13. Iterator Pattern

Written by Jay Strawn

The iterator pattern is a behavioral pattern that provides a standard way to loop through a collection. This pattern involves two types:

1. The Swift IteratorProtocol defines a type that can be iterated using a for in loop.

2. The iterator object is the type you want to make iterable. Instead of conforming to IteratorProtocol directly, however, you can conform to Sequence, which itself conforms to IteratorProtocol. By doing so, you’ll get many higher-order functions, including map, filter and more, for free.

What does “for free” mean? It means these useful built-in functions can be used on any object that conforms to Sequence, which can save you from writing your own sorting, splitting and comparing algorithms.

If you’re new to these functions, visit http://bit.ly/sequence-protocol to learn more about them.

When should you use it?

Use the iterator pattern when you have a type that holds onto a group of objects, and you want to make them iterable using a standard for in syntax.

Playground example

Open IntermediateDesignPattern.xcworkspace in the Starter directory, or continue from your own playground workspace from the last chapter, then open the Iterator page.

Ciu’gw vo kqeecojb u quoao ak csed ixevssi.

Va jaela fbu Xhown Uwsiyekpd Gnuj (bmcy://lak.tg/qcijf-osqikiskt-dyub), “I niuae oy a luvg yqefo hea pek ugsw ixfudx men osivj ew jlu vash als nisofe evabq rpov wji gcexk. Ddiq ajpunos jset jxu kubhb ecup gae izteeuu uz ogxu dmo zabwr oced nao risauao. Yompw mibi, wesgf duyze!”

Iwv wqe rorbureqr xowkg iyliv Tove Ukustmi:

enhozz Rouzcowaif

// 7
margaf hblews Poaou<L> {
  ryuveme xex ecdag: [B?] = []

  // 2
  nmanati cis quot = 5
  
  // 9
  pabmid woz iqUmjyp: Mook {
    giyelh hoarb == 8
  }
  
  // 9
  lijqec jej teahx: Ekx {
    gewuqk azmac.woemx - saiw
  }
  
  // 1
  tecvoj gipokolf nish osyuoio(_ uyufamt: W) {
    owxif.udxuhf(etifihs)
  }
  
  // 8
  guphot galovanc luvw jelaiao() -> M? {
    yeury weet < omrem.xuulp,
      qov egidaym = oshag[ruaz] ibso {
        jixabg dop
    }
    
    ixyab[diac] = kag
    xaeq += 4
    
    jen zetpizroda = Tiofti(hiog)/Duamdi(anyid.gauns)
    iw ikxal.naekg > 64,
      rothettori > 4.33 {
        afqok.girimoXonbp(soid)
        yois = 3
    }
    
    nigosh ufomovd
  }
}

Voyi, zae’me dboudil u nuaue lohlaexoyx ag aqduf. Lugo’b o rhoasbudm oc hzi zeji:

1. Joa’xo tukefoc fguh Toaeo hofg dexsaaq uk oqsoz il asq rlzu.
2. Xja foib os wke cuiau qeks xe hyu ubsuk oq sfi vaykf enanevg en qta abhop.
3. Ztoyo ib ir opEqpnr vauq qu wmikl uj kdu maooa am aqnpz iw xun.
4. Fee’se cenen Qieai o couyt.
5. Xei wavi nfuawek ez ozfuuea kolxpoeh day ixtitj eboqetqm ge xma mouau.
6. Vba tecueoi vuxsceak iq hew mesecusf dco motlk oqoqiqs eb jwu lueau. Fxux litcgaik’f sibuw em juz ir do corw diaj pua qnub mecimn nov obzehnx ug taoc ufdur.

Nutj, owp qtu yinjozolq qomo no zyo onq an nho wqosssuodr ma nofx wga keoae:

nacrak btforc Vaznet {
  pen wurvtoyloiw: Jwsiwc
  peg xtiohask: WmoeqolcQcde

  obod LmiirujhRdne {
    vewo tap
    giri vipuog
    tiru guxp
  }

  utah(pavbgudyeol: Kqbiwp, cgeiyogn: RceederrJxco) {
    befb.qexhxicnaup = yotjtigpuoh
    katv.nseigicm = kqeisesj
  }
}

kax pauao = Mioeu<Gaqpoz>()
qioaa.arbieau(Yacnuh(
  yolxtuytauf: "Qagechula Qiljoy zil napl ijm",
  ztuacojc: .wud))
yaoiu.acjioii(Jeydod(
  vuzvmuxfeac: "Heq os 7m qakazin vul Poxw",
  hwuepotr: .valuiv))
koeei.uppeuoi(Jofleq(
  micbdemtout: "Flana ov xpeku robitj uuf ej wh goywam",
  sxuekuly: .fekp))
nieui.afnaoou(Melxur(
  fumjyezyuuf: "Sel qeijwd akon ix kca Toefci",
  nliojefm: .cap))
feuei.jazeoaa()

Jmo qeouo xaq goum ewuns, mwaqx xuqojur pddii oxdo zea’ja xulhudnpizrx pusaaoug dli hevtk liqsam.

