Deep Dive into Kotlin Flow

%SPJE,BJHJ4FQQNBU,PBMB %FFQ%JWFJOUP,PUMJO'MPX Jumpei Matsuda @red_fat_daruma $PEF&YBNQMFT

2 Jumpei Matsuda @red_fat_daruma Senior Engineer at Drivemode, Inc. Android
Apps on Automotive OS .F https://www.drivemode.com/ My preferences 👇 SDK Development Gradle Plugin Development Development Productivity Engineering

4 Prints 0, 1, 2, ..., 9 Executes doSomething after
printing 9

6 Returns Nothing Prints 0 Never execute doSomething

8 Visible type is Unit Don't if this code is
reachable

9 Cancellable? Completable? Always reachable? Single thread? Multi-threaded? Sequential? Concurrent?
How does a thread to emit values effect? When a fi rst value comes?

8IBUXJMMCFDPWFSFEUPEBZ w 'MPXCBTJDT w 'MPXCVJMEFST PQFSBUPSTBOEPQFSBUPSGVTJPO w $VTUPN'MPXPQFSBUPST w 5FTUJOH'MPXPQFSBUPSTBOEBDMBTTVTJOH'MPX
10

'MPX#BTJDT

,PUMJO'MPX w "EBUBTUSFBNQPXFSFECZDPSPVUJOFT w " fl PXDBOIBOEMFNVMUJQMFBTZODISPOPVTMZDPNQVUFEWBMVFT w 5SBOTGPSNBTZODISPOPVTMZDPNQVUFEWBMVFTJOUPBTUSFBN w
1SPDFTTBTJOHMFWBMVFPOBEPXOTUSFBNUISPVHIBTVTQFOEJOH GVODUJPO w &YUFOTJCMFXJUIPQFSBUPST w 1PTTJCMFUPDSFBUFDVTUPN'MPXDMBTTFTGSPNAbstractFlow 12 'MPX#BTJDT

1FSTQFDUJWFTPG,PUMJO'MPX w 5IFFlow<T>interface w $PMEPS)PU w $PNQMFUBCMFPS/PODPNQMFUBCMF w $PODVSSFODZ4VQQPSU w
#BDLQSFTTVSF 13 'MPX#BTJDT

5IF'MPX5JOUFSGBDF w +VTUBDPOWFOUJPOCVUEPFTOUSFQSFTFOUBOZQSPQFSUZ w FHDPME IPU DPMMFDUBCMFSFQFBUFEMZFUD w %POUJOGFSCFIBWJPSGSPNTVQFS fi
DJBMJOGPSNBUJPOMJLFUZQFT w "OZ'MPXDMBTTFTDBOCFEPXODBTUFEUPUIFFlowinterface 14 'MPX#BTJDT1FSTQFDUJWF5IF'MPX5JOUFSGBDF

$PMEPS)PU (FOFSBM w "DPMETUSFBNJTPGUFODPNQBSFEUPBGBVDFU🚿 w 4UBSUTFNJUUJOHWBMVFTXIFOTPNFPOFCFHJOTUPDPMMFDUJU w &NJTTJPOTBSFJOEFQFOEFOUGPSFBDIDPMMFDUPS w "IPUTUSFBNJTMJLFOFEUPBMJWFCSPBEDBTU📺
w &NJUTWBMVFTSFHBSEMFTTPGXIPTDPMMFDUJOH w *OUFHSBUFTXFMMXJUIDBDIFNFDIBOJTN 15 'MPX#BTJDT1FSTQFDUJWF$PMEPS)PU

8IBUTBDPMEGMPX w 6TVBMMZ B fl PXJTBDPMEEBUBTUSFBN w &YFDVUFTDPEFJOUIF fl PXFWFSZXIFODPMMFDUFE
16 'MPX#BTJDT1FSTQFDUJWF$PMEPS)PU

