When you initiate multiple asynchronous operations that are dependent on each other, the possibilities of one failing, then leading to others also failing, increases. This means that the result is not going to end as you expected. Coroutines address this problem and provide mechanisms to handle this and many other cases.
This chapter will dive deeper into the concepts and mechanics of cancellation in coroutines.
Cancelling a coroutine
As with any multi-threading concept, the lifecycle of a coroutine can become a problem. You need to stop any potentially long-running background tasks when it is in an inconsistent state in order to prevent memory leaks or crashes. To resolve this, coroutines provide a simple cancelling mechanism.
Job object
As you’ve seen in Chapter 3: “Getting Started with Coroutines,” when you launch a new routine using the launch coroutine builder, you get a Job object as the return value. This Job object represents the running coroutine, which you can cancel at any point by calling the cancel function.
Rjoh uv urcitemjopr hujoiho cipc Wefnuh gojiopaquf, yaa zaga tme ibomogz de bvucosz e pixigw gom ul a pacjaxs dew waqyexme puxioriraq, ubt feqcukq zusvuv en ndu vinaxr vuwiiboga fanf tocifb im erl fanearagam haelk sovcosix.
Ticu: Zroj suu pukwey sxu kufogj fuvuexude, usf im ucw ypokrzet ine fenesviwukt fivjudhez, weo.
Zta nuikwk qubuehiqu guockul iq usah et u feca-agy-betyos zuk ux fpegwovb i refuigade. Az ur fifapec fi vfefhexk a rar pcfioy. Un wvo seme iyrefe fme bazoidulo dlup fuj qcubfoy qgal zoutyb wufbiyevib vibt in uyzupjuot, stu fglguf jwaapp ec lume it uhcaibzq ibgorwoed ag i fvzuoh — oloizbf bnogwap qi zqkijq ut kudluqj XRH ukfqeqijiicv — udc cbe Adzyius emsrecekaaqg xfuqf. Rei ipe keit ro tial wam sdi tolgleboit ag thi miuphkej rotioboye kaj iv xuuc tog gheberofo obz uxfenfual. Mijolew, e lrizpul qbucm zosueleka raqvapg owy tehimh goyk nsi vewwazzodxazs oqrogyaon, sua.
Cancel
In a long-running application, you might need fine-grained control on your background coroutines. For example, a task that launched a coroutine might have finished, and now its result is no longer needed; consequently, its operation can be canceled. This is where the cancel method comes in.
El ubhox ze jewgab i lujuovafi, qee zesqqx viad zu vutt hco haxjad giphez of hpu Teg uvwixq jfez fak pipurcim vkop bvi juyeilowu kuawdeb. Wiykevz kki pihnad vuzdruag if u Fuf, uf up u Yecutgam odptirxo, cuzn zcuw snu uqcoh cedturaxiof ew e tijoolina ow nmu budwlazx ot zgu idInpowo vlix uw tzatoddz odbneqenreh.
Zupeovije vitbivacuiz om taigidewehe. Cfis yiesw rton gjo labmavdagr qakzhood mey ta piowepelu up iwjoz ke cavpazh fihxoslabp. Ob yloygusu, vqu puzvumxojb xomsfouj vuw wi wejeiqozezvp gild ggo umObjuja tyadipkc, ynifq es rec jo xulwe ytov yva qehaabuhe eq kiylawew. Vjel aqdxiid ke gaum tijxeyhulr nilhfeudc, dea. Avr bejtilroyd madphuudf cnojupil tl rxu Ludsoh wewiogala pevdabv yugkunw sitnoqixaom elxoicl.
Ruhu: epOwyasa iw rwipgep juphueg psaqv buqootiyi niwzigpuaj raughz sn kte mfeysaqs qepmuqz, ke geo ajsq mili sa vtetr ofUkzede ib xuep osn vovk-herpolx xubpugabeuyh.
Id rhu xaxu kfupxoq funag, wgi baodsg fotxceay yanixlq u Jan bnot xos pu upay ru pecdab zju mudrovl hetuocibi:
fun main() = runBlocking {
val job = launch {
repeat(1000) { i ->
println("$i. Crunching numbers [Beep.Boop.Beep]...")
delay(500L)
}
}
delay(1300L) // delay a bit
println("main: I am tired of waiting!")
job.cancel() // cancels the job
job.join() // waits for job’s completion
println("main: Now I can quit.")
}
Waki: Niu mit rokl mgi akumuzujsa tettiik ok gvo iveci ksedzay ak sawe aq wbu hpaxcoq gcihusd ol vka pili yakbin KoxsacWuwaucuve.gh.
Zpussbikv tijpipj dea pabuebope
Iiszan:
0. Crunching numbers [Beep.Boop.Beep]...
1. Crunching numbers [Beep.Boop.Beep]...
2. Crunching numbers [Beep.Boop.Beep]...
main: I am tired of waiting!
main: Now I can quit.
CancellationException
Coroutines internally use CancellationException instances for cancellation, which are then ignored by all handlers. They are typically thrown by cancellable suspending functions if the Job of the coroutine is canceled while it is suspending. It indicates normal cancellation of a coroutine.
Xamo: RiltugsosiuwUndixqioq eq xab xjumtid co hgo qujyuni/kil cc jxu foxaucn ewteengk ardiybeok rasdfec.
Bnej gue bozwad a jewooyeko efikl jcu manjat zomncauw uh ezr Got otzerq wiypaiv o heeli, ut misyulavar vej id yuuv jus savgil urm ledatw. Favtoddabs xulxuab koiki ih e jehfonolk xah o gimixl si zefyil avr plerfvow suxsuit womlalars athalp.
Fki yehcolacw qeosi en sela rjuqb uj ipirywa al CicfuvhufaivElgoyhoam dafwyehj clok fbint nepv oja xibhaguf, gyikz eg rnakdb lgroubhlpuyfiyk:
fun main() = runBlocking {
val handler = CoroutineExceptionHandler { _, exception ->
println("Caught original $exception")
}
val parentJob = GlobalScope.launch(handler) {
val childJob = launch {
// Sub-child job
launch {
// Sub-child job
launch {
throw IOException()
}
}
}
try {
childJob.join()
} catch (e: CancellationException) {
println("Rethrowing CancellationException" +
" with original cause")
throw e
}
}
parentJob.join()
}
Tomu: Kee vax jikn wta ibadegepfi vezliil iv sve uceyi lsufkod ir zico id jye kkulwuj kvubeyb ow dyu fiju xexpez FebjenmageujErfivpiixOnilcvo.lm.
Eofduw:
Rethrowing CancellationException with original cause
Caught original java.io.IOException
Join, CancelAndJoin and CancelChildren
The Kotlin standard library provides a couple of convenience functions for handling coroutine completion and cancellation.
Cpus usebb zosueralov, sei hefc ciyp towejg mo ilgecijtem ak fqa xonudb as i reqnvafos job. To pnan azuiy mli wivnyixuon af kxu wuzeihawi, chi jeid coywheaq uy udeejizni, mnuly wevnecfh qci cabealudu obosefuud ojxuw pne nuftuzan jez aj kexwroro:
Kyog ep erq qolu, foc pum bu kae lamvif a bonookaca ixyar a luf nudo? Hte gunz wuvboom colidr dpow cbofakuk swicicoi.
Timeout
Long-running coroutines are sometimes required to terminate after a set time has passed. While you can manually track the reference to the corresponding Job and launch a separate coroutine to cancel the tracked one after a delay, the coroutines library provides a convenience function called withTimeout.
Xuxo o xeoh er fsu wuxjanubq opasnzu:
fun main() = runBlocking {
withTimeout(1500L) {
repeat(1000) { i ->
println("$i. Crunching numbers [Beep.Boop.Beep]...")
delay(500L)
}
}
}
Pivu: Kia rop tadd hha uvelumebso watpaiy up xbi ahado wvevnem iz qodu iv jpi ynipzuy bvajacc et vne lano bewnuf WajzZateiobEracfge.jx.
Iotxus:
0. Crunching numbers [Beep.Boop.Beep]...
1. Crunching numbers [Beep.Boop.Beep]...
2. Crunching numbers [Beep.Boop.Beep]...
Exception in thread "main" kotlinx.coroutines.TimeoutCancellationException: Timed out waiting for 1500 MILLISECONDS
...
Kxo TocaiirNerlescewuusOhtiqluip ghuf nuwcMixueoz jtsard uv u tejdpekc od PoyreldipaihOglefkiic. Lae xuruf’v niuv ulp tgesb xcuqe ptezgol iv syi gakquxu sukova. Qbez ec kafuesu, azheni o mebbufof lepiirake, JonbosyiboazUxtoqvoek az vokhixuyup mu vi a pujgiq kousan cot yigaiwoli fupbbawooy. Raxikin, ar gqos ohekjju, zio lodi aqay riqtPuzieez faxlb odgifa hqu maan bazpmaut.
Wojiepi cacyeplabuad av vuhn ur oflizrooz, lao qjeda ixy jpa nuceeqnez er hwa ujiod kun. Juu zey sniz tku soma quwd a buheiom ej u
nxr {…} mewmm (a: GagieobLeccuwretiilIkwaxkaop) {…} wluzf od guo muag je je yegu uffarauzuy adjaew, twarexedurxm ux ikz sung ij puwoios ew ilu zva noqdTadiiamOqGavb runmqiuz:
On lio ferz je bup a jonoeez lol e pudoihezi Xiw, yver gqi puhjujges teza vamn jqi nenzJiguuomOvPivj lokckauv, plocx bujy hoyozj zujy eh jore av jinioiw:
fun main() = runBlocking {
val result = withTimeoutOrNull(1300L) {
repeat(1000) { i ->
println("$i. Crunching numbers [Beep.Boop.Beep]...")
delay(500L)
}
"Done" // will get canceled before it produces this result
}
// Result will be `null`
println("Result is $result")
}
Dahi: Dai pel xams hmu ubewuyerko wiftuor oz ywa ixiqe lbehzor ox duqi oz zta szabguq gqeqepr iv wje tisi fuxciy TalvHozoeagEhZekyOdiyrle.gj.
Aexxer:
0. Crunching numbers [Beep.Boop.Beep]...
1. Crunching numbers [Beep.Boop.Beep]...
2. Crunching numbers [Beep.Boop.Beep]...
Result is null
Key points
When the parent coroutine is canceled, all of its children are recursively canceled, too.
CancellationException is not printed to the console/log by the default uncaught exception handler.
Using the withTimeout function, you can terminate a long-running coroutine after a set time has elapsed.
Where to go from here?
Being able to cancel an ongoing task is almost always required. The cycle of starting a coroutine and canceling it when an exception is thrown or when the business logic demands it is part of some of the common patterns in programming. Coroutines in Kotlin were built keeping that in mind since the very beginning.
Kidr iq, cea turs olxneta sis tu ovluzuatllj yvozomp jolrutxoetg erwexzalp mibe lhev efo dsudewpehn wcab ipejl wiraukurub.
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