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
m

Marc

01/31/2023, 7:23 PM
Hello there, I'm having an issue with handling state in a ``ViewModel``. I have an app which has a Barcode-scanner to add products to my cart. The items which are in the cart should be rendered by CartList (and the CartItem subcomponent; which isn't necessary to show here). The products in cart are represented as a list in the viewmodel. Well, when I add a ``Product`` to my ``cart`` in the ``Cart``-Viewmodel, the ``CartList`` does not re-render. Is it because it is a list of products and lists are somehow not observable? I use the ``cart.update`` function. I use the google barcode scanner to add new products to my list. This function is nested in a FloatButton. ViewModel
data class Product(
    val barcode: String,
    val name: String,
    val price: Double,
    val category: String,
    val image: Image?
)

class Cart : ViewModel() {
    private val _cart: MutableStateFlow<List<Product>> = MutableStateFlow(listOf())
    val cart: StateFlow<List<Product>> = _cart.asStateFlow()

    init {
        val product = Product("123456789", "Test", 13.99, "Test", null) // test product
        _cart.update { it + product }
    }

    fun addProduct(product: Product) {
        _cart.update { currentState ->
            currentState + product
        }
    }
}
My composable:
@Composable
fun CartList(padding: PaddingValues) {
    val cart: Cart = viewModel();

    LazyColumn(contentPadding = padding) {
        items(cart.cart.value) { product ->
            CartItem(product)
        }
    }
}
QR-Code-Scanner which is called from a floating button:
@Composable
fun FloatButtonScanCode() {
    // Local context
    val context = LocalContext.current
    val cart: Cart = viewModel()

    FloatingActionButton(onClick = {
        val optionsBuilder = GmsBarcodeScannerOptions.Builder()
            .setBarcodeFormats(
                Barcode.FORMAT_EAN_13, Barcode.FORMAT_EAN_8
            )
        val options = optionsBuilder.build()


        val gmsBarcodeScanner = GmsBarcodeScanning.getClient(context, optionsBuilder.build())
        gmsBarcodeScanner
            .startScan()
            .addOnSuccessListener { barcode: Barcode ->

                if (barcode.rawValue != null) {

                    cart.addProduct(Product("123456789", "Test2", 1.99, "Test", null)) // <-- this is where I add a new product to the cart, but after calling addProduct the element with the item list does not re-render
                    println("Added product: Test2 to cart") 

                    println("Cart ${cart.cart.value}") // proof that the new product is added to the cart-array
                }
            }
            .addOnFailureListener { e: Exception -> System.err.println(e) }
            .addOnCanceledListener {
                System.err.println("Cancelled")
            }
    }) {
        Icon(Icons.Outlined.AddCircle, contentDescription = "Add")
    }
}
i

Ian G. Clifton

01/31/2023, 7:36 PM
I’d recommend going through the state codelab to better understand this (here’s the specific page but the whole thing is helpful https://developer.android.com/codelabs/jetpack-compose-state?hl=en#10). In short, Compose doesn’t know that you’re adding items to the List, so it doesn’t trigger recomposition.
m

Marc

02/01/2023, 9:27 PM
@Ian G. Clifton just wanted to drop a thank you for the link, it helped me a lot!
i

Ian G. Clifton

02/02/2023, 1:02 AM
Glad to hear it!