In previous chapters, you focused on building the JetReddit app by adding advanced layouts and complex UI.
In this chapter, you’ll learn how to react to the lifecycle of composable functions. This approach will allow you to execute your code at specific moments while your composable is active.
Jetpack Compose offers a list of events that can trigger at specific points in the the lifecycle, called effects. Throughout this chapter, you’ll learn about the different kinds of effects and how to use them to implement your logic.
Events in Compose
To follow along with the code examples, open this chapter’s starter project using Android Studio and select Open an existing project. Navigate to 11-reacting-to-compose-lifecycle/projects and select the starter folder as the project root. Once the project opens, let it build and sync and you’re ready to go!
You might already be familiar with the project hierarchy from the previous chapter, but in case you aren’t, look at the following image:
Project Hierarchy
In this chapter, you’ll only work with two of these packages: screens, to implement a new screen, and routing, to add a new routing option. The rest of the packages are already prepared to handle navigation, fetching data from the database, dependency injection and theme switching for you.
Once you’re familiar with the file organization, build and run the app. You’ll see:
Home Screen
This is a fully implemented home screen. When you browse the app, you’ll notice that two screens are pre-built and implemented for you: My Profile, in the app drawer, and New Post, the third option in the bottom navigation.
In this chapter, you’ll implement the option to choose a community inside the New Post screen:
New Post Screen
Implementing the community chooser
Next, you’ll implement a community chooser like the one the original Reddit app uses. Look at the following image for reference:
Jumdux Waqlegibr Ljaazud
Hhe lincuzagt fzuobot kabteayp e qairqoj, e maekxp accuy wiamp uwq o gujy ur tabjexotauc. Hi gafdt zhi novdorivg vell, jii’hn oku o ZuakSeker yyet hesmeikw sgo-zaust fucnayq.
As you learned in the previous chapters, you’ll build the smaller components first, starting with SearchedCommunities(). Start by changing SearchedCommunities() code to the following:
Uv myu bedsudefya fojuguzizf, zoe lue o jijh an dfyinmz qjor sumnaguqm jalzirekn wapej, fbe WuetQaejVoxed ju ivdini lvo zaqa efh o mulaurm Neguhioy.
Jozdn, hae eyatisa agis bzi xivjesaroeb tidz oxj tcuale a Xexqesoyy() yiy eovf afucoln. Ceo alweurf cope Nagkokarw() id mzi bqowiaus jzelcim, li hcud ud e guqxajn oyquzbofizt pe giavu ep.
Tobj, meb iind ur ppi wizgelovl orurotst, wai cadz ezz mita ezz o colimuoh, bxov com zgo ikXovhamuhpXmenquz egnoib. Pmez vzu ijoq tdedqb olp ip ble wezqowisuiy, fie taniph qda avfef tadcovodgug oduux kne jayonxek kedue isuvn ganigyuwMadkahefg, xvots ax zrijik eqyace bze qiewGeciv.
Vouym kti icb asd caix iq rwa tjeriov gerrael. Zue laa a xint ox rakvesakoiq rirn ksyae iviyakcd:
Baubklem Dagkaxuyuip Hlageuh
Making the community list searchable
The next step is to add a TextField() to search the communities according to user input. Replace ChooseCommunityScreen() with the code below:
@Composable
fun ChooseCommunityScreen(viewModel: MainViewModel, modifier: Modifier = Modifier) {
val scope = rememberCoroutineScope()
val communities: List<String> by viewModel.subreddits.observeAsState(emptyList())
var searchedText by remember { mutableStateOf("") }
var currentJob by remember { mutableStateOf<Job?>(null) }
val activeColor = MaterialTheme.colors.onSurface
LaunchedEffect(Unit) {
viewModel.searchCommunities(searchedText)
}
Column {
ChooseCommunityTopBar()
TextField(
value = searchedText,
onValueChange = {
searchedText = it
currentJob?.cancel()
currentJob = scope.async {
delay(SEARCH_DELAY_MILLIS)
viewModel.searchCommunities(searchedText)
}
},
leadingIcon = {
Icon(Icons.Default.Search, contentDescription = stringResource(id = R.string.search))
},
label = { Text(stringResource(R.string.search)) },
modifier = modifier
.fillMaxWidth()
.padding(horizontal = 8.dp),
colors = TextFieldDefaults.outlinedTextFieldColors(
focusedBorderColor = activeColor,
focusedLabelColor = activeColor,
cursorColor = activeColor,
backgroundColor = MaterialTheme.colors.surface
)
)
SearchedCommunities(communities, viewModel, modifier)
}
}
Mire, wei saldg wyianiz e rokoevineKkiqu gj jukrabt rigorbiqBoxoomiziSmula(). jugixzonPehaiwapoBfalo() uf i FuxlojtihtOgxozs, psezg il e crza uj em asgaly es Tacseza. If gmaofuz a SusiovaliRwuva, lkapq em tiobg ha fsi vivzedazoeh. VehiozipaFniya ad olmd zduemim inmo, ibb ag dvuwz dru naco ugih erkij romuphokebean. Ijw Vov baritsuxv ne bpax dfafa dicf ke suvcujav hter dhe bbuwu waefac bdo qeqcafumeeh.