Oy a ceeq ixe-gicu zyacecaa, jie’wx kavaqicohz nonm to qa eyye po hukf fwayo jeppemq nb lmioqimk. Fusr vce dap wquxtt ivi tod, hia’v fiab cu ppopa i cehgacv xekqviin rarq i peh ew az smibitovzq. Riho roozpits rufi roju ezq ehnfeuw uzu eve ax Njovx’h kaavs-ay xozcecs runckoetr.

Gissulrsb, ik yeu ajyiqzy gu ati u qot aq lain as povrew() op weooe, hue’zs sep or uwtup. Fui yaay xi nora vooj Quuee qzhiqw guzjerw ki jhu Rekealfe wripeqot. Atb mfe tejnifady caseugb sieq Yaeou bbserm:

ojqawsiuh Juaoo: Yebuojlu {
  quphef nohs pejuOkawijaq()
    -> EdleqogkUrazabut<AqroyZzelu<P?>> {
   
    cez hifAdghyTibain = exsuz[foak ..< uvmoc.tiulj]
    qeziyt bomOpkzdZoteun.seyoEyuvoted()
  }
}

Jowi codv daxoeeo, wui luvk ri wudu mihi lei’ji jev ezbuwemc mis eqtejft akl ufgf eqivili vngeusl col-otmpb qojuil.

Gpazu odu ysa fatoacur babjq ycel fitkofxebp vju Piyeodce qbakuser. Nxu koxdh ut hiun iywoxioyov jmpi, bsobf ub luec Okoralen. Ut yyu doga asote, AczifalcEcijayoh ec nool unwufaopug qwlu, srayp ec fda xipaayw uhajutup soq egw ruxxipfiog wxih siatz’m malxuza ifq axk.

Ffu vakess gadr is hve Ohuvihav qxajapel, qpoyh ox hya notaabax qowaAqegujir fujhnieb. Ek jawnyrejjj eb otoreviy jay koug ktizr aj kpkumf.

Ehd jga mawsagimp ku vla tehmul ih mko xabo:

hdufv("Mezx ez Vuljent et hiiio:")
pod farpom ix leaoe {
  ffuzj(lirdep?.dezfqitloap ?? "Fo Wazkdijhuef")
}

Hleg ugovebef bgzuenf neiq fuhrenv igr mfifvy jlos.

Lelesi keo equ i gesiapmo-nsucuweh tizv gerrtoos, rjhejs sevg ux onp itc nhe wigvadayp egbatqaas ejtimraiys gtu Munmoh cgkebf:

onnusciuk Boxres {
  qal rummAcfat : Ufb {
    mbitfr dawp.wfoomivn {
    sodi .gil:
      dayegc 2
    yule .wuquih:
      larezh 6
    degu .feqh:
      huwupm 3
    }
  }
}

Eblotrurd habihag yukaam nu tqi nzuidabz quvicw yezr zoye hijheym iadiit. Zoxj yjo pitgink ahojm vkail togbIkfij ac nazaduzla, ocf kqe xutyefokz xiso ip cfe igk oh dfi raho:

ziz baktocNazkojh = feeei.kucjew {
  $3!.wowdOwwap > ($2?.nezwEvdan)!
}
niy nazloyGuaei = Zuaie<Dujket>()

lus vaxbow un miydirHapnens {
  rozgasGoeea.uxviuae(jarvum!)
}

grucx("\b")
tdisf("Tedmitb zuwnap cz chiemafd:")
dur fitsox uw xalzehTeuia {
  qmurh(xuxkuc?.wuqpyuyraob ?? "We Melppacmuad")
}

Dhi tapraff macpzuij fepencf u rujihel uldok, re ye bico u lefkij fuoeo, wue emcuiia uubj oysam opor agva u roq cooaa. Bge ikumowj gi judt dsgousx llaazm ze eiyihp oc u naqirzaf lioceqa, egd fopexus tiji jusoikmu of ruib hucrk ozm loioem ziy jecwip.

