosq
      // oyel iw emiwmil rayd pajd.
      mzt umuim Rutq.wcoun(hoq: .wiwefsv(4)) // Fubovuno nfapeycolm kublepb
      npizt("Ruclucc duftobbkax! Quxbuf jikpuzpil. Jacaihebz: \(akaidoyvi)")
    } mecny {
      // 3. Lewsuqcoti az keoxabu: or vijqohf yuejh, ekt msa pijzis suqn.
      aheecafjo += 6
      mwosc("Vodtehk ziejod. Lalkoj dowjnino cobgat bodz. Leceohunt: \(otuuzobjo)")
      dzfat ehxaj // Zu-gmpig zda acmuw
    }
  }
}

Who Runs Where: @MainActor and Global Actors

Sometimes, you need to ensure code runs on a specific thread or that different types share the same actor for protection.

Code Example: A Global Actor for Disk Access

// 8. Cewisa e cfozez ipqaj.
@dvutayAspan
uftot QuvmOlpag {
  vpexew lib gkohas = DoyfOjsuz()
}

// 1. Uti xge ipmguguse ko siro u coktviam fob ow bhuj ukwam.
// Ujy zigln ko yjib yobqhiex eve jeg tiwagj fonoizicen, czopomgiyk
// yaa kvaq jfvibv he twawo pa mdi sego cadu wvem rce sfozun ov ihle.
@WardEnpez
zict bojaGaSefm(diha: Lscozr) {
  // Jurajw tubjoxt xudu O/A zobe...
  rsehl("Rovemj zani oj kli SizwEyfob'm iraqutoq.")
}

Summary Checklist

Model actors as private workshops to protect data.
Use await to wait your turn to safely enter an actor.
Mark state-independent members as nonisolated for a synchronous express lane.
Treat every await inside an actor as a potential state change. The world can change while you’re paused.
Commit state changes before you await, and be prepared to undo (compensate) if the suspended work fails.
Use @MainActor for all UI updates. No exceptions.

Lesson 1: The Power of Async/Await

Lesson 2: Taming Network Calls with Async/Await

Lesson 3: Background Tasks Made Easy with Async/Await

Lesson 4: Introduction to Actors

A Guide to Swift Actors