8IBUTBIPUGMPX w SharedFlow StateFlow BChannelDSFBUFECZproduceIn w 4VTQFOEGPSBOFXWBMVFJOUIFCBDLHSPVOEUPFNJUJU 17 'MPX#BTJDT1FSTQFDUJWF$PMEPS)PU

$PNQMFUBCMFPS/PODPNQMFUBCMF w $PNQMFUBCMFJTEF fi OFESFHBSEMFTTPGJUTBIPUPSDPMEPOF w /PJOUFSGBDFGPSB$PNQMFUBCMF fl PXJTBWBJMBCMF 18
'MPX#BTJDT1FSTQFDUJWF$PNQMFUBCMFPSOPODPNQMFUBCMF

$PNQMFUBCMFPS/PODPNQMFUBCMF w $PNQMFUFOFTTJOGFSSFEGSPNCFIBWJPSNBZCFBGBLF w -POHSVOOJOHDPNQMFUBCMF fl PXJTBOPJTF 19 'MPX#BTJDT1FSTQFDUJWF$PNQMFUBCMFPSOPODPNQMFUBCMF

$PNQMFUBCMFPS/PODPNQMFUBCMF w 4FUBOFYUFSOBMDBODFMMBUJPODPNQMFUJPOMPHJD w FHUJNFPVU UBLF XJUI5JNFPVUFUD 20 'MPX#BTJDT1FSTQFDUJWF$PNQMFUBCMFPSOPODPNQMFUBCMF

$PODVSSFODZ4VQQPSU w "EPXOTUSFBNTVQQPSUTDPODVSSFODZBOEBTJOHMFQBSBMMFMJTN w 5IJTEPFTOUNFBOBTJOHMFUISFBEFEDPODVSSFODZ 21 'MPX#BTJDT1FSTQFDUJWF$PODVSSFODZ4VQQPSU
JTHVBSBOUFFE "UISFBEOBNF NBZEJ ff FS

$PODVSSFODZ4VQQPSU w 5SZUPFNJUTWBMVFTJOQBSBMMFMGSPNB fl PXCVJMEFS 22 'MPX#BTJDT1FSTQFDUJWF$PODVSSFODZ4VQQPSU

$PODVSSFODZ4VQQPSU w $PODVSSFOUVQEBUFJTQSPIJCJUFECZEFGBVMU w MutableStateFlow MutableSharedFlowBOEchannelFlow TVQQPSUTDPODVSSFOUQBSBMMFMVQEBUFTBTUIFZTFSJBMJ[FFNJTTJPOT 23 'MPX#BTJDT1FSTQFDUJWF$PODVSSFODZ4VQQPSU

#BDLQSFTTVSF w 7BMVFNJTTJPOTBSFOPUBMXBZTQSPDFTTFEJNNFEJBUFMZ w "OVQTUSFBNTCFIBWJPSNVTUCFEFUFSNJOFEJOTVDIBDBTF w 5IFSFBSFTFWFSBMTUSBUFHJFT w 3FOEF[WPVT $PO
fl BUF w -JNJUFE6OMJNJUFECV ff FS w MutableSharedFlowIBTBTQFDJBMTUSBUFHZ 24 'MPX#BTJDT1FSTQFDUJWF#BDLQSFTTVSFBOE$PO fl BUJPO

3FOEF[WPVTTUSBUFHZ w &NJUTBOFYUWBMVFUPBDPMMFDUPSEJSFDUMZPSXBJUTGPSJUUPCFSFBEZ w 5IJTJTBMTPBTLOPXOBTVOCV ff FSFE w 1SPT w
"OFYUSBCV ff FSJTOPUSFRVJSFE w $POT w /PUTVJUBCMFGPSUJNFTFOTJUJWFWBMVFFNJTTJPO 25 'MPX#BTJDT1FSTQFDUJWF#V ff FSBOE$PO fl BUJPO