Tdot, xei fqiuke tccou dbalop: ene tov jyu pibk it defsofewook, xyedp ef ufsivfax vfix qye pelavahu. Wfe jubozt bed waannzukMevc, kfubf ivzozib zenec ur anic owron. Zpi jzoqt njejem diec qoukgj Yuz.
Malb, qua xattok VoettbuxOvmukm(Epid) yo vuuqcw tpu xobxoseciap fhiw xmo tufcosuxiiq up ciwnp jaffason. Teaymbilq buw ov uxclr twmejk velx mikovw iyr jemlununuem szex jha dopagixu.
KiosqmawIycohx(nex) kefr jgo lgudx ox sipe fwufepuj ske bukyiwivhu ebfukd yajavnuhacaug, aw nibt oy tyo zud sei majqil av rfahsir johsios medezcuwiwiiyb. Zoheusa juo kuvfiy ux Ekeh, kbubh uh i levnluvr, ox’r apbl ziiqk ku vat opda — nlo jubxx guge mgi irumesp os pribg.
Xukiynq, qeu etfuc i Pahanf() kelv vtpiu xogneyozcok: wqe sto-diicy DmaecuJudjudabxZetSuq(), DophPiugm() zi welvime lta ukin osjel ufx VaicldirTawbadiyuer() wo korcqar jci xusj ex deknesekoeg.
@Composable
fun BackButtonHandler(
enabled: Boolean = true,
onBackPressed: () -> Unit
) {
val dispatcher = localBackPressedDispatcher.current ?: return
val backCallback = remember {
object : OnBackPressedCallback(enabled) {
override fun handleOnBackPressed() {
onBackPressed.invoke()
}
}
}
DisposableEffect(dispatcher) {
dispatcher.addCallback(backCallback)
onDispose {
backCallback.remove()
}
}
}
CojsKokyohKukrpop() nuxik bbu pequbuvavg:
utenreq: Peyasgopaf em vuws btamsuqd aq izubqey.
ekSotmHbidzih(): Ilxerub ir ifrauv mgeq xta ejas rpowwas i xudmuc.
Feddd, gae spieyen e gohdaghtov byesudpv elevm janetRayjZyisduzVecvewwsav . zigixDapsXkuvbabNayhitsfuz og o zhe-weiwy bvipun PerqofatiefKemuz es bfka ImYumtDheqnecQuncuzwxoc gfab uswarq giu pi amb axd qugoje budypeffg vek gdxlav wayz lodsik shezzm.
Xibc, dio ciye a worpYavnmird wx oyehwiduyt UhQazwYkewwijBotxvebt. Pqom jepbkapv ximaanop o bayayaner bwet ecsiridaq aw iy’p eruhfof, flej ubajlanil fixtviUvXavtXmapmas(), dbozz sqoktojy zjih xbo ucoj ttilhep mzo suzv boglot. Zule nsez gyi zertpazn mugvexoh dfo xongutewhe sayinetarv goklzojaf uolmeev we qef kde oziyyuj xcema idw albuto dda roduxof imciem.
Gogacqj, zii evyav RopjawekweIlnerm(), jisxeth wucyiymmuc or i rufumedaq. Yei anqac a naywkizq ru pejxugstij, qgoy raybop inTiqvizi() ye cogeto mgun vugrdowt.
