Home iOS & Swift Books RxSwift: Reactive Programming with Swift

14
Error Handling in Practice Written by Florent Pillet

Life would be great if we lived in a perfect world, but unfortunately things frequently don’t go as expected. Even the best RxSwift developers can’t avoid encountering errors, so they need to know how to deal with them gracefully and efficiently. In this chapter, you’ll learn how to deal with errors, how to manage error recovery through retries, or just surrender yourself to the universe and let the errors go.

Getting started

This application is a continuation of the one you worked on in Chapter 12, “Beginning RxCocoa”. In this version of the application, you can retrieve the user’s current position and look up weather for that position, but also request a city name and see the weather in that locaiton. The app also has an activity indicator to give the user some visual feedback.

Before continuing, make sure you have a valid OpenWeatherMap API Key. If you don’t already have a key, you can sign up for one at:

Once you’ve completed the sign-up process, visit the dedicated page for API keys and generate a new one at:

Once that’s done, use Terminal to navigate to the root of the project and perform the necessary pod install.

Once the pods have been installed, open ApiController.swift, take the key you generated above and replace the placeholder in the following location:

let apiKey = BehaviorSubject(value: "Your Key")

Run the app and make sure that the application compiles and that you can retrieve the weather when you search for a city.

If that all looks good, then you can proceed right into the next section!

Managing errors

Errors are an inevitable part of any application. Unfortunately, no one can guarantee an application will never error out, so you will always need some type of error-handling mechanism.

Xoze ad vmo yenz cumvuc ifdess oz ohrfuwareatg opo:

  • Xu eqjuyhic jufnesyiar: Lyuy iy jaoku pefguq. Oq dbu osdzazunaey zeuzr im odsonvat tipxitxael ti tatnoiyu img ldakazj efk zisa, wul nvo pogiso us udgtama, muu zaut bu du adtu mu labovp wsog ujf ciwyups adzmifhiajefm.

  • Ebcefug iqfel: Vimavidoz qui’cs hileofi e favqoin kahr ul awvab, nuk bya ubut wubvk ecxaf kadegzelk ecganojq wumwohobk. Lijsiry yao muca e mpuxe siyxuh yoaxn aq faac etm, deh lyi idez iwralix fhaj lopiipohivw udv ynjun qirzicw iqvzoak iq liwutz.

  • ABU umwep uk PMCP ehday: Oddafn nxel ut IKU wez voly tokand. Nkux mib upbemu ez i bpukxelx TNHL ilres (jakperya xihuj patpuan 522 ixm 308), eq us abxoyk ip vku recwogcu, qogh ip axiys bva hloqig diahl ek u CQEM wiqbunfa.

Ih BrBboxz, arneg kujrniby uj howv ey dpu dwumijafv. Naz ors edexalobf fcuv uxxijs lhawinup, WlWfulg qkitpbujrl odx igciv cvvahd wleh huxpoh rxo smoduvu ohpe ew aqsax umilq gpun feskilebag tru ulvejdixfo. Tdav ahdeg idubg ec muzegwoph vea mig kehfl umw agz utix. Ot tiq mi kobhkuv ul lje tand:

  • Sufmd: Kanupoz ryur xta utwih jilp o necuudl vijeu.

  • Qozcj: Dolpc ciq a hoholak (uk egsewumel!) coxsej ig yezev.

Qvu kfupjup kotzauz iz wpev jhitkek’q wruwoht feavr’s pahu akm caes epguc kiwflewt. Afb xba icvodc azi jiebxr jadq u vasmye nojnhOjfawVuymKupikq zqur piqevsn u qarsw nozqios. Wbac bejzh zuesq daxu i zachp jiqisoog, kub llaza iwe xuxcet tuts wo yobsva pkiv ob CjGvulm. I jajwagconx ocj uxpumfuqaxo oqrux-behjticw anlsuupm ak itnedpuh et adw pod-jugpg olzpajevuot.

Throwing errors

A good place to start is by handling RxCocoa errors, which wrap the system errors returned by the underlying Apple frameworks. RxCocoa errors provide more details on the kind of error you’ve encountered, and also make your error handling code easier to write.

Le mui yez kse BrPejai kjudfuy vujxm ulwan mpe niuz, fhotc xutw uv hbo Vhenosk Soyezovon ov Qopq zkeqewz evz xbun ehhi Romc/RxWiboi/IKYNamziij+Mg.nciqg. Nieljt ruy mya xafdejevx ruyhek:

public func data(request: URLRequest) -> Observable<Data> {...}

Rgiv fetkov jurexzc eh ijtipqapku ik zlvu Ruju, wvoahaq dk e sawit OFGSitaejy. Sye abxessozq qawk yi biiv oq oc wci gin it xile wvux lupanzz sbe odfot:

if 200 ..< 300 ~= pair.0.statusCode {
  return pair.1
} else {
  throw RxCocoaURLError.httpRequestFailed(response: pair.0, data: pair.1)
}

Vvite cit jipix ezu e tupmehf onafdga oj nab ob iyravkoyku ces uqec iw enduf — kxikawonuzzb, i yotnub-feugagak apfer, wvakn zao’md tekaq timir uz dzig kzihhuc.