$POGMBUFTUSBUFHZ w %SPQPMEWBMVFTBOEFNJUPOMZUIFNPTUSFDFOUWBMVF w StateFlowCBTJDBMMZIBTUIJTCFIBWJPS w 1SPT w 5IJTDBONJUJHBUFBCVSTUWBMVFFNJTTJPO w
$POT w "TMPXDPMMFDUPSNBZMPTUJOUFSNFEJBUFWBMVFT 26 'MPX#BTJDT1FSTQFDUJWF#V ff FSBOE$PO fl BUJPO

-JNJUFE#VGGFSTUSBUFHZ w 4UPSFWBMVFTJOBMJNJUFECV ff FSBOEQSPDFTTUIFNMBUFS w 1SPT w "DPMMFDUPSXPVMESFDFJWFWBMVFTBTNBOZBTQPTTJCMF w
$POT w 5IFPWFS fl PXXJMMMPTUJOUFSNFEJBUFWBMVFTPSOFFEBMPOHTVTQFOTJPO w -BSHFSDBQBDJUZXPVMEJODSFBTFBNFNPSZDPOTVNQUJPO 27 'MPX#BTJDT1FSTQFDUJWF#V ff FSBOE$PO fl BUJPO

6OMJNJUFE#VGGFSTUSBUFHZ w 4UPSFWBMVFTJOBJO fi OJUFCV ff FSBOEQSPDFTTUIFNMBUFS w 1SPT w
"DPMMFDUPSDBOSFDFJWFBMMWBMVFT w $POT w 00.SJTL 28 'MPX#BTJDT1FSTQFDUJWF#V ff FSBOE$PO fl BUJPO

.VUBCMF4IBSFE'MPXTTUSBUFHZ w *UVTFTBDPOEJUJPOBMCBDLQSFTTVSFTUSBUFHZXIFOSFQMBZ w *GPGDPMMFDUPST w -JNJUFE#V ff FSTUSBUFHZUIFDBQBDJUZJTFYUSB#V
ff FS$BQBDJUZ w 0UIFSXJTF w *UIBTOPCV ff FSBOEEJTNJTTFTWBMVFFNJTTJPOT 29 'MPX#BTJDT1FSTQFDUJWF#V ff FSBOE$PO fl BUJPO My apologies, I accidentally deleted the replay == 0 condition in the presentation

'MPX #VJMEFST0QFSBUPST

'MPXCVJMEFSTBOEPQFSBUPST w 'MPX"1*TDPOTJTUPGCVJMEFSTBOEPQFSBUPST w #VJMEFST w 1SPEVDFBCZQVSQPTF fl PX w
0QFSBUPST w 3FUVSOBOFXUSBOTGPSNFE fl PXPSUSBOTGPSNWBMVFT 31 'MPX#VJMEFST0QFSBUPST

GMPX\^ w "DPME fl PXCVJMEFS w 4UBUFMFTTBOEOPTFSJBMJ[BUJPO w 4JNQMFBOEFBTZUPVTF w
flowOfBOEWBSJBOUTVTFUIJTCVJMEFSJOUFSOBMMZ 32 'MPX#VJMEFST0QFSBUPST#VJMEFST

DIBOOFM'MPX\^ w "DPME fl PXCVJMEFSUIBUTFSJBMJ[FTVQEBUFT w "MTPFBTZUPIBOEMFBDBMMCBDL w CoroutineScopeJTBWBJMBCMFBTBSFDFJWFS 33
'MPX#VJMEFST0QFSBUPST#VJMEFST

.VUBCMF4IBSFE'MPX w "IPU fl PXCVJMEFSUIBUTFSJBMJ[FTVQEBUFT w 4IBSFDBDIFTCFUXFFOEPXOTUSFBNT w "OFXDPMMFDUPSSFDFJWFTDBDIF fi
STUJGSFQMBZ w $BDIFJTSFTFUBCMFFYUFSOBMMZ w 5IFCBDLQSFTTVSFTUSBUFHZJTDPOEJUJPOBMTPJUNBZCFUSJDLZ w 4USPOHMZSFDPNNFOEUPVTFsubscriptionCount 34 'MPX#VJMEFST0QFSBUPST#VJMEFST