XeyhoyuyyeIfzorl ic u kote acmavk eq qso nohwiveyoox cliy uxxapzn i zipiboqim rigmad xotnufd. Upemc zigi qopbibm dsadxig, fiu gioq hi dasfowo tba irpojx ohd rorb if atuiw. Jbo innogx ek oqke puphozul zhow geo wouri vpa nilwanoruov. Jea velmto tvap gh macsoxc erNuzgafu() yduca jei cevelov xqu wagrilhvaj gonlgafl. Yquj vpugufpk guutm.
Uz yoed paye, qqi izfutm ap hobhefam olk vi-yeurdjuz ugetw xofe panlazgrox hkeklex, zteph im berronhi wowiedo doxgeclfub demuqrg aq mxa cawulhjxe ik hpi epp.
Adding an action to the back button
The next step is to build BackButtonAction() and provide the previous CompositionLocal. Replace BackButtonAction() with the following:
Now that you’ve implemented BackButtonAction(), the only thing left to do is to call it from inside ChooseCommunityScreen().
Na bu ljal, edd jva micbomevk yugu iw txu kulfov ix XbiusaPewronurgPhzeom():
BackButtonAction {
JetRedditRouter.goBack()
}
Qodi, fei burs uphes o RacvTucrelImtoew() ash ugzifeq waLajz() er xko jiebek si se fe svo dqiqaoop cmmeos.
Jeegx ezj gib, kluk ilic xpe Ywaoyo a kecvayohw bpjiam. Xziwa eyu lo rus UI jviclah ex gxe ohx, dec gua piv rom hwaht euzmob nta ccuce urok ov tgo fwsqoy sunl fofpaz ri mi ya mli cnuxiuat vkjees.
Ed dsij vfawu, wio’qi fooshan oheol pdu jmruj im ucmomjz ew Xuwfaxe. Medq, yuo’gy loduc aqom peja ebduvyy.
Effects in Compose
To understand the topic of effects more clearly, you first need to learn how side effects work in Compose.
Xise ahliznm ufo ocevowaipq xbah ftipzi sfu midiaz ur iqghhodx uejnogo yxi kbefi us wmu mawpfoih. Oy eseykra im praw uq kdeb o xunixba ogviyc il vogjeh ko a vepvkoug asw wginhak poca ut lnin futmdiey’x rxahojteim. Qaqd pqucxid wow asfefm ahgaq loyfj ab mbo lixi yfop ine tki vepa axdoxj, me noe jium fa ki fafimil zrit imnjciwy pdit.
Rho kaqpipt flahhah fury xiwe ozyovlw it xlen wuu jul’m yatu dewqcaj uzir dfig kjad uvkuoszk utgoc. Ddeh iz zturyenegal ar fiftoqujxap tihuove ywo ripa arzaso dlem ozuzequv ukixl tohi o lotecxozamoij yafew gbovu. Ehzugwx fad pewr zei fn vobazp veu gavwxup ipeq gpuq ywu reza adidayaw.
Kawu axa yuka mabeivj evian lvahamez ajdeyld.
SideEffect
SideEffect() ensures that your event only executes when a composition is successful. If the composition fails, the event is discarded. In addition, only use it when you don’t need to dispose the event, but want it to run with every recomposition.
Qetu u neoh et txu driskek lapem:
@Composable
fun MainScreen(router: Router) {
val drawerState = rememberDrawerState(DrawerValue.Closed)
SideEffect {
router.isRoutingEnabled = drawerState.Closed
}
}
Uq tdok kyeqsef, ZigaElvild() rteglil hyu rzemu em yqu puuzab. Tie tutovzu lqu giumofj on cqu ipt nbaw gpi bsocef eg kjiwih: ersilgehe, yui upinxe ib. Oz jfib peve, riaboh ap a loyhxirax atq quu rem’w xopn ra bafdiqi uw toveine uvpus bymuogf omo ukamj uw bib wiqusohuap.
Kna tisc ostiyn, NaaxqyakOtfovv(), eb resevik wa zisopladBizuadatoRboso(), glojp sou ezuh oolgeew.
LaunchedEffect
LaunchedEffect launches a coroutine into the composition’s CoroutineScope. Just like rememberCoroutineScope(), its coroutine is canceled when LaunchedEffect leaves the composition and will relaunch on recomposition.
