https://kotlinlang.org logo
Channels
100daysofcode
100daysofkotlin
100daysofkotlin-2021
advent-of-code
aem
ai
alexa
algeria
algolialibraries
amsterdam
android
android-architecture
android-databinding
android-studio
androidgithubprojects
androidthings
androidx
androidx-xprocessing
anime
anko
announcements
apollo-kotlin
appintro
arabic
argentina
arkenv
arksemdevteam
armenia
arrow
arrow-contributors
arrow-meta
ass
atlanta
atm17
atrium
austin
australia
austria
awesome-kotlin
ballast
bangladesh
barcelona
bayarea
bazel
beepiz-libraries
belgium
benchmarks
berlin
big-data
books
boston
brazil
brikk
budapest
build
build-tools
bulgaria
bydgoszcz
cambodia
canada
carrat
carrat-dev
carrat-feed
chicago
chile
china
chucker
cincinnati-user-group
cli
clikt
cloudfoundry
cn
cobalt
code-coverage
codeforces
codemash-precompiler
codereview
codingame
codingconventions
coimbatore
collaborations
colombia
colorado
communities
competitive-programming
competitivecoding
compiler
compose
compose-android
compose-desktop
compose-hiring
compose-ios
compose-mp
compose-ui-showcase
compose-wear
compose-web
confetti
connect-audit-events
corda
cork
coroutines
couchbase
coursera
croatia
cryptography
cscenter-course-2016
cucumber-bdd
cyprus
czech
dagger
data2viz
databinding
datascience
dckotlin
debugging
decompose
decouple
denmark
deprecated
detekt
detekt-hint
dev-core
dfw
docs-revamped
dokka
domain-driven-design
doodle
dsl
dublin
dutch
eap
eclipse
ecuador
edinburgh
education
effective-kotlin
effectivekotlin
emacs
embedded-kotlin
estatik
event21-community-content
events
exposed
failgood
fb-internal-demo
feed
firebase
flow
fluid-libraries
forkhandles
forum
fosdem
fp-in-kotlin
framework-elide
freenode
french
fritz2
fuchsia
functional
funktionale
gamedev
ge-kotlin
general-advice
georgia
geospatial
german-lang
getting-started
github-workflows-kt
glance
godot-kotlin
google-io
gradle
graphic
graphkool
graphql
graphql-kotlin
graviton-browser
greece
grpc
gsoc
gui
hackathons
hacktoberfest
hamburg
hamkrest
helios
helsinki
hexagon
hibernate
hikari-cp
hire-me
hiring
hiring-french
hongkong
hoplite
http4k
hungary
hyderabad
image-processing
india
indonesia
inkremental
intellij
intellij-plugins
intellij-tricks
internships
introduce-yourself
io
ios
iran
israel
istanbulcoders
italian
jackson-kotlin
jadx
japanese
jasync-sql
java-to-kotlin-refactoring
javadevelopers
javafx
javalin
javascript
jdbi
jhipster-kotlin
jobsworldwide
jpa
jshdq
juul-libraries
jvm-ir-backend-feedback
jxadapter
k2-early-adopters
kaal
kafka
kakao
kalasim
kapt
karachi
karg
karlsruhe
kash_shell
kaskade
kbuild
kdbc
kgen-doc-tools
kgraphql
kinta
klaxon
klock
kloudformation
kmdc
kmm-español
kmongo
knbt
knote
koalaql
koans
kobalt
kobweb
kodein
kodex
kohesive
koin
koin-dev
komapper
kondor-json
kong
kontent
kontributors
korau
korean
korge
korim
korio
korlibs
korte
kotest
kotest-contributors