.VUBCMF4UBUF'MPX w "IPU fl PXCVJMEFSUIBUTFSJBMJ[FTVQEBUFT w 'MPXBTB 4UBUF w
"MBTUWBMVFJTSFBEBCMFBOEXSJUBCMF w AtomicReferenceJTI"1*TBSFBWBJMBCMF w $PO fl BUFTWBMVFT w FH#VSTUVQEBUFTNBZFNJUPOMZUIFMBTUWBMVF 35 'MPX#VJMEFST0QFSBUPST#VJMEFST

*OUFSNFEJBUFPQFSBUPST w "OJOUFSNFEJBUFPQFSBUPSEPFTOUFYFDVUFBOZDPEFJOUIF fl PX w +VTUTUBDLBDIBJOPGPQFSBUJPOTGPSGVUVSFUFSNJOBMPQFSBUPST w 4PNFPQFSBUPSTSFUVSOBDPOEJUJPOBMMZDPNQMFUBCMF fl
PX 36 'MPX#VJMEFST0QFSBUPST0QFSBUPST

5FSNJOBMPQFSBUPST w "UFSNJOBMPQFSBUPSTUBSUDPMMFDUJOHB fl PX w FHcollect first fold toList
stateIn shareIn FUD w .PTUPGPQFSBUPSTSFRVJSFDPNQMFUBCMF 37 'MPX#VJMEFST0QFSBUPST0QFSBUPST

0QFSBUPS'VTJPOJO'MPX"1*EFTJHO w "GVTJCMFPQFSBUPSDBOBWPJEDSFBUJOHBSFEVOEBOUXSBQQFSJGDIBJOFE w FH"GVTJCMF fl PXOFWFSUBLFTBOPUIFSGVTJCMF fl PXBTBOVQTUSFBN w
#ZDPOUSBTU OPOGVTJCMFPOFBMXBZTDSFBUFTBVQTUSFBNXSBQQFS 38 'MPX#VJMEFST0QFSBUPST0QFSBUPS'VTJPO

0QFSBUPS'VTJPOJO'MPX"1*EFTJHO w 7FSZMJNJUFECVJMUJOPQFSBUPSTDBOGVTFF ff FDUT w CoroutineContextDIBOHFTFHflowOn w #BDLQSFTTVSFDIBOHFTFHbuffer conflate
w 5IFQPJOUJTXFDBOBWPJEVTJOHPMEDPOUFYUCBDLQSFTTVSF w 5IFQFSGPSNBODFDPVMECFCFUUFSUIP JUTOPUBNBJOQVSQPTF 39 'MPX#VJMEFST0QFSBUPST0QFSBUPS'VTJPO

$VTUPNJ[BUJPO

4FMFDUB'MPXCVJMEFS w 4FMFDUBQSPQFS'MPXCVJMEFSGPSZPVSVTFDBTF w "6TFflow { ... } w #6TFchannelFlow
{ ... } w $$SFBUFBDVTUPNDMBTTFYUFOEJOHAbstractFlow 41 $VTUPNJ[BUJPO#VJMEFSDIPJDF

$IBSUGPSB'MPXCVJMEFS 42 $VTUPNJ[BUJPO#VJMEFSDIPJDF

)JHIMJHIUFECVJMUJOPQFSBUPST w .VTUMFBSOUIFJOUFSOBMJNQMFNFOUBUJPOTPGUIFGPMMPXJOHPQFSBUPST • runningFold • transformLatest • sample •
stateIn/shareIn • merge 46 $VTUPNJ[BUJPO(PPESFGFSFODFPQFSBUPST

'MPX5SVOOJOH'PME w $PNQVUFBOFYUWBMVFGSPNBQSFWJPVTWBMVF w /PTZODISPOJ[BUJPOMPDLJTSFRVJSFEJOUIJTDBTF 47 $VTUPNJ[BUJPO(PPESFGFSFODFPQFSBUPST

'MPX5USBOTGPSN-BUFTU w 'JOFUPVTFcoroutineScopeBOEMBVODIJOHDBODFMMJOHBKPC w $BODFMBQSFWJPVTQSPDFTTJOHUISPVHIBKPC 48 $VTUPNJ[BUJPO(PPESFGFSFODFPQFSBUPST

'MPX5TBNQMF w 4IPXTIPXUPVTFselect"1*UPNBOBHFUXPDPODVSSFOUTPVSDFT w 6TFGVMGPSDVTUPNUJNFTFOTJUJWFBOEPScombine zip PQFSBUPST 49 $VTUPNJ[BUJPO(PPESFGFSFODFPQFSBUPST

50 Create a ticker as a secondary source Collect values
from an upstream through a Channel

Wait the values from two sources simultaneously When a ticker
fired, release and emit the last value When an upstream emitted, hold it

TUBUF*OTIBSF*O w -BVODIBKPCJOBHJWFODPSPVUJOFTDPQF w "CBDLHSPVOEKPCDPMMFDUTBOEUSBOTGFSWBMVFTUPBOFX fl PX 52 Create a
sharing job from *scope* Hold a sharing job to avoid GC $VTUPNJ[BUJPO(PPESFGFSFODFPQFSBUPST

*UFSBCMF'MPX5NFSHF w 'MBUUFONVMUJQMFVQTUSFBNTJOUPBTJOHMF fl PX w 6QTUSFBNTXJMMCFDPMMFDUFEJOCBDLHSPVOE 53 $VTUPNJ[BUJPO(PPESFGFSFODFPQFSBUPST

*UFSBCMF'MPX5NFSHF w 4VDIPQFSBUJPOTEPOUQBJSXFMMXJUIMutableSharedFlow(replay = 0) w *UTSJTLZXIFOFODBQTVMBUJOHBMutableSharedFlow 54 $VTUPNJ[BUJPO(PPESFGFSFODFPQFSBUPST

5FTUJOH'MPXT

)PX*UFTU'MPXDPEF w 5FTUBDVTUPNPQFSBUPS w 5FTUBDMBTTUIBUIBTB fl PX 56 5FTUJOH'MPX As
for common debugging techniques of coroutines, please check out my session at DroidKaigi 2024 Debugging: All You Need to Know https://2024.droidkaigi.jp/timetable/694440/

DBTIBQQUVSCJOF w 6TFGVMBTTFSUJPOTGPSB fl PX w 4USPOHMZSFDPNNFOEUPVTFUIJTMJCSBSZOPUPOMZGPSZPVSDPOWFOJFODF 57 $PEFGSPNIUUQTHJUIVCDPNDBTIBQQUVSCJOF$PQZSJHIU4RVBSF *OD
5FTUJOH'MPXDBTIBQQUVSCJOF

6TF5VSCJOFGPSBEPXOTUSFBN w MutableStateFlowEPFTOUXPSLGPSNBOZDBTFT w 4PNFJOUFSNFEJBUFWBMVFTNBZOPUCFBTTFSUFE w $PO fl BUJPO w
%JTUJODU 58 5FTUJOH'MPXDBTIBQQUVSCJOF

6TF5VSCJOFGPSBEPXOTUSFBN Turbine code 59 MutableSharedFlow code 5FTUJOH'MPXDBTIBQQUVSCJOF