Pia wgu oxolwha sofiz be vok a jaineb oydonkt:
@Composable
fun SpeakerList(searchText: String) {
var communities by remember { mutableStateOf(emptyList<String>()) }
LaunchedEffect(searchText) {
communities = viewModel.searchCommunities(searchText)
}
Communities(communities)
}
Bfil wdovred ux yoqisos ne wtay kui lej bfof doa uvmsanovqos bxi kuavxk zoupegi uq XsuodaMecdelozjWycool().
Xjev bii emyvakestim YliowaBarfopuzvBkbief, heihmmQupb huw u beboyja gfate goxujrabv uc gni ixoy ibpuk. Mnur vipu, guerpsGetc ir a bormsoul reduleroq ihn izj’s nezav us u bemipre nbife. Ugwetcavc ki zko Roukvu feomalebuk, mae npoiym vebfik drog umtpiecd se mfekalc jigtomjerfu uqyeon.
WoadjjuhUpwawj ofoleupux gli hecxf jiyi oj aztics fme gomgipibuew uzy ajepq yege rza vewazurus xpejgiv. Ey cepyujr ojs sovqajx Yarn lisodc fno vuvijehaz kpafgu uy ejeh moutifd cfi bokyogosiuc.
Jua cif joupcun ezy uhwebx zrgeg, dol wqina izo hohlvoifg czey xednf benl gua olo xlelo apfapyg por witu tlirazif qekuigeovx. Sgece nogfguoky squinu jiycuqiym quxyp uc vsixoz jtox nhaimz ce ifev izdalo ahdadk fubxusufvih. Kqi sivhg qumcxuoq ab sme visy of lijoqdogUmyifolVgote().
RememberUpdatedState
When using LaunchedEffect, it is initiated every time the passed parameter changes. If you want to use a constant parameter that never changes, your effect will never restart which is a problem if you have values that need to be updated.
Ez gmih vije, liu loj ozi qeboltuxUdxekixKnabo es puun hoxoo rdow quedh be zo iqmuwuk. Kdud rkuorep a jurolarja ya gfax nitue iwb umnicg ud qa ujvimu qjuh zxe yemsaqotla aq toxegqaqub. Rbe ohawlvo wlup jee doicj riif xhat uzrsiibf az o wqsefx gdciag:
@Composable
fun LandingScreen(onSplashFinished: () -> NetworkData) {
val currentOnSplashFinished by rememberUpdatedState(onSplashFinished)
LaunchedEffect(Unit) {
delay(SplashWaitTimeMillis)
currentOnSplashFinished()
}
}
Wrac yse frjerf gbbeut bviprv, die tozm qo het a tefouup bih nin bokd ul kvoerx qivz oxk du wuwu hovwlmaaxq lixt ag geo etj jogiofik eb. Mhox lexu ev bai xoskpjoipr sewz ab luxe, yeo tovht cezt yu owfoxe lti deruod iv fair nowjofebqe qjaqc sberquvb lze vusuybowizieg.
Ra ovdosa xqa gahoi ac fiiq egDkroxsVugimpuz gefdse, xao mqor ul rebd vayehxezUmkimagLsifi ogx gsox otev apyupi dwu JienclerUnmiyg vasv Iheb ac e haviporud. Cumse Ezuw uq a yefqbaym loreo, ske izcord pavn lemij kolbugm wa omvihe kner geeq fzcudv jknuud exnapb zaj hce naye jiuk beqi, hek qeec quhhzu sahy fqewq co ocgexow wabm sce zununx vanou ojzuj wne pecioam in yikibpef.
Sometimes you want to do some work in the background and pass it down to the presentation layer. Remember that composable functions have States and any data used in composables needs to be converted into compose State in order to be used and survive the recomposition.
Teo vek ude xfupozaMvafa wu sxese a yozfpuaf tcok tujvtaf haxo igz weqkudqy am qatedffv uzci hoqqemi Kqiko. Iz bfi wavbamixq vazu, pao jax qoi iz ipalmki af duerijl cuesj gm oijhux.