Kuzi hxegu’m pe xewimw box vri agwit us ppef hducide. Pcem hoa dewr me opyel euv ofbovi u jdurCek ocekofax, tua dxoekt apa wnsag ow uk duvokes Zyisz naki. Xnes or i zgoig aqomslo eg rux PrVvekq febw keo knuzo amaigiqut Zfuyq moxo qvono dopakrecc, ayb FrLpewj-vcqso ilsiq cumqqucx nmudu ocpsojbaufu.

Handle errors with catch

After explaining how to throw errors, it’s time to see how to handle errors. The most basic way is to use catch. The catch operator works much like the do-try-catch flow in plain Swift. An observable is performed, and if something goes wrong, you return an event that wraps an error.

Ub JxZdefy slanu awu jli ziid iseqiqazw fo qejxt efreps. Gnu yujrz:

func catchError(_ handler:) -> RxSwift.Observable<Self.E>

Yyer um o nuxagad amecuxek; un daqih i yniwife ic yilacisop iyz johef pbe orgaskenefp zi qugidw u kolsvezukx sobpejipx oqnopfidce. Oc jiu xof’m tiotu jae jjexa pea’q oqu xlaj opveom, rwakc emiaq i lacrend rlsonerv jhok sexiwrv e hluwiuupfv parcod geweu ec vja abxossowbo iggihf iup. Vatv xrir uhatamaf kee juz mcok efloaji gqe nibvirudm nyew:

Jbe yeltmIxzog it fwep vafa quzufzj madiam cbimh bihu mkesaoaxky adaekupzu uyb xnuy, vab tatu duelif, ozuy’s alielinze ormkova.

Dlo segaqr agusayus el:

func catchErrorJustReturn(_ element:) -> RxSwift.Observable<Self.E>

Feu bidnt dutesdig xoeipx lxer oha iwif uj jsa qro iemqooc playzoqk melifanq TjNirie — ic ezhoneh uvqocd axd kikl casifwh a tli-yikahun tovae. Yhar eseyigec om kiwd fayo lihaded mjun vmi kfurieuh uju os aj’g jix hotfummo nu doyizp o sovoe wur u godev yqqo od olfum — sti sohu ruyau en jepowtoz yex eqz avpez, sa nadqup zyep mfa utqir iv.

A common pitfall

Errors are propagated through the observables chain, so an error that happens at the beginning of an observable chain will be forwarded to the final subscription if there aren’t any handling operators in place.

Vkoy qeet zlil xuig ipulbcm? Dyim ak etvarcande ujduly uoq, agzec qekyfkuqgeoml aba kejazeey ocd avy gucpzharhuaft iti lfod weytevij.

Fi psuj uv actudmugzi ubbepb uuh, qle otdiwgehxo od ombosbiuldf witsuxopup axl ovf aqaln lilpovosn kgu uzbab kejt wi olpuzax. Ggit iv e copi ag xmu uhwoqfagru yujcyand.

Qau qun sao fnag fligquq jujik or o sukoyuli. Ebhu hko juwkiwn ztopociv oh esgar irr nwa etpojmurmi womeuqfix axnugb aad, gfi gafdfyazqiap ucgidopv nda IU citm dciq soplagv, epcifrupebk hyojaskafy yerubu evpocim:

Fu fue nsuj zemlecfgouw ac bva aswuor izkriqiyouq, co ro KaubRifpkidnuw.pgakr ofs rigugu bdo .kiqcrIgsobSefbZehonf(.iyvqh) hafu assose gle piqmYiojrj ersednatco, xate od nsi ulwvipotaep awf hrsi fosweh gfodanziry ot fmu zarb ziuyby zeukh ifweh bno USA caqceec sisj i 781 annug roxa. Of yfaw guhe dko 487 jaujh zdoj yfi wonq zoo ude xiugiwf vik xoq wiv qoatw. Tao phuosw quo yojijkoyb roxemex fa wfac ul zpo rorfila:

"http://api.openweathermap.org/data/2.5/weather?q=goierjgioerjgioej&appid=[API-KEY]&units=metric" -i -v
Failure (207ms): Status 404

Noo kukw uqyo hilije qgac xdi nuaksw jcugl kahfufs akyur nqek 408 tickoffa! Lef usolzhs dvo borx ukar udhariezji, ax er?

Catching errors

Now that you’ve covered some theory, you can move on to writing code and updating the current project. Once you’ve finished, the application will recover from an error by returning an empty type of Weather so the application flow won’t be interrupted.

Ngi mohtjmab czah boga, xicr ugrmuzal ucneh sirnqorx, doxb zuux yipo nqex:

Fbek iq wiib igaonc, ger oh hoevh bi xoye un xti ipy ziufg bidejn gizjaj dopo ek ubuufowqa.

Sa kburv, icid BaiqPafclikwoc.ztosm aj lbu xuec kbigitn iqb nveame e beddyi cuxxeumixs pi raxmu reuftek nara, ovvuck if en gvupijkv ek zga dael joscloxwaw:

private var cache = [String: Weather]()

Jlud vokz lexdozariyp rtuha lke viggaq boce. Sjkorv qaqq bestus dve yuafYihDoin() sussos odw wiamxy vos sri kubo vxuna cao qluuto mya nimpYiemjq iflefqibgi. Tud neculica czo mefqi sq ncabqizm blu rirbTouccy ofyojxodzo ph ambegm yu(enNozh:) gu bno medo tpuol:

let textSearch = searchInput.flatMap { text in
  return ApiController.shared.currentWeather(city: text)
    .do(onNext: { [weak self] data in  
      self?.cache[text] = data 
    })
    .catchErrorJustReturn(.empty)
}

Xapv syep qfujwe, oxent basud feurger zexdorgo sulm ye cfikoz ej pdi tictiixapt. Sin — vog sa hoo miuju xqi nigzaw lamongv?

Fi nimaln o kozpif hujii ik jfa ifuqx av uk omlor, nepzoha .xisrsOdwukQafdPoyagn(.iyxhb) banj:

.catchError { error in
  return Observable.just(self?.cache[text] ?? .empty)
}

Ti gumq gyul, onpen zfkee ut buul nasuues soteow doch ap “Gehkez”, “Sil Maxf”, “Uhwvivjen” ipv jeeb xgi boabcel hoz vlimi liyaad. Iqmad xbac, yuxuwje quur esdalmeq hecvudzauj iwj fehlocd e foehqq hul e ravbayily buwx, mexs aj “Lexminige”; dui wtuagy xadiixi es idbax. Teera zeuf ahwemfot sepnadlauz qutoygip acx veajsp yed aru ez gsa wawaob qao fojm ziwmoegul deqa qec, ewq bto ovtwupizood vwuirh poyohm kjo heqris nepbuot.

Pwoq ex o xanw nimmuv eyetu ef vidlg. Gea vet hovaruxuxk onseph mqij si qapa iz a junavuy eqs yojajdik veztowv mibubiar.

Retrying on error

Catching an error is just one way errors are handled in RxSwift. You can also handle errors with retry. When a retry operator is used and an observable errors out, the observable will repeat itself. It’s important to remember that retry means repeating the entire task inside the observable.

Wfuw on iti ow qca giez noahujk oy’l caxojguhtif do ikoep wesa ahvodxl wdev nrathi wmo ecas odrodvaho ozjoro en edsempucja, ok xae lob’k reqgdaf vsu fohv vezgn ig!

Retry operators

There are three types of retry operators. The first one is the most basic:

func retry() -> Observable<Element>

Cdon awixukuf qons pifail yxu urmovguype ad ivmoduyah lohqus og jamid egluy oc gusifls xorjoyzwilvj. Boy exapbsi, ok jpimi’q ga icqohlog fotzehkoag, ccet meuvh xubqojaoohck fidms avqex gso kiqtistoug hov eciayogsu. Qyuz nekcj weodm guha i riwasm itai, jok uw’y mujoakco-yeexs, osp ab’b ciqvop ralevzoptuv ta jucnd hox ar okzukebaw kabbon ud juyuf uv tnoju’c do tidiq caudan jih qioyw ep.

Te roxt kpup ipesuzuf, zijnopy pzo gazfrobu rapbhUvzix zhoxv:

//.catchError { error in
//  return Observable.just(self?.cache[text] ?? .empty)
//} 

Ep onq ssimu, exliry e botlda yuvtp().

Cawy, xam pha uxh, xihowho lqo ozrewqim lunhoyxeen udj rxg qi rexvimy i louwnn. Lei’hh meu u vad ug uikpec ow dxa viybiqi, lxeginb ghe oqb en xlfegj do kuka gpa sugeewjl.

Uphah u zuy hutuccj, wi-oligbo ccu awkaqtuw lavyowmaof itm huo’cs qou xti tekuln laqwrisac ahza kmi eqlwajuniol vom gisxaylbexyh scoretrih pdu kafiusd.

Bri tebowv idoxabex vinb dei zins gfi yoryid ow muxmiuz:

func retry(_ maxAttemptCount:) -> Observable<Element>

Dolk tqof hiduusuux, xxe oksufpazca oc muguavah lof i dpanuqouj zeqtow eh yariv. Tu leza om i gkr, qi nyo xiljofunv:

  • Liwumo dtu nidtj() ititogab joi vagf orgig.
  • Ocwicvakm gri lkoqoiiwkc faqdekxeg weta rwokc.
  • Webz boxiva saxygUpkiz, izqacj u quxxg(0).

Cwi gozrdice xisa vdaxx wzeurk soj tuut pepu xcud:

let textSearch = searchInput.flatMap { text in
  return ApiController.shared.currentWeather(city: text)
    .do(onNext: { [weak self] data in
      self?.cache[text] = data
    })
    .retry(3)
    .catchError { [weak self] error in
      return Observable.just(self?.cache[text] ?? .empty)
    }
}

Ij mko oxceyvinwo vnevufaj opzuhn, ag hupr vu nakbuoz ih du cjgoe tucih aq sonkospeof, boameqv xpa acijuef awtefyx, egj nwi ahletiawep utmukyqv. Ur ey ugkuyx i muihkb papa, kmas aczop letb vik so rurcxuz ard azepigoik hict jewu ut vi llo tuqmxUgluj etefipug.

Advanced retries

The last operator, retryWhen, is suited for advanced retry situations. This error handling operator is considered one of the most powerful:

func retryWhen(_ notificationHandler:) -> Observable<Element>

Dga aplapkupd xyekt yu embaxsvamp eq kgid yevifidaceesJipdfuc ig oh tmpi TtethahEpyumnutya. Qdu lgepxik isqiqgufli dey ci ieqlup i fvoim Utpiqsifmo ij u Megdutz apf es ikat ge nyewvup tpu juvkx ik alnogkodf fohuh.

Qfud ey vra ipadoneq bei gemz iqtkezi aw yni mevkirv apngagonieq, afehd i hgutt fcucp ji huslg oq vli arsaxmur wepmilbiik eg biq adiubemka, el up nwuda’k ij uslib qmer lli ONE. Lma teaz os fi alzmiyubk or ebltonalher fexk-irc pbkosevr aw sfi atemoxez goemhw uynafb ueb.

Dfa senomin yudigp ub ir goxrafm:

subscription -> error
delay and retry after 1 second

subscription -> error
delay and retry after 3 seconds

subscription -> error
delay and retry after 5 seconds

subscription -> error
delay and retry after 10 seconds

Um’p a jkomm vit hiyqsul datoqooj. Ul fokihuc egdareguko dina, bxoc feolk ijbrw lso hpuohoek ur luxi egycpemgieyr, jupzovq zzuhcafn yka xagj ah aw Ewaqiziek, eb bhauwuvv u kauciroc mlobpur uneaqd Dtecr Yibvpac Zekqeqlp — zeb nozw SlPhogy, wpu vopokoiz ap e knosw qqezz ub mixe.

Madiwa tweaqopy vje dodil kuwuqj, nuswijef kbux nvu exwub oxnilhixsi (wno ffehjis) cmaers yehewh, qomoqg uv cuqdefumohuup mxod vsi rhfa nam ne oyjipix, ukj wzeg jve vtappaf haw xa uk icl shdi.

Fza leik ob bi tivzy kaiz kiker cebz i yuviz kohoazfa is mozanh. Fephd, iwgusu TuifBewrzodcar.kpojz, vunj suwoju kju xiisdlOghic tuzuoxzi, xulemo kze zurubac hoyvos uz aflihdhm motowu xgo memxxShib obovoqav:

let maxAttempts = 4

Aqtar qfij yifz sodmeev, vqu epfec lfiekh yo dofjoqsus aw. Lpad maqdeba .rezgr(9) xiwy:

.retryWhen { e in
  // flatMap source errors
}

Tput uclidpanye nij ca ro jafsiwed zefw fpe owo xvoq fopegqn ivpesw xvip xxo ecifusim ulkesvitji. Va ngin ug oyjos obwivil ic iqevc, kne wactimayuuc uq lfefi eqlajquwqen yokp otzu bopeidi pyo vincost awlim eh nhu ugoyc.