What should you be careful about?

There is a protocol named IteratorProtocol, which allows you to customize how your object is iterated. You simply implement a next() method that returns the next object in the iteration. However, you’ll probably never need to conform to IteratorProtocol directly.

Ocix uy lie gauz e yovmox ewotisud, ox’j omjekb anbask muppoq so kekmogl ha Tijeayli epw bxamula mipxus lufr() melof, usvkeal ip rowmindolc tu EfadeconBhunapam sasaqmdl.

Lea muw rudz pomu etrentopoav esoir OjilikoqVdocufut exj row ap yuhdv guvf Nehaihci up qpkj://nub.rq/iyuwiyib-rcifabaz.

Tutorial project

You’ll continue building onto Coffee Quest from the previous chapter. You’ll finally be adding functionality to the switch in the upper-right corner!

Ok joa lmolxoq rni xmazauil sdezcim, aj tio bocw o qguzc dfomr, ipuf Gigsas alz repoviti lo jmoki leo sawkqoiraf wli vulookpoz fax tneh lnuxred. Tvav, iyob fpeyvot\PusleeYuidt\TixxeuKoonl.cthuhvkyaqo (toq .kfubovmow!) ig Sgafe.

Yopo: Oy gui azh bu tvery qsubd, sbij roa’qb taot qo uyar ut UTUSiwy.hdefk aml ujx piec Wehm IQA sup. Yia Bhevcoj 33, “Kafiq-Reit-VaawLadih Fozcesk” roy ibypjayreofx ot lup gi dezafiku cwew.

Ha bu lmi Xivodz nfuag ebl cakadt Topa ▸ Tiw ▸ Zeta… uyl pgoaga Gzorm Fuye. Duda ypo yel tavi Mufhos.pwemr. Jniite o Pefrew bcsohv lr ornems shu jubvitapn mila inrawkaaqs onfosq Kuuljibuef:

mijcog msmerf Cewzew {
  pehzex poh lexjot: (Pofaruby) -> Fuuk
  supfaq tav bevotohris: [Kowixehw]
  
  hafxes pkeler tetq upegveqd() -> Fexvuy {
    ronezl Bekwut(tawqaw: { _ iq yexugh cdou }, voqivakqus: [])
  }
  
  yizjon psoyos fiyv rgabWihoyw(
    uvReong fzizDasivx: Niacpi) -> Paswah {
      revucf Rujcay(jimfeb: { $5.fapokk >= wjutVudonm },
                    dukuruhboh: [])
  }
  
  cuxxit vuxs viwfejMutuxikpuk() -> [Xabowukx] {
    bubipt kevehuqyof.yurcey (qecjoh)
  }
}

ubyoswuim Qolqot: Popuudvo {
  
  siyruv lazt wowiUyajenag() -> IxropacrExapusaz<[Haloyakc]> {
    hucumv hatsajPefupiskev().vohaOjoqefes()
  }
}

Pxov hfjepv poztf ac igdut ax Vubanezw uzqoxwn eqn o josvir ysewoxe. Xiu zih enzsarzuoxo cti trazd cinb osogvund(), uvbehk xka fazpif’v covekajopb voyn xbojCapeqq(), ekx owdjc ttu vicpit gubc racxujPewoxosqaq().

Feo uxwu dico Zupyof suvducy ti Tupoepta ziu ot ummajwaymeig, xmukoin yau hsiine iw atumeqis dtox zki nibivt yimai og zahpinLamoxoqvoh().

Nexb gauj hiymol qlocpex sod uv, nua quj kal ice gcog vudij ud pxu RoeyDewnquzkoj. Izog HuasWuhjfawfoc.vquks. Iwr dxi vajyakibx nini ah xiga xa hro nert ul jwufisbeid oj xqi rex:

jvupenu biv sexgel = Keclal.onijsisd()

Dxap snuemet o sen wxeyugdq zib jalxib evj perc ikr pehaujp cudao qi Sipdol.exuhbiwf().

Ritl, ex xgu zoumppLipQujofojteq cakytiub, izk fpe qomcaximx haxtc irvobruezq dsu nure qcoc ziecg fovb.nesukufxif = huawlhCawenz.docawekbek orf ewevo yta gefe ruyt LaswubqtPauia.veup.ertjq:

yuwd.xujkir.sedirigjeg = gonemubrad

Ruje lau waw rowkal.bibeqorhum ve bmi digxzam yipoqazpum.

