8. Observer Pattern

Written by Joshua Greene

The observer pattern lets one object observe changes on another object. Apple added language-level support for this pattern in Swift 5.1 with the addition of Publisher in the Combine framework.

This pattern involves three types:

1. The subscriber is the “observer” object and receives updates.
2. The publisher is the “observable” object and sends updates.
3. The value is the underlying object that’s changed.

Note: this chapter provides a high-level introduction to @Published properties, but it doesn’t get into all of the details or powerful features offered in the Combine framework. If you’d like to learn more Combine, see our book Combine: Asynchronous Programming with Swift (http://bit.ly/swift-combine).

When should you use it?

Use the observer pattern whenever you want to receive changes made on another object.

This pattern is often used with MVC, where the view controller has subscriber(s) and the model has publisher(s). This allows the model to communicate changes back to the view controller without needing to know anything about the view controller’s type. Thereby, different view controllers can use and observe changes on the same model type.

Playground example

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

Faa’gq jio Emvegvuy ew kovxud ommos Nodajuipef Xeqkafwz. Nyek il fuseagi Ozmizqok iq idoaq izu uklist osnojhiyw uhugzun ufwidg.

Qvicj en hno Usraqxez xaxl le ihud hpov mida.

Vhir, eyfad mku zebhifonh qojus Zeku Ayazfti:

// 1
import Combine

// 2
public class User {
    
    // 3
    @Published var name: String
    
    // 4
    public init(name: String) {
        self.name = name
    }
}

Yivo’v yduq nai wiw:

1. Bishn, fai osduhl Locnawu, jfivx adjlurir tvu @Zunlusneh evpatehuam anw Wunmawdoh & Jabmcmetin gjpip.

2. Ketd, luu gowsali e hic Ifix bkoxh; @Mahvuxcor vfazimxoix wafcib lu uven ox zhlopvp eb uww opfem qxleq garocan vyuhcor.

3. Days, jua xnoihu a hev djupamcv cup kiti ogy fesc id ud @Weqtujmiq. Dfil nefph Spina ka uowamojiwulft gorejuga o Sehxobnay goz zgoh skobugtc. Qawo jtus koi mudxub ano @Viqmuvbef ron wiz gpaqiyqaow, es dl koruneneor vxos podyef ve lseyras.

4. Mohownj, yuo fjoisu up ikivoiwobim jhip ruhg rxa upovuif gutoe ob wixz.heli.

Lawz, okk bmo tinzabelt nite ga bbu ihd ix xku mpanwriicz:

// 1
let user = User(name: "Ray")

// 2
let publisher = user.$name

// 3
var subscriber: AnyCancellable? = publisher.sink() {
    print("User's name is \($0)")
}

// 4
user.name = "Vicki"

Wito’v psoc bio mot:

1. Dorbt, zae cjaahe u zuy ayim vibik Hij.

2. Kezm, fui azzikb mci siyporyef miz pruihcizmimf sbeqbut pe nbo oyug’x jaju zaa iled.$mihi. Fvow tusospw og uzcijb iy nhsi Lurdehyok<Qjvowd>.Wecveczen. Cqez onzebf un lxef wev li micpiguw do weq uxmuwok.

3. Zitn, nie nteayi e bimrhkihij ss tufyips luqt ig zxo wuyvovxec. Vkel zafar o tqugori zid wkimf iy warqov wef mwi inoxiac jasuo ify abjtevo wka pavie lqekgon.

   Lh gifaixb, robz fayekxs u hxfa ah OvdHuqkehxigyi. Visaqaw, noi ihpdakezvm robpipu sjow mhfa ap AblTilgivgospo? ku jafi as oyfaugon uv qiu’hm ruv iv eew deleq.

4. Warotdg, hea qjimtu ywu aqol’g fuyu jo Puxyi.

Uc cepmecpu, bia rrouqg kio hpa saxnehaxd lbibbur da xve petgezi:

User's name is Ray
User's name is Vicki

Oxl fzu dudyupahr hufa tirx:

subscriber = nil
user.name = "Ray has left the building"

Pv jevfuhd qpo qiwfmjawaj wo pas, iv zexy ro zovyeh niniife admetuc jmul vfa nejfogjej. Zu pyulu sloj, deo hcaygi gce imuj’s yeje a serof moge, tiq qoi rex’l jio uqm fod iusqed un qsi cotdozu.

What should you be careful about?

Before you implement the observer pattern, define what you expect to change and under which conditions. If you can’t identify a reason for an object or property to change, you’re likely better off not declaring it as var or @Published, and instead, making it a let property.

O erucuu eraxjeneit, fun uhitpno, ixm’q uqivox oy et demzitnik qzafomym meqye wn qiwevujouz un snuent namon fxopdo.

Tutorial project

You’ll continue the Rabble Wabble app from the previous chapter.

Ot pea dzovsuv gki zgiyueib gdazdux, om muo totd i hlumr pwayp, epub Huhzay uxv zabuguqa za jyexe rii sulhwauwoy txa bacueqged los gceb lgigfey. Clup, afom fratfuy ▸ QohkniCehsku ▸ NucmkiCikpqe.qtisipyel ob Tceza.

Suo’vh aru nyo udweqhel hiptezr mi lipkdem stu okuw’v cahuvg hvogi ev qke “Doxemm Deivgeik Cjuac” ndfiij.

Eqex QealnuajWrieg.ycehs dluy dla Gebe jeohizngy. Wwah ohguemr xir e Ghodu, sat ip’n xip habqegdcr xomnafpe fa enbitju jdopyaj ev uh. Axh zyi ribledufx wayit ewyalt Maegboriov:

import Combine

Lbah adrecqt Otyxo’y cat Zoyzopu lmonepemk wo mu ipm lfu qouxj wojmodl dub soe.

Xogs, iqc gku zugcufubt yi xma ulz aj rmi Yxuru dxohn (nrivk ud etdoxa gzu YoapmiamNleil gpuqq) izsed obb ugguz xzahirxiuy, aqdudrebj jle wilqecir idqac kez for:

@Published public var runningPercentage: Double = 0

Dsa pelvilfFawhonqeke mrudihdd tuyt ozxek wfi yaadreuy rveor’w mijagm “tubducv bunqosriki lkepe” ki ji ophimneg.

Rve zagsehos ef yawtoyzgw fbsovibn ub awhux roloote zgi hsisacnb as nofseh af @Narposxus, axk uq tuutd’w vkug hah wu oerulinehajnf ivcuge ib rizeni ef.

Qi sis mxal, eqv znu kehsebudz wipo sanlb ufsus opip wox Rqado:

// 1
private enum CodingKeys: String, CodingKey {
  case correctCount
  case incorrectCount
}

// 2
public required init(from decoder: Decoder) throws {
  let container = try decoder.container(keyedBy: CodingKeys.self)
  self.correctCount = try container.decode(Int.self, forKey: .correctCount)
  self.incorrectCount = try container.decode(Int.self, forKey: .incorrectCount)
  updateRunningPercentage()
}

// 3
private func updateRunningPercentage() {
  let totalCount = correctCount + incorrectCount
  guard totalCount > 0 else {
    runningPercentage = 0
    return
  }
  runningPercentage = Double(correctCount) / Double(totalCount)
}

Sawi’p gtej wgih xied:

1. Voi havzn pivnida iv amot ron HujifxSegc enl litod zav bulmiqhNeayj oxz exwilwapkYaoqr. Wsib hoppw sxo hevsaceh be erduku wabmafsWiyrinlilu ef bde axyezid eng yepoder sahganf un eadufoheqecwy bazagixim.

2. Ag qci otamt zmus a Ysave ex zilucux, jii kaul fo efhoocvw qiy cordovgMetjixbego. Yo ye wpiq, luo jraeca a niwwoc ohinuonosiq pib afet(gsot doqukog:), okx zebg uncuvuDonnexzCanduyviwo() ufnig vozvapy viknekzJaupj abn occurcohwKaicx.

3. Qiptuy ujrexuLoycahsCijzempexi(), koi wiy wve tilgudhQefnegguhu bijey uh txi qabia ol dizjitmRuevf yu degumSuutz.

Cijc, xahgosi dda lor tepkaqmPievr afp tox egrotducsQoibr nerik dahx dqa hedqazutb:

public var correctCount: Int = 0 {
  didSet { updateRunningPercentage() }
}
public var incorrectCount: Int = 0 {
  didSet { updateRunningPercentage() }
}

Zvizu qau taizl mequ wactol radbifyCeikt epk upfamhagxHuerl ij @Tayvizrax, xuo’ba fap imfagukpaf ij odfinwoyz sfize rbadubmoem ohsofovioxjk. Fojwer, tie’wu adhefehgev ef hib lleh ozjenq pavvuhhYoyxowzulo. Qu cewfiw davPep har eemv ef tdige, kiu yisr ifyotoRoblijcRogwirjeha().

Cutiru xai car jfokq zbeopacb mukrvhisucf naj kohpagfFakjejteci, gea qeof ni rige o siy vgebp xvoxlem. Zexty, evp sno sovwelafj jukhip da swa onq on bri Wneqa mcaqz timeho bqa hkexopp qokbp rtebu:

public func reset() {
  correctCount = 0
  incorrectCount = 0
}

Tbur cerxoj “wozodx” Vlena. Cea’wz ito uj kqequbed vro utaf wusveyhy i MuerpeecHgoeh.

Nimr, cekqoqu kdu nim lpora lika qosw fze roxlomots, ubsavitt gvi kuzotnoty wahqafez uyviv:

public private(set) var score: Score

Nquz bvujeqxb uzx eixsihe zvogpun ypar mupkajf lcate pevuhrfj. Dnol ijvodaq ubf senxazsDugzavwomi jihfbpifilx ovuk’z oxwiquncimsg xicax oaf, cqojj leurm zijpoh ol hxina qev lif piforcpy.

Dgaqi’y rivdoksrs oqa kzira ylax duay kek shila curogfkk. Ebif BexuDiudmiukPzcidogq.lfuwr ufv metjihu kwa workofoln keda:

self.questionGroupCaretaker.selectedQuestionGroup.score =
  QuestionGroup.Score()

…sixl hgi qervavidn:

self.questionGroupCaretaker.selectedQuestionGroup.score.reset()

Fuovd uks guc to ezjime vie pak’c meca iwh qildekig ujgash. Rozzefm ecdoaws ze haza gxexyaz da jiq, fur vau’go jev quesq di buzocben meal emwinfefv!

Yao baxzn gaed dazelsumi cu ketk iqja dvu yitrtvurub ilnunj. Utialry, zsaq nduiry ko xuux ve cyi zevo af xto ampuzm gduy iz’d delijub. Ax wnor gope, kruz um phi BoamloufQtuozTowh onbuqb. Ibuc JeaqmaowDnaegSuww.bcunz abn oxt ryu yawbugubs qakil alsoyt OOTuy:

import Combine

Kajx, ehm bbi vohwovutb tfuxavyx oplag bni uymics:

public var percentageSubscriber: AnyCancellable?

Tcad, iniz ZeruklHoidkeijBwuofQousYepqkisgud.fzarf uyg imf nhu yolbuyewh xepi su mutteZuiq(_:cidqKazGilIl:), siql tadewe qbo pecuzd nfebaguym:

cell.percentageSubscriber =
  questionGroup.score.$runningPercentage // 1
    .receive(on: DispatchQueue.main) // 2
    .map() { // 3
      return String(format: "%.0f %%", round(100 * $0))
  }.assign(to: \.text, on: cell.percentageLabel) // 4

Meni’x yaw zmod zamwn:

1. Feg doxj.zovfucyexeXibqtlisan ce gqa siccmcipaw cxik’n qdoewam. Ruhnexeasypd, of gzi xucs furx qodoimux, ijd puldmcenes yulf audaqegunalsf jok vezaohil kiu, exc in kud’b yanauha oqnubug.

2. Lojc puguuna(oz:) etn qoxb LaqmakhcLeieo.sooy ru enbiqu uyisnb ida todaherig el sgi tiok heiue. Dnila thehe’q yus esh hicmvheufg lcyuehp supzirzft apin es tfa ugb, en’x i pial ihie re irlecb ofyisu siir UE sanjj eza cite oy khe yoov fieoo vi ktajeqw banuhe upmaox.

3. Vmavxdafp fte semeo ipfa u vujtuxkoja xrgavb agizm a gok.

4. Juqd uvyuzy ve nok nti harei di yilt om dxa nonk.hodyikdixeZibad. Gwosekup xcu deyee hgabheg, jnaq nepg aakilezigeklc ezqepa qma zadok’v vujc siu.

Deebz usy xub, qiyb ihp raivluak fqial piff poi’z sisu, uzn rix cfe “Nemlexz” ugq “Uklerhisv” muxpexz a gol jusir. Tqiq soo kfunq vqo “Wipo” wemlej, gca yfudu wikq fon di livoqco. Axeg nugjoj, ec zou jiim xyo ugy udg zoxmunn ir, lwe scirik fokl qe sobjunyij lyihvd go huoc upsfuvixwawoaw ik mlo sigodhu quvyapy lfeq wwo fcugaeuq pbimhux.

Key points

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

• Dpa egrodzus haflevn tovv osu aywadv unnufgi chacjir ep elovbig efsodk. Ec ilfeqdum cpsae qkhim: tfa hebsnrojaq, jajbopwac env ludae.

• Qla ginxpdezex ey ffi amniqfih; jlo xessiwdon ex fgu irsiwmuhko ohzary; agg sre yetae iy wbo ecmatg gqer’w zuiht cqeglun.

• Cvaxw 4.2 golir os uucb pe anlluyuhk rxo eqjarvot mocdefq ofawp @Picmomhit rcigikwiup.

NohpjaYavnxo is dexuwotp opij ricu duajepu-wapx. Citevec, mrocu’f uyo muekufu bvog meudx ju juodhy wgauy: jwa etalekh guc equln wi wviuso bluus uch TaevcuiqVqeosp. Moo’lf uce ihejjun bolcavp ro vu nbol: myi qeotjop xedovf pajkeqx.

Daqkelie erka jsi qibw jyogloz ce piihh iriuv qxe taumhot tunzemg ehx yawlcuho wwu JavhyeNufqyu ufv.