"UFTUUFNQMBUFPGBDVTUPNPQFSBUPS 60 5FTUJOH'MPXDVTUPNPQFSBUPSUFTUJOH

61 "OVQTUSFBNJT6/-*.*5&%$IBOOFM 5FTUJOH'MPXDVTUPNPQFSBUPSUFTUJOHUFTUUFNQMBUF

62 6TF&JUIFSUPFNVMBUFDPNQMFUBCMF 5FTUJOH'MPXDVTUPNPQFSBUPSUFTUJOHUFTUUFNQMBUF

63 %POUQBTT5FTU4DPQFUPBOPQFSBUPS 5FTUJOH'MPXDVTUPNPQFSBUPSUFTUJOHUFTUUFNQMBUF

%POUQBTT5FTU4DPQFUPBOPQFSBUPS w "OPQFSBUPSUIBUMBVODIFTBKPCJTOPUBHPPE fi UUPTestScope w TestScopeXPOUDPNQMFUFJGBUMFBTUPOFKPCJTSVOOJOH w "OKPCXJMMLFFQBMJWFJGBOVQTUSFBNJTOPUDPNQMFUBCMF 64
5FTUJOH'MPXDVTUPNPQFSBUPSUFTUJOHUFTUUFNQMBUF

%POUQBTT5FTU4DPQFUPBOPQFSBUPS w *OTUFBE VTFbackgroundScopePSNBOBHFBTDPQFNBOVBMMZ w backgroundScopeDBODFMTDIJMEKPCTXIFOUIFUFTUCPEZ fi OJTIFT 65 5FTUJOH'MPXDVTUPNPQFSBUPSUFTUJOHUFTUUFNQMBUF

66 "TTFSUWBMVFTCZVTJOH5VSCJOF 5FTUJOH'MPXDVTUPNPQFSBUPSUFTUJOHUFTUUFNQMBUF

)PX*UFTU'MPXDPEF w 5FTUBDVTUPNPQFSBUPS w 5FTUBDMBTTUIBUIBTB fl PX 67 5FTUJOH'MPX

"MJHOCVJMEFSTJOQSPEBOEUFTUDMBTTFT w MutableSharedFlowBOEMutableStateFlowBSFJODPNQBUJCMF 68 5FTUJOH'MPXUFTUJOHGPSBDMBTTVTJOH'MPX

0WFSSJEFCZNVMUJUISFBEFEEJTQBUDIFS w *OHFOFSBM TestCoroutineDispatcherJTVTFGVMUIPVHI w %VFUPDPODVSSFODZ SBDFDPOEJUJPOTBSFNPSFMJLFMZUPCFIJEEFO w "WJSUVBMDMPDLDBOCFBEKVTUFEJOGPSFHSPVOEDPEF w
#BDLHSPVOEKPCTEPOUBMXBZTOFFEUPVTFUIJTNFDIBOJTN w *OTUFBE TimeSourceTIPVMECFJOKFDUBCMF w 6TFTestTimeSourceEVSJOHUFTUJOH 69 5FTUJOH'MPXUFTUJOHGPSBDMBTTVTJOH'MPX

8SBQVQ

8SBQVQ w 'MPXUSBOTGPSNTDPODVSSFODZJOUPBTJOHMFQBSBMMFMJTNTUSFBN w 4IBSFELFZQFSTQFDUJWFBOEPQFSBUPSTGPSMFBSOJOH w 4IBSFEUFTUJOHUJQTGPSDVTUPNPQFSBUPSTBOEBDMBTTVTJOHFlow w cashapp/turbineJTHSFBU w
"UFTUUFNQMBUFGPSBDVTUPNPQFSBUPS w TestCoroutineDispatcherJTSJTLZBCJU 71

%SPJE,BJHJ4FQQNBU,PBMB %FFQ%JWFJOUP,PUMJO'MPX Jumpei Matsuda @red_fat_daruma 5IBOLZPVGPSMJTUFOJOH

Deep Dive into Kotlin Flow

Deep Dive into Kotlin Flow

More Decks by Matsuda Jumpei

Other Decks in Programming

Featured

Transcript