stateIn and shareIn

Cold to Hot

stateIn and shareIn turn a cold flow (e.g. a Room query) into a hot one that is shared by all collectors, so the database is queried once, not once per collector.

SharingStarted	Upstream runs
`Eagerly`	immediately, until the scope is cancelled
`Lazily`	from the first subscriber, until the scope is cancelled
`WhileSubscribed(5000)`	while someone collects, plus 5 s after the last one leaves (survives screen rotation)

stateIn

Example

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*

fun temperatureSensor(): Flow<Int> = flow {
    println("Sensor started")
    emit(28)
    delay(100)
    emit(31)
}

fun main() = runBlocking {
    val temp: StateFlow<Int> = temperatureSensor()
        .stateIn(this, SharingStarted.Eagerly, initialValue = 0)

    println("Right away: ${temp.value}")
    delay(50)
    println("After 50 ms: ${temp.value}")
    delay(100)
    println("After 150 ms: ${temp.value}")
    coroutineContext.cancelChildren()
}

Output

Right away: 0
Sensor started
After 50 ms: 28
After 150 ms: 31

shareIn: One Upstream, Many Collectors

Example

Two screens collect the same ticker, but the upstream starts only once.

import kotlinx.coroutines.*
import kotlinx.coroutines.flow.*

fun main() = runBlocking {
    val ticker = flow {
        println("Upstream connected")
        repeat(3) { delay(100); emit(it) }
    }
    val shared = ticker.shareIn(this, SharingStarted.WhileSubscribed(), replay = 0)

    launch { shared.collect { println("Screen A got $it") } }
    launch { shared.collect { println("Screen B got $it") } }
    delay(450)
    coroutineContext.cancelChildren()
}

Output

Upstream connected
Screen A got 0
Screen B got 0
Screen A got 1
Screen B got 1
Screen A got 2
Screen B got 2

In a ViewModel

class ProductsViewModel(repo: ProductRepository) : ViewModel() {
    val products: StateFlow<List<Product>> = repo.observeProducts()   // Flow from Room
        .map { list -> list.sortedBy { it.price } }
        .stateIn(
            scope = viewModelScope,
            started = SharingStarted.WhileSubscribed(5_000),
            initialValue = emptyList(),
        )
}