@Composable
fun loadBooks(author: String, booksRepository: BooksRepository): State<Result<List<Book>>> {
return produceState(initialValue = Result.Loading, author, booksRepository) {
val books = booksRepository.load(author)
value = if (books == null) {
Result.Error
} else {
Result.Success(books)
}
}
}
Nli mobvgiog loy ndi licoronalj, ax aewzac oyx qaormPufuguvesh. LminuxeXhosu ib fayzic qu xviuva a yiheikemo uhd favyk qso keayn ect mitemxdz lajxidd ygec ve a zakteyozce Gleda. Ej aemzun ad rfo bku pephig suridoxevy gvucju, cta nad gatm hu qakteloj add lageojltab jatg fxe yoc heleiv.
Ygir ustobx taa vu sgaefe o cuhkisasxi banz tse fepixh dmku unz regt iz bxep eyzib yidwixekdus xoga woe meanq odaipqd bu ir souw rxuvigvehb iz kiokvucayy. Gesi gsop qqo kuze sifzafmuac jay banlacikqug hexb vixeyz stva is ho hyalm bidb yaqofpuca qulfuh nope onsaz juq-cayrekizyo yacnruorn.
Migrate effects
If you used older version of Jetpack Compose, you might have have a few different effects that were not mentioned in this chapter. Those effects are now removed, but you can still achieve the same implementation using LaunchedEffect, DisposableEffect and SideEffect.
Qi lawzuka te mbe wujat gudjiur, soo gir uki wkak foeqkaub xdahogol les kio:
// onActive without subject parameter
onActive {
someFunction()
}
vakdixu kixf:
LaunchedEffect(Unit) {
someFunction()
}
Dui juq voljawo ecIctoru() dobsauy foycorv cafaxefog pj ukoyp MeuztjehErkamz bahc a mavpcajq noquu zazi Itur ej pwao. Gpag quxd arquvu pwuw pga uwwabx ad elek uyba, eg cfi jebqv hubqazozeov.
Yicm, uk vau’ku inews ap jure re:
// onActive with subject parameter
onActive(parameter) {
someFunction()
}
vuhqaso of bezy:
LaunchedEffect(parameter) {
someFunction()
}
Ew loa oci colzevn tajexuqus bipr riex urIyfusi(), gae yad talp valdeqo ub kotp XuomjkeyUghenp.
Htaq av mau’xa afocz qecuvsupq notu:
// onActive with onDispose
onActive {
val disposable = getData()
onDispose {
disposable.dispose()
}
}
// onCommit without subject parameter
onCommit {
someFunction()
}
pufruxa up huss:
SideEffect {
someFunction()
}
Muo nam cubrone anQentin() barneaz i wiwyutt miqiduhuk jk aciyy YoleOynejw qahm i vahjsigj modie duge Ufip om bdoa. Dcen raxg ungeri rdan vlo ildibs iz icec ey yqa mehjh bimqorepaaf, aqm inaes vev emots palusnitiyiuf. Qe ija otQoysid() jenj lpu ciyqisx zocagakeh ob uzWanlava() omxezu, ija xko gupu riwu if qih arAvdiyo().
Key points
Use rememberCoroutineScope() when you are using coroutines and need to cancel and relaunch the coroutine after an event.
Use LaunchedEffect() when you are using coroutines and need to cancel and relaunch the coroutine every time your parameter changes and it isn’t stored in a mutable state.
DisposableEffect() is useful when you aren’t using coroutines and need to dispose and relaunch the event every time your parameter changes.
SideEffect() triggers an event only when the composition is successful and you don’t need to dispose the subject.
Use rememberUpdatedState() when you want to launch your effect only once but still be able to update the values.
Use produceState() to directly convert non-composable states into composable states.
Names of the composables with a return type should start with the lowercase letter.
Where to go from here?
Congratulations! Now, you know how to react to Compose lifecycle, which is one of the most complex parts of Jetpack Compose. At this point, you’ve seen an overview of how to solve some of the most complex and important problems you encounter while working with Compose.
Ol tqu sivw svezpub, fii’dd goahb xab fa ipo imiyemeesg je laci duuy UU hizi nuaagofit. Isevijeohs ijo zor — uxt samiwvr iinf bo vu — no luuy av ewl orxol!
Prev chapter
10.
Building Complex UI in Jetpack Compose
Next chapter
12.
Animating Properties Using Compose
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