StateFlow

@ExperimentalCoroutinesApi interface StateFlow<out T> : 
    Flow<T>
(source)

A Flow that represents a read-only state with a single updatable data value that emits updates to the value to its collectors. The current value can be retrieved via value property. The flow of future updates to the value can be observed by collecting values from this flow.

A mutable state flow is created using MutableStateFlow(value) constructor function with the initial value. The value of mutable state flow can be updated by setting its value property. Updates to the value are always conflated. So a slow collector skips fast updates, but always collects the most recently emitted value.

StateFlow is useful as a data-model class to represent any kind of state. Derived values can be defined using various operators on the flows, with combine operator being especially useful to combine values from multiple state flows using arbitrary functions.

For example, the following class encapsulates an integer state and increments its value on each call to inc:

class CounterModel {
    private val _counter = MutableStateFlow(0) // private mutable state flow
    val counter: StateFlow<Int> get() = _counter // publicly exposed as read-only state flow

    fun inc() {
        _counter.value++
    }
}

Having two instances of the above CounterModel class one can define the sum of their counters like this:

val aModel = CounterModel()
val bModel = CounterModel()
val sumFlow: Flow<Int> = aModel.counter.combine(bModel.counter) { a, b -> a + b }

Strong equality-based conflation

Values in state flow are conflated using Any.equals comparison in a similar way to distinctUntilChanged operator. It is used to conflate incoming updates to value in MutableStateFlow and to suppress emission of the values to collectors when new value is equal to the previously emitted one. State flow behavior with classes that violate the contract for Any.equals is unspecified.

StateFlow vs ConflatedBroadcastChannel

Conceptually state flow is similar to ConflatedBroadcastChannel and is designed to completely replace ConflatedBroadcastChannel in the future. It has the following important difference:

StateFlow is designed to better cover typical use-cases of keeping track of state changes in time, taking more pragmatic design choices for the sake of convenience.

Concurrency

All methods of data flow are thread-safe and can be safely invoked from concurrent coroutines without external synchronization.

Operator fusion

Application of flowOn, conflate, buffer with CONFLATED or RENDEZVOUS capacity, or a distinctUntilChanged operator has no effect on the state flow.

Implementation notes

State flow implementation is optimized for memory consumption and allocation-freedom. It uses a lock to ensure thread-safety, but suspending collector coroutines are resumed outside of this lock to avoid dead-locks when using unconfined coroutines. Adding new collectors has O(1) amortized cost, but updating a value has O(N) cost, where N is the number of active collectors.

Not stable for inheritance

StateFlow interface is not stable for inheritance in 3rd party libraries, as new methods might be added to this interface in the future, but is stable for use. Use MutableStateFlow() constructor function to create an implementation.

Properties

value

abstract val value: T

The current value of this state flow.

Inherited Functions

collect

abstract suspend fun collect(
    collector: