Kotlin Flow

Hot Flow vs Cold Flow

Type	Description	Example
Cold Flow	Starts emitting only when collected, each collector gets its own stream	flow { }, flowOf()
Hot Flow	Emits regardless of collectors, shared among all collectors	StateFlow, SharedFlow

Cold Flow

Created using flow { } builder. Each collector triggers a new execution.

fun fetchData(): Flow<Int> = flow {
    for (i in 1..5) {
        delay(1000)
        emit(i)
    }
}

// Collect in composable
LaunchedEffect(Unit) {
    fetchData().collect { value ->
        println("Received: $value")
    }
}

Flow Operators

flow
    .map { it * 2 }              // Transform values
    .filter { it > 10 }          // Filter values
    .debounce(300)               // Wait for pause in emissions
    .distinctUntilChanged()      // Skip consecutive duplicates
    .catch { e -> emit(default) } // Handle errors
    .onEach { log(it) }          // Side effects
    .collect { /* use value */ }

Convert Flow to StateFlow

val stateFlow = flow.stateIn(
    scope = viewModelScope,
    started = SharingStarted.WhileSubscribed(5000),
    initialValue = emptyList()
)

SharingStarted Options

Option	Description
Eagerly	Start immediately, never stop
Lazily	Start on first collector, never stop
WhileSubscribed(ms)	Start on first collector, stop after last collector + timeout