kotless
kotlick
kotlin-asia
kotlin-beam
kotlin-by-example
kotlin-csv
kotlin-data-storage
kotlin-foundation
kotlin-fuel
kotlin-in-action
kotlin-inject
kotlin-latam
kotlin-logging
kotlin-multiplatform-contest
kotlin-mumbai
kotlin-native
kotlin-pakistan
kotlin-plugin
kotlin-pune
kotlin-roadmap
kotlin-samples
kotlin-sap
kotlin-serbia
kotlin-spark
kotlin-szeged
kotlin-website
kotlinacademy
kotlinbot
kotlinconf
kotlindl
kotlinforbeginners
kotlingforbeginners
kotlinlondon
kotlinmad
kotlinprogrammers
kotlinsu
kotlintest
kotlintest-devs
kotlintlv
kotlinultimatechallenge
kotlinx-datetime
kotlinx-files
kotlinx-html
kotrix
kotson
kovenant
kprompt
kraph
krawler
kroto-plus
ksp
ktcc
ktfmt
ktlint
ktor
ktp
kubed
kug-leads
kug-torino
kvision
kweb
lambdaworld_cadiz
lanark
language-evolution
language-proposals
latvia
leakcanary
leedskotlinusergroup
lets-have-fun
libgdx
libkgd
library-development
lincheck
linkeddata
lithuania
london
losangeles
lottie
love
lychee
macedonia
machinelearningbawas
madrid
malaysia
mathematics
meetkotlin
memes
meta
metro-detroit
mexico
miami
micronaut
minnesota
minutest
mirror
mockk
moko
moldova
monsterpuzzle
montreal
moonbean
morocco
motionlayout
mpapt
mu
multiplatform
mumbai
munich
mvikotlin
mvrx
myndocs-oauth2-server
naming
navigation-architecture-component
nepal
new-mexico
new-zealand
newname
nigeria
nodejs
norway
npm-publish
nyc
oceania
ohio-kotlin-users
oldenburg
oolong
opensource
orbit-mvi
osgi
otpisani
package-search
pakistan
panamá
pattern-matching
pbandk
pdx
peru
philippines
phoenix
pinoy
pocketgitclient
polish
popkorn
portugal
practical-functional-programming
proguard
prozis-android-backup
pyhsikal
python
python-contributors
quasar
random
re
react
reaktive
realm
realworldkotlin
reductor
reduks
redux
redux-kotlin
refactoring-to-kotlin
reflect
refreshversions
reports
result
rethink
revolver
rhein-main
rocksdb
romania
room
rpi-pico
rsocket
russian
russian_feed
russian-kotlinasfirst
rx
rxjava
san-diego
science
scotland
scrcast
scrimage
script
scripting
seattle
serialization
server
sg-user-group
singapore
skia-wasm-interop-temp
skrape-it
slovak
snake
sofl-user-group
southafrica
spacemacs
spain
spanish
speaking
spek
spin
splitties
spotify-mobius
spring
spring-security
squarelibraries
stackoverflow
stacks
stayhungrystayfoolish
stdlib
stlouis
strife-discord-lib
strikt
students
stuttgart
sudan
swagger-gradle-codegen
swarm
sweden
swing
swiss-user-group
switzerland
talking-kotlin
tallinn
tampa
teamcity
tegal
tempe
tensorflow
terminal
test
testing
testtestest
texas
tgbotapi
thailand
tornadofx
touchlab-tools
training
tricity-kotlin-user-group
trójmiasto
truth
tunisia
turkey
turkiye
twitter-feed
uae
udacityindia
uk
ukrainian
uniflow
unkonf
uruguay
utah
uuid
vancouver
vankotlin
vertx
videos
vienna
vietnam
vim
vkug
vuejs
web-mpp
webassembly
webrtc
wimix_sentry
wwdc
zircon
Powered by
Title
l

Landry Norris

05/20/2022, 6:50 PM
I have an interesting use case I’m looking to support. I’m currently using a MutableStateFlow of a single data class to represent the UI state. I have several video controllers that control several videos. Each one exposes the current timestamp of the video as a Flow<Double>. The user can select one video, and the seekbar should set its position based on the timestamp of the selected video. My current solution is to collect the state flow, get the current controller, and collect the position inside of that. I can then update the state based on the collected value. Is there a better way to handle this?
general structure: state is a MutableStateFlow that holds, among other properties, a list of video controllers (state.value.controllers), and a state.value.currentController, and I need to collect state.value.currentController.positionFlow, which is a Flow<Double>, even if I change state.value.currentController
m

Michael Marshall

05/20/2022, 7:59 PM
I think some extra code snippets might help here
l

Landry Norris

05/20/2022, 8:03 PM
Sure. I have a class that looks like
data class UIState(val controllers: List<Controller>, currentController: Controller? = controllers.firstOrNull())
and a class like
class Controller {
    val positionFlow: Flow<Double> = MutableStateflow(0.0)
}
Whenever the position changes, I call positionFlow.update { value } In my code, I have
val state = MutableStateFlow(UIState(...))
. I want to collect the currentController’s positionFlow, but I want it to update whenever I change currentController
Right now, I have
state.collect { currentState ->
                currentState.currentController.positionFlow?.collect { position ->
                    //Code that uses position
                }
            }
I’m sure there’s some more clear way to do this without nesting collect calls. If I add other flows to the controller, I’d like the solution to scale well.
I’m also considering moving the controllers list outside of the state, since it isn’t really ‘UI state’, but I’m thinking I should keep the currentController part of the state class.
👍 1
m

Michael Marshall

05/23/2022, 4:54 PM
If the Controllers are actually just data classes then it’s probably okay to leave them in the UI state. If they interact with lifecycle and UI events then I’d move them out
currentController could probably just be an index of the list, so you’re not emitting the same controller from two places in the UIState?
Are you looking for something like this?
fun reactToControllerAndPositionChanges() {
        // I assume you have some scope
        val scope = CoroutineScope(Dispatchers.Default)

        // Old way
        scope.launch {
            state.collect { currentState ->
                currentState.currentController?.positionFlow?.collect { position ->
                    positionCode(position)
                }
            }
        }

        // New way
        state
            .flatMapLatest { state ->
                state.currentController?.positionFlow ?: emptyFlow()
            }
            .onEach(::positionCode)
            .launchIn(scope)
    }

    private fun positionCode(position: Double) {
        TODO()
    }
If the UI only shows one video at a time and doesn’t need to know about the whole list of Controllers (which at this point you could call
ControllerState
) you could instead just use
val state = MutableStateFlow(ControllerState(...)
and switch out the controllers info in the business logic behind the scenes instead.
^In this case you could just have
data class ControllerState(val position: Double)
and update the whole
state
whenever the position changes
l

Landry Norris

05/23/2022, 5:28 PM
It will show multiple videos side by side, but the position should only respond to changes in the selected video's controller. I'll definitely look at using flatmap.
👍 1
I guess the best I could describe what I’m looking for is being able to state.combine(state.selected.positionFlow), but of course the combine method wouldn’t be able to update what it’s combining with automatically as state.selected changes.
m

Michael Marshall

05/23/2022, 5:33 PM
Yeah combine is for transforming the results from multiple flows, flatmap definitely makes more sense here. In your original code, I believe you might have been creating memory leaks by collecting a new flow each time the current controller changed, but never cancelling the old ones. Flatmap handles that for you.
l

Landry Norris

05/23/2022, 5:34 PM
That makes sense. I need to study up on the more complex flow operators.
❤️ 1
Using flatMap seems to work perfectly, but I’m noticing my Compose UI takes about a second to react when I select a new video.
m

Michael Marshall

05/24/2022, 6:56 PM
Could it be that the point flows don't emit for a second, so the change doesn't happen until then? You could try using a StateFlow for the point flows in that case
l

Landry Norris

05/24/2022, 6:58 PM
The positionFlow shouldn’t be updating its value when a new video is selected. The positionFlow does use a MutableStateFlow under the hood.
👍 1
From my understanding, when I update state, it will trigger onEach. Is this correct? That should be what causes the positionCode to run.
Now that I think about it, I know Compose Debug mode is supposed to run a bit slower. I’m having CI build a production version now. I’m wondering if that’s the cause.
👍 1
m

Michael Marshall

05/24/2022, 7:04 PM
When you update the screen, the chosen position flow will change. When that position flow emits, onEach will run
It could be debug causing it
l

Landry Norris

05/24/2022, 7:06 PM
Just added logs when I call state.update and when onEach runs. The difference is 0.002 seconds, so I think it’s safe to say the delay in rendering is Compose, not the flatMap. Thank you for helping me with this.
m

Michael Marshall

05/24/2022, 7:07 PM
All good ☺️