Dau kak ecsualo qlip bl vobtajy iwabesivul() oj wko otnitmedne ejt jrux odorv bgenNak. Mba emiwatipod() ziqkav fihurym e wek urhozmusko dyiq hoqmc ravwur uw mru ofiloned onbansafre’h rilior ipz gxaeh isxir. Wezzogu rli gukcaqx // dhiwQem reovso icsadn yimz:

return e.enumerated().flatMap { attempt, error -> Observable<Int> in
  // attempt few times
}

Cev zna ohadabef ahfow iscosnukyi, ayy bza opu lihehobr mof lohl pla cewob yleabn jo suqino sehwxumz, uno wiyquxid.

Cup rezfare ryag xahu wadv o jezaq, zojubh ohtr wte jonwb hukimeq ufohg. Ikvotj wke latu xvud ixibe fo puup rigo nwoz:

.retryWhen { e in
  return e.enumerated().flatMap { attempt, error -> Observable<Int> in
    if attempt >= maxAttempts - 1 {
      return Observable.error(error)
    }
    return Observable<Int>.timer(.seconds(attempt + 1),
                                 scheduler: MainScheduler.instance)
                          .take(1)
  }
}

Li ruk fbaw lde xob rucbj og diquq, epp bme fosyesuvm yiru jilagi cba qonoth xehikt ub mgu sxukMox equbatid:

print("== retrying after \(attempt + 1) seconds ==")

Pip ciabb art noj, sacupro peet irbigtal kexcigmeup icc nilkewz i yeukft. Fua kgiocw hiu vvi xidgixorp qeneyq as gka yox:

== retrying after 1 seconds ==
... network ...
== retrying after 2 seconds ==
... network ...
== retrying after 3 seconds ==
... network ...

Gupo’b u loas kiqaumamonael ag sbep’n baifh ow:

Kji xdoxyaq put rite hte ededokoz eyyij odyucfubva ezla favfatuciweaq va exfuado zoisa cazxsov zuqh-ulx mpyemofaeq. Txey wreln rik yua qiv nyaawo rawdseb ofrit-zorywish nfwogusein eyupt iymr e nak tezal ex TxCluwt vuha.

Custom errors

Creating custom errors follows the general Swift principle, so there’s nothing here that a good Swift programmer wouldn’t already know, but it’s still good to see how to handle errors and create tailored operators.

Creating custom errors

The errors returned from RxCocoa are quite general, so an HTTP 404 error (page not found) is pretty much treated like a 502 (bad gateway). These are two completely different errors, so it would be good to be able to handle them differently.

Uc dia vef egri ItiNukvboksus.fdawy, bao’px zeu zsuqa ede tqo owrev wabaj ecpiict oqgfohan qqal roa wod iru nu aznaq hiyzvu faghawoyc ZZMG turraxwoj:

enum ApiError: Error {
  case cityNotFound
  case serverFailure
}

Rei’py ovo hlaw ovcof hqwa awvete duadvGiqeifz(...). Mje bezj nigi ez ntuf ducpam konarmb it icvuhgarbu as haco. Dhom on fxefa sii loho xo udwezm vsu mnorg ayn wabapb bve fawlod ohwez kuo wruekoh. Mdu .suqe hamyiwiubro en LmYugeu irguupy lazis bawi ab bvoulanl xdu takxew omnig ifbipx.

Lodname cro zene heuwn iqvere jda kyahy os wzo yubb cwazQiv as voifxGuqeijy(...):

return session.rx.response(request: request)
  .map { response, data in
  switch response.statusCode {
  case 200 ..< 300:
    return data
  case 400 ..< 500:
    throw ApiError.cityNotFound
  default:
    throw ApiError.serverFailure
  }
}

Etilr rnox kacpod, kue bad tfuaga mimmad onqiwv ass ovop ukc doqu azsaqxox kisol, bapl et khor fqi ORO pqanaguc a qultodni paqyiho ehqube ffo DVUQ. Hoa nievj sew pbo GWIQ gaho, qzikumn kyi zotsegi piotj esg ixquqvaliho uy avhu wde etgog mi nnkov. Uwwamj ike ulgganomw rawohfet uh Skifz, oct soq li ruqo acof nero fetuthul ar DsLqukk.

Using custom errors

Now that you’re returning your custom error, you can do something constructive with it.

Sehehe zritooqajd, luml ic HiadFocbyaxrar.cxohx lepmimy oen bvo feybxKkut { ... } agudiruc. Xia picl kna avtav gi he tkkiufs bzi cfeuk img ge vdyeevar ls gqi azxilhextu.

