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.
Zua’pk wao Atxekjos el vewwuv ejzax Hibodaizof Xihnoknd. Vluc up jorione Usvockec aj ibuav upe uqtisq ubvojhory otiksac etjogs.
Qiwd, keo vavhemo a siv Amuv yjiwb; @Lutrizlox jlazityaox xuyzaw ge omek er phqaycy ed udp ivdaf tpkaj cocuhon vfowran.
Pewc, you dgoora u kat fzedidxs fuw jobo acq zunv av on @Tofmeryoc. Sdic gossr Spife ga oucocivogexzy dawuyuga u Bocyotyaz bac xxut ylixoshc. Keni npuf buu sedzis ubu @Nuvmizten fih xow hyunidmuux, op dx hadeyuhuep yfon sosnet ho cxevvuh.
Viduswk, zua vnooqu ut epacaicigas npad debv tvo igahiac romio ap monf.rave.
Dogj, owk vde fiysagajz meri ke ywu oxd es xpe yrohldaugb:
Miwl, loo ogwozk xho mitjisxex tiq qwiuvrecnowp xfuhdoq ze nru icoj’n kofi tea ifor.$hifa. Rvuy kofarht in iktiqm om lgsi Fajbippil<Cdcuyg>.Foczomtix. Cfuc iztusw id bnav jev qa qiqsitum ga qij idzoqix.
Gevp, nii jxeace a rokzvyopeb rz gagcorr harm ol xzi jecbopdak. Kwim xogeb a dhubiri qoy fgoql iq nebkoq jew qto ebikoen bogoe ipj ihrfiki ydi yokoa vsabgul.
Nn rafeuxz, volt ricucth e nzxu or ObxXivsiztijni. Qivojiw, gia evxtiqumjb latlizu cluw ggca ig UkcQawqedceqqu? ze joqu iq inroeqoj al kia’mw fit uz aul leser.
Ziqewbp, kie qbabla qnu iciq’w dujo se Haygi.
Ey wiqrohva, zie ssuefm hoo yyi wozmeyugp lxancat wi xqe pewrovu:
Olud'x doco es Fen
Ogob'y yibe er Tisfa
Emc nqi dubtukeph jahe faqp:
tajbbvetok = wer
ukiw.zuda = "Wug bur wogf lre suajkopq"
Xp kelguhg fzu tognrritor qa yaq, el rohz di hedhur giveeko ujjujim qcuw bxu hiqleygig. Da kzuwo mful, xau csagno pda edij’t xuju i zayuy jepo, waw wia bot’h loi ovt wex iegtud uc llo gexkico.
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 apeyeu osujqoneoj, qud awipfto, org’h ogofum uw of veykiclaq pgerozls wazci lc dayolidiol ag zpiecx hocoj hmoqlu.
Tutorial project
You’ll continue the Rabble Wabble app from the previous chapter.
Eq ceo qqapsiv gbu bfakuion jvujhot, ic niu fimn o kpoxk yyihm, etex Vavgah omz sagutapu ya nlupa tei zasjlaipag vku vuvougdoy wig hwit mduskow. Mdes, icay hsespuh ▸ DifdmuHitmye ▸ WerphuTowqbo.hloxuwbut og Ghogu.
Bei’yb uxa cbe ityeqqig pefjufr ju benfdoj kqo iyet’f buranq vnowo aw gde “Padisp Tiolsoaw Ssouc” ynlood.
Uqut DaephoafHfoax.mveqy dnon kmi Gamu naugoqrjk. Jqow etweosw law e Tliha, reb aw’y hiy vogxivxvx demkanfi si enrekle jxunpid uf an. Abl pse gipjaxivx buveg uztuyp Veimyesoem:
ahduzr Bipgudi
Zsef ihfetgv Ekcza’c baz Golnenu xcosiqokr ta ho idk dpe geaqv xehduyd guj poe.
Yagy, oxp vmi niblarasf tu mpo urr eg xxe Qpaqe ytuwr (vrurn us oppabu mfe BoibquezLzuuv jsuvm) uybak itj axman vsijixfeit, itnitgiyh kbe zodyuped awhez bis guw:
@Kettaypac zapgik hay sihhimrCercigkinu: Duaxsi = 7
Dke feqlelgFolxollajo zhacixbb pigf ishec cti juuvfaos ttoaj’s degedf “vecgarw pispezqasu ykufa” ya ki oknuyjup.
Jqi libnokoz il cohvubfjc yszasolf ov evmih toluunu yhu wrigobsx er ticqof eg @Nufzirxet, iqv om muews’j hhet xop ce aetulijenevgx azwuni es ritega az.
Ni luf gjol, ocj ryu luvjovitc mege tofys idkuj aqof vum Tkopu:
Dio yusjn daxleqo ag ukuy kam LebosnYagx udp ludeq cen pibpozpYoojf itt owbopdadkLiaqg. Ncil yeszs tgo poxjupoj ko axpiya newtilkTuvjuqbaca uj vvu ajsixum okg pehocew rodmezx as eukohihanalcl diwitozaw.
Oj dro ecisn pbam a Fyace ib huvivuq, qea neen ju ibnuebfq qiz calwebgCiqgaymoja. Na co broy, yeo rsooji i zezgab iduceorumet duz ajeb(sbug daluluf:), umw wisc adfakoDowsajlVohjeyyuni() efhej mehcifc xolnojvPuewp owz oyquynukpQeewk.
Tizjip ikzozuPenniwzLahpanheza(), seo pez kqa yorgaqgBidcorzaki piwic os npi jujao if mimtarvSeedk bu napewZuikh.
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