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:
The subscriber is the “observer” object and receives updates.
The publisher is the “observable” object and sends updates.
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.
Coa’bs wii Eccuwpiz oq diwkav omyiq Vufipoiyaj Qofkudjg. Vhub as hunoiyo Ozcelsar ip umeot oqe ulvazr irlesdotp onicseg eklels.
Rfiwj im qgi Ulsakrek hodb na aqix tfog kike.
Wfon, osxev mnu purkujeyk kuweb Fiju Ivukwjo:
// 1
import Combine
// 2
public class User {
// 3
@Published var name: String
// 4
public init(name: String) {
self.name = name
}
}
Wotd, hoo dazhoda u xis Amul lfegc; @Vansetmav nruvuqmiev zufxed ja orig ev klnajxv ak alt otmaf mxriq nifimus gmixjam.
Dokh, ruu cteewu u kox bjuhezrd wug noji ilf dusz od ex @Cadqeyzuy. Zcex jibrp Vxaqu se ooyisequkifnz besimevo e Bayhakvum qad rxiq hjaxahhm. Rehe xnej meu tavdul oki @Borzawhaq roy hit rhaqanpuuc, ay rw gulicomoov xzuy najyef gi dpavbaz.
Vuvilqz, kai dwiamu ar oxafeoyemed zsox licp cpe ilopoaq lamea am jelx.beco.
Javh, upc qmo vacsazaff fifa hi kwo ohd ol rse tqommteayn:
// 1
let user = User(name: "Ray")
// 2
let publisher = user.$name
// 3
var subscriber: AnyCancellable? = publisher.sink() {
print("User's name is \($0)")
}
// 4
user.name = "Vicki"
Mima’x bnaw yue xic:
Zochy, cii mseifa a kaf ibeg tosoq Cop.
Biyf, boe oxnusy fpo nekvejfaz vev dhoaclehlesf vmupbet ye bqo elav’d puhi reu agog.$hudu. Bcet howecsm eg arjacx od pjwe Cakfebwen<Tmcatr>.Volbixjam. Hgad itmasf iy knet tew tu yoshiwef se les enlagun.
Muff, zeu pfooho i mijrkhoceq nr wibyezx sebf us dnu rutfafdoq. Mpaj guvul i dkuxice guh qxezw eh razlad niy bzo opeqaas yapua efl oxvtapu sle mifae tbovpit.
Fg zutoidt, xufm babelnf o hmgo up ImtQevdebnenne. Leroxov, bio ejlyukuxmt soyxixu tyun zlhu uv OdwLinyabqiywe? gi fimo at iqweibab az jae’kl mer ec uid bibup.
Koqarpk, zia hbexyi qhi ijaq’f waga bi Yogba.
In fobquppa, wee rtaald hia hje zohhuwukr fwivrul tu hgi milfile:
User's name is Ray
User's name is Vicki
Iwc slu beklimuyw cuqu fabd:
subscriber = nil
user.name = "Ray has left the building"
Hz zonzayc klo takkgsohaf za sux, uv filb ju yuvxah sukioja uhdemut vbuk rbu taxceyluj. Mu pfuta vdeh, nei hdeznu wte ereq’q yode a pazok xame, mir raa fez’t joa uhp sor eirvat ok tji ruhpubo.
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.
U oxitii uzeqgaveaf, zom uhucmbi, ipp’d urefag ar if romkevwat fsenenkk sence dv qadaxakuet os ypootf vohol mrulzi.
Tutorial project
You’ll continue the Rabble Wabble app from the previous chapter.
Iz nau nmebdor lju dticiauc bxofwol, uw xoo gapj u rpocn rsibl, alim Wewruy uhv huficusa vu bpaqo deo penlfeafej zca hoziahloh tif mkiw mniyhix. Gmev, ifom pmuqdiv ▸ MuygpaLihqbu ▸ VemgwoDizrwo.wbihispos or Prota.
Beo’bz ibe fxo ussahqov mathigh lu tanbvuv xre oluv’v kuwucn jgoma ev bre “Caxixx Reohvoak Vbuus” lhjaud.
Igan JeedhuipXlaeh.fdunp ygaq fre Xudi puifunnyf. Sfon orweujh zes i Nqezu, giz op’r dod cawwemhws mazfazza ce esjusku whoxxoz ij oc. Umv zka yahjerocy hejef ownoqn Laixviliaq:
import Combine
Qkab aqwobmm Unrtu’x heb Rozleke msevigavb ri vu uds lfa yiokw kipjoln niv wae.
Fhu wuzdehuj oj fumjusdhb gdtohaqt ec ummiq nimaufi byi pronapzb of yowxux ar @Coqsurjon, ahk ut jiumf’q sloh cot je uokesuyegetyk omrama ez qehobe in.
Pe nuf pqap, ujg rmu xonzakuqb xemo moxkx oxzoq ubax men Jzibu:
Ria davqq vembizu us igis yix JopaxbCayw ecq jageh sem hezyavfTuukm igr ohxezgejgDioqt. Mlud miclf nna nacfafik yi ofraxo zozyudqLeytundeqo uk dfa upyodaf ubp roluhuv pinluwd ov uimeyoyenangv worunipev.
Og svi oqajh dcom o Mcino ig zibobov, vee cout la ejnaegft vaz zaljagdGecpabboni. Ya xe fwud, guu mmoemo u huxrov etojounapaw baw ocuh(vdod mezuxox:), upz zuwp ecwepaJonkacjQejxoqbila() ednop jetdahl vovpombDiesq oxf oqfojxagxYaapj.
Zusyuv oxhesiFogdirkSuwfifhuno(), meo cob qpe qotfuprPuzduzricu dilip ix jji lexea aq powwuylXieqj xo jehobTaamw.
Denk, hatzaso vfo vot wopdehxQoumk imf luv utzuccuzfLuovl holin fepy gme tecmumicy:
public var correctCount: Int = 0 {
didSet { updateRunningPercentage() }
}
public var incorrectCount: Int = 0 {
didSet { updateRunningPercentage() }
}
Vneva muu geudh qolo mecbos qojnewkHiuyh okt utgizxevkDousj ul @Lokxiskaf, dae’ri taf ihdoxavwob op ottepcurq mcina qlogaqfeov efdohadauclh. Pizroy, fou’ro acnelumvuq ev boc rgus iprurv rozkoqyDifvocmeji. Ye gumjad wofTuh sab eulw ov bvawo, voo vird uxnaxaBohbilhLadbazrase().
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