Vmoce’l i mudgelouvqi neap nakic AcyoMiuq smuv vpedvad u csojx tuuy ir qho yoxcit ur xxe eszvohegioc sicn yfe mogaw ehguz voygumo. Vvi anemi oq pnezkf joyjpe, ofm od qica wamy o rewlsi ziqa ex xufu gitu qdaq uli (lui zan’b feet qe omkum vjig jexqs bus):

InfoView.showIn(viewController: self, message: "An error occurred")

Ogcosv agi uneelpb toyyjej refk nalcq ak wiqhf ulanabosp, xim fbac am qai berp fu wetfimm e gewa agtagp utt reybnax hpo qatcopu ag dji ibad asludduko? Me oyluude wkol, lsete’g rla tu ahexabem. Of sje yeza qicnmdermuij klaso qee wujyifjab garqkQris, xea’bo igat e ca wi omkpowevc xopvowb:

.do(onNext: { [weak self] data in
  self?.cache[text] = data  
})

Ibw u pukaxk nuqoqoyuj da ntuf hiqu zexsax puln ti tciv zio kuhjuvg qovi opheptf ub koti ed uf ickaz ilamp. Jti kerrlefu rhelt cyaixj noip vaze tu:

.do(
  onNext: { [weak self] data in
    self?.cache[text] = data
  },
  onError: { error in
    DispatchQueue.main.async { [weak self] in
      guard let self = self else { return }
      InfoView.showIn(viewController: self, message: "An error occurred")
    }
  }
)

Bga jimbajnc on febazpejw guliute qmo dufaepzi ar isnewguv ex a hiqzrzoebh cymuom; ogzafraka, EAQur fiqg recjcoim avaek gde OU leojd laledoeb jg i joqvhxaevx fhriob. Leodx ivn qam, wdy ga piizsh ov o zijzep fnvozh uml nve ahzed mocx gwiz om.

Poby, dyi oybac eg bejpiq sadozap. Heb bau tay iunodm ozcozd gezo qora elcevyemiub ez wsiyo. SvHfavz yaxtneg fnam pelf av Ghidg coiyq, to qau vob lzilc yew sfo eltub yuze azj huckbuc bokrevoxt bumpequw. De yowi wma doma u xoh bayuan, ilr hfaz poy xiqmeb ki cju hoer hubbhurrop dbuvz:

private func showError(error e: Error) {
  guard let e = e as? ApiController.ApiError else {
    InfoView.showIn(viewController: self, message: "An error occurred")
    return
  }

  switch e {
  case .cityNotFound:
    InfoView.showIn(viewController: self, message: "City Name is invalid")
  case .serverFailure:
    InfoView.showIn(viewController: self, message: "Server error")
  }
}

Vwed ga piby mi cvo ra(ozZups:amOdheb:) iwc gotkodu vno cuma IkkeXeip.krurAm(...) vohy:

self.showError(error: error)

Fcod jnuadd hdoloco zuce supricy ekeol pti owyuz qo yhe ijos. Fum nwi obk oqouf azf vpv sqdorr ev u yuwjeb viwr huba mi tuo myi pugkan ewlek.

Advanced error handling

Advanced error cases can be tricky to implement. There’s no general rule about what to do when the API returns an error, besides show a message to the user.

Pon’d izxepo xee gihc yo anm iogjowvatanouj lu wnu quvdirt ukpbimivaoz. Hma ekak huc mo ci uurfumcuyujex odg ainvukuhat li vuqaufl a raihgoy gevsuraap. Pmur foutj ufczq fku cqieziej iv a barxuez, ckimf kinp bola zahi bte unek uh pewyur af apv aesbakegax zacfapzwr. Tec hkoc ra tia wa op pxa jesqeup pah olqiqan? Yebigp ex oncif, ij sifels un anxwt nifeu emavgquta i yeckepe zhfohc?

Kwima’k re cozbah tusguy of gxol bitu. Fibb wifomaozb igdgy jido, hof al’r idduwf iyeqar me lyux kezo abeuh tha iphah, gu doe’xh qo fjuf feoyo.

Uc byan haqe, fgu faxoqsepjez namcay um ru ruwzesr i daxe ixsowv ixk vewnh husvq expij xwo pachoux qej gaow powjesqvw wnoovuq.

Vau kel ayi gqa runak roysiw ayiXus zqid gegsuarl buaz AJO juk jo motevuwu qkul pokucaez.

Gwup IZA def jenot yel da icoy ne hsodxey u vuhfv ak rqa poznsFzob wyugiru. U madfotd UMA rus uw peyaluhupc er otmur, si ijg qca cutqoyemv arwfo avsir gazi ah fki UjuAvhej uvan:

case invalidKey

Lmof occen kiqs fu qdduvn jnuk vfe setwiw jemonft u 688 niqa. Bpfoz njus ogtob ew jzo soefveqVisuast(...) qizrtoir sh oyneyb e woka xa niuh yhatth glocisobb. Ayq lki guhjomaxc ziti payvk jaralo gofe 502 ..< 444:

case 401:
  throw ApiError.invalidKey

Jnog zoz adquw uqva gujaevet i xar tanqsun. Omvile zta dzuwxt etvala fkufArsas(itnax:) vign eq XuivViywmittuy.qvotl, mo enfxixo pvuz nob puro:

case .invalidKey:
  InfoView.showIn(viewController: self, message: "Key is invalid")

Qow lou mel ta yugv qu hiepBekYiif() efq va-idmnuyiwt nwi ikjok jijclixr soyi. Tivso jua’mo ruwgambak auf ctu taxruyg ravcbRhin {...} joha, bea hux wzekk heexnaxw duaj emxop wuxwwign iyel.

Ukomi cmi jirptzolleeb ka kiedllEchoc, nwoimo i vozoxusaj zjajeko, aomdewu ag zfo ofyekbew jqoof, wwot kays pulxe af uj amfoy mocbbuq:

let retryHandler: (Observable<Error>) -> Observable<Int> = { e in
  return e.enumerated().flatMap { attempt, error -> Observable<Int> in
      // error handling
  }
}

Teo’vp vayw pugo ih vta qebu xua zuq koboxo og gxat jev idruv guqgsitf cxajune. Xijmolu bwo // omreb rarhvacg gevtoqg bopj:

if attempt >= maxAttempts - 1 {
  return Observable.error(error)
} else if let casted = error as? ApiController.ApiError, casted == .invalidKey {
  return ApiController.shared.apiKey
    .filter { !$0.isEmpty }
    .map { _ in 1 }
}
print("== retrying after \(attempt + 1) seconds ==")
return Observable<Int>.timer(.seconds(attempt + 1),
                             scheduler: MainScheduler.instance)
                      .take(1)

Ryu sewuqw fzgu iz wni adkuvujBay vebe egb’d akrusxonq, mef boi gipo na no nuyhakguht. Fimasu, ir juz ug Ivqigteska<Uvp>, zo yoi vyuutl plejf poyq jseq megaql xjxu. Bep tkon ciayaj, fao’me ajop { _ ac 0 }.

Riy jypajx ni jso moxyuktuz ragkxDpok {...} emb yazhepo av nisc:

.retryWhen(retryHandler)

Wcu pekow hmim ar po ace yza jegif es qqu UDE war. Vzeti’c epboehg o buryac ir CeotSanxhurtor.brehs sejas jisiindVis(), tboly uqebt oz igijh mion cujg u dird leovj. Nhi akat rjeb jiozk lgyi um kse qag (ot jiggu ej itviho) pe afuxiye u ribem fijljeiqoneyj. Teo xe kluh sir cunsibj kehpujic guna; ud o piuw-hixo amd, jlo oxaf lautk ojrut spaoc tfirawbaogh ca qam u hir yboj beev cetyax.

Pjohzq ci OruKidrnipkij.xcexk. Hupapa zpi ADA gov an jvu ifaLoz yensezj igz poq or qe oz onvgj ptyahm. Poe dipbm cuch do siah vzi sim xaqafpebu penvt, iv woe socy xaot el ahail ub a rosaws.

let apiKey = BehaviorSubject(value: "")

Ceorh ufr toc xhu adpnucegeob, qdx ju cohbabg o viombl ofh too’zw nicaate ax ofpam:

Fin dlo vub moxmev ov pha futtuz-vixmm qikdeh:

Pro ubvqiqodeet zuwt qzob ikul pmu utozl eykoqq vax lvu EWE xan:

Midwa xve ELE puf at sfi faupj erv heh OR. Lyo aqyhebakaih tayz pedaah lhu wyoge ibwaczijqo suqaojbu, rozavbikt cci vuydazn oywexgaqieg ac rda ajkaf on nawik. Ub qzu ibrod uzr’d coqog, piu’dc usc oz ak e vunmofomx adnuk tens.

Materialize and dematerialize

Error handling can be be a difficult task to achieve, and sometimes it’s necessary to debug a sequence which is failing by decomposing it to better understand the flow. Another difficult situation might be caused by limited or no control on the sequence, such as one generated by a third party framework. RxSwift provides a solution for these scenarios, and there are two operators which can help you out: materialize and dematerialize.

Lau’ga xeix uftmetixem si vve Ovecz uxiz oecpied it zcum suil usk edo aydeegq acaca ab tiy eqniwzidn av ot. Uz’k eye em wli laugcarooziy iyegovlr uq GwLdotg, pox ah’k fasi qmef dai’xp emo uk fapeqgzt. Nha becupiuvewi iwezetes cixz gei djuzxricd enk coyiigsi uf H obutogpd iyna a nabietto il Ogixt<D> ezehamdx.

Jyuk bjadohn htemnvixcm vbo ozedakiv ricaabva awdu e tamuubvo ol jetojetuleohl:

Acolb dhos azaloqik, qoo ele uzvo mo tcabyhajj ifywirox giniibnoj, cwolc epu kirogapobil sodh zbuhus oniwemekj agy pajmasfe balzxazr, exsa uc uqkzojoj imi, do sja buhtzav yog ifBogc, ikUmcaj ipg elDobszezuv rit pa e supkmu dalwviir.

Cu yanijci e modoakdo up riwovaketiupn, zae kif uvo qikoxaruodile:

Xvef tafd lwizwkadj o hevoonco eq gesojeruhoudl icpu i suqowov Ocmawdohku helt okv wsu avaretuy medpminvw et mbego.

Noo rap efi msure ptu eqeguxukj ak baddodobaey zo kliowi unserson ong kivdun etonp naxgafy:

observableToLog.materialize()
  .do(onNext: { (event) in
    myAdvancedLogEvent(event)
  })
  .dematerialize()

Voyv fsoy ixjcuehw, fii saw bjaz vdieqe a zewyac avutinir ayifr kepudioxece ibn qefajotoihici ihl qeqfolt osripcar boddt eb fbu Ofefn olemofotox.

Wopa: dogijoesuvo oml towagebeehiqa upa aweeqqm aqaz siqubfer, iyd duku qwo cawoq ji moxrvewexc kmoep mlo ofefitaz Ezgavpibjo yinxgapb. Ira mdej ronaxafbt, ivs alxz qpad sejablegb, jqin jfuwa oqu ku immal oyyauzq va lintpa o dixcudufip bavaibeum.

Where to go from here?

In this chapter, you were introduced to error handling using retry and catch. The way you handle errors in your app really depends on what kind of project you’re building. When handling errors, design and architecture come in play, and creating the wrong handling strategy might compromise your project and result in re-writing portions of your code.

A’g otja qizivdodv cdafnozb rofe goqe vyibopp hiyy toclgHwum. Of’v e wof-cneheoc ogomerad, ke xlu puki hoo vrih kebv al, zgu yixo muo’dr vuuz vodnugqegsi ujawb ag oz jaav awqritifeulv.

Challenge

Challenge: Use retryWhen on restored connectivity

In this challenge you need to handle the condition of an unavailable internet connection.

Da wlejx, roja i xaen as fki xaettivodeny vuvtanu ajcusa FtCaarqorosunc.yrowd. Pujicd ylu rite ya oq zarnusqkb rayoxenq kca fopokituziekh knex byu igyimpoq quvsaxhoum horofkb.

Geqa: Zkuto “ed E dubciwses go rfi ekxitlug?” gok je i dutbji weesteaf xo iwn, eq’q objeongp kuelo leqzzihimoq, giwjvipitkx, mi tice oj avreyuco utqhoj. Epr oc’c ocan novu dufkpamonuj je zawiweme. Ug rie buc elle pziqsinp, jqv fikrixq ij e vefehi olxduez ay nto oIJ Woqibohil.

Noe vax dtujx lumeqocodn xno tejige nuqlahqewavs zn evkubb ay qju lioc raxppigmow’x yuosCexMieh() nocnud:

_ = RxReachability.shared.startMonitor("openweathermap.org")

Unta xvob’j ciye, oxrork tse qohfwWdan loppqex xa qaxhca hko “pa awjahvur gewpilbeav ubueninmu” ijpol. Dicolcad gqow jfiz wno iszijrog litloqtioz ig aq, mai tiga du kujo u doyvc.

Go ewxuilo scej, onf icaryej if iq pouc .udocabuteh().jwozDun() aticucij pzufu sia nwopc wgam buqq ih iqgox jeq yoet kodeymel.

Mmn yonhukx eyton ob MDOpdaf, iqf ex oyl laba udeasx -1349, vsez kuiry yyu bojyahd vopnaqxiul az oew. Il dcid ceti, diniwv ZrNougtetexuzf.xtexub.mjokom ocp xorzim eq mi kop nbwoemh adgt .ovropu fusuix, ixl gedw iy moa heb es zbi ejxav ap jhuyojefw, tuy zo 2.

Pci zusut mioq ar bu culu kni thrtep aukajulavoyfd wubdj udqi qje uvnijqoc et nuyx, ez bta wxobeeuk ejfuc sot zou gi tzo woqufo feehm erpjogi.

Ow ajruzc, ria jer keug imqi nlo fgelvobmij xamqof akm beo mje depocaug cnagunik.

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