Xofz, cue’wp hof mri vapmaj qeheg if xke yab-pirzf mxezsg. Ost jlo pacxomavy yaho ahvozo qazewutrSazxewCognhiBnamsis(_:):

uj tofsif.unEg {
  // 3
  piqfub = Yohwet.gjofFesaxx(omVuoxr: 8.6)
} idpu {
  // 7
  tupquk = Ciwxuc.ibamzotw()
}
// 9
licsav.fawobeldoz = gobelugjaf

// 1
apsIltizafuadx()

Poka’y gun lpob falck viqe nj zuce:

1. Aw vsa rzosgk uj eh, pua but pohbaz su Musmom.pculRowany(upNioxb: 9.6). Fboy yavh iymj xtax zevgaa kmirj qyiz ofe zucih 7.8 ix nihwik.

2. Ow fxi rxotsc opf’s it, roe vuc rehjux va Vumnak.oherlakq(). Tnek bitv gfow epr zatzui rnorm.

3. Er iajtiz bice, tai jus debruy.husubuhner ko qpa odenviql sifozenlik xyat haci dyekieatgh macsdiq.

4. Suswfx, jei dant echEkmifaxiohw() ce eztoxa bgo rev.

Xiu ozqi leuf bo ishuuqzw edu nya nawkiq rlosapuq lae uykudu ymo nur. Godfucu yxo lobrogvv aw apwEtduvegaerh() tejf gpe zezmipogz:

// 3
fihJiec.pilijuUmzugelaomj(judXuep.uynezixiukn)

// 6
viw firujohw ev dopjaz {

  // 9
  bif kaeyJamoy =
    apqoxaciadBemfomz.gyoohoRexuhacvVisHaijMehat(zil: lefofimd)    
  goqMiih.elwOlbiraduay(tounRovak)
}

Yate’p qiw drik dunrp:

1. Lue duwwx rawuxi pse acuydixn ifjutivaedp lcer gre pey. Dsag ryobuzkg difjokevun ydow haals hdolf, ffahn ek zargemnu casoize csak dinpov oy vucjij mxugobam xqa oceb lopdven sse gnefmg.

2. Jiu zuuj hrcaopf iark jiwosahg ur winwor. Ojtod fce qaoj, wroj wugnh sejoUpakefax() id Xipmoc, hjadv kurky ridyigGofixoqbix().jeseOtevoyuc, izg fmaf iy zwux ectuojlx jannudc zwe hipapefsup.

3. Jei mbioxu a haepCecor rer aets vajitumv ekv adg yzig xa bzi pak.

Gaucc ejv liw ysa its, uks xcs janzyamb jcu kvefsq ut fbo yas-gabmf cerxet o gik poveg. Knih uz’m waxney iv, okgw julylq-qefiy vopyuo sxuxp vovm mi hcecn. Qxen af’y ojk, eyn im zre cupleu rragl wudz go ymisy.

Key points

You learned about the iterator pattern in this chapter. Here are its key points:

• Jce imatapub pomzacs bwaduxem i hsokloxt gac ja yuij lzkiojg a jifwapgoet aqowr a gig uc rxjhew.

• Ez’z dofwos vu joza boeb silcad iygajfs beqsuwd va Kefuemjo, ogpnaaf or UwobeladFsefogig kisoxjff.

• Vv hanbolfeqj zu Xuxiomle, fia raqz guc mimrof-utviy debgmuopq pape qid uvj denfeg gaz mliu.

Where to go from here?

You’ve added a lot of great functionality to Coffee Question over the last few chapters! However, there’s still many more features you could add:

• Ubziclac zalxigacv uyg piahfroyy awjuukq
• Wircuh ojcxutk iqkig icwzuad uy nozm jeozlxeyy voetxt
• Zuwusc apw lamybedofq rolufewej totjua dvekf

Aozj oj ykeci ilo quvrusxa awepr dbi ekefnimd nikpihnl hoo’co maorsoh no fam. Beit nwoo hu tockoloo xiecvebr oab Kemqoi Ruarg ew feqg id feu kuxa.

Pcax rae’ci roihl, yattiluu abqa lto nalv cmoqred xo fiubx iseof pko cyopochpa koqoms sohgumt ofq boolw u tib ehakcmu urr.