What is the difference between StateFlow and SharedFlow?

StateFlow always holds the latest value (requires initial value, replays 1). SharedFlow is more flexible — it has no required initial value, configurable replay cache, and can be used for one-time events. Use StateFlow for UI state; SharedFlow for events.

Why expose asStateFlow() from a ViewModel?

Keep MutableStateFlow private so only the ViewModel can update state. Expose read-only StateFlow to the UI to prevent accidental writes from outside.

Intermediate 8 min readKotlin 2.0

Kotlin StateFlow — Observable State for UI

StateFlow is a hot flow that always holds the latest value and emits it to all new collectors. It is ideal for UI state in ViewModels.

What You Will Learn

Create MutableStateFlow
Read and update state
Collect StateFlow values
Compare StateFlow with LiveData
Use StateFlow in a ViewModel pattern

MutableStateFlow

MutableStateFlow holds a value and broadcasts changes to all collectors.

Basic StateFlow

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

fun main() = runBlocking {
    val counter = MutableStateFlow(0)

    val job = launch {
        counter.collect { println("Count: $it") }
    }

    delay(100)
    counter.value = 1
    delay(100)
    counter.value = 2
    delay(100)
    counter.value = 3
    delay(100)
    job.cancel()
}

Output

Count: 0 Count: 1 Count: 2 Count: 3

MutableStateFlow(0) creates a flow with initial value 0. Setting .value = x emits the new value to all collectors. The collector in launch receives each update.

Best Practice: Expose StateFlow as a read-only StateFlow from your ViewModel: private val _state = MutableStateFlow(...); val state: StateFlow<T> = _state.asStateFlow()

StateFlow in a ViewModel Pattern

Use StateFlow to model UI state.

ViewModel Pattern

kotlin
import kotlinx.coroutines.flow.*

data class UiState(val count: Int = 0, val loading: Boolean = false)

class CounterViewModel {
    private val _uiState = MutableStateFlow(UiState())
    val uiState: StateFlow<UiState> = _uiState.asStateFlow()

    fun increment() {
        _uiState.value = _uiState.value.copy(count = _uiState.value.count + 1)
    }

    fun decrement() {
        _uiState.value = _uiState.value.copy(count = _uiState.value.count - 1)
    }
}

fun main() {
    val vm = CounterViewModel()
    vm.increment()
    vm.increment()
    vm.decrement()
    println(vm.uiState.value)
}

Output

UiState(count=1, loading=false)

UiState is a data class. .copy() creates a new UiState with only the changed properties. _uiState is private (mutable). uiState is public (read-only). This is the standard Android ViewModel pattern.

Practice Exercise

Exercisemultiple choice

What value does a new StateFlow collector receive immediately upon subscribing?

Quick Quiz

How do you expose StateFlow as read-only from a ViewModel?

Kotlin StateFlow — Observable State for UI

What You Will Learn

MutableStateFlow

Basic StateFlow

StateFlow in a ViewModel Pattern

ViewModel Pattern

Practice Exercise

Quick Quiz

Frequently Asked Questions

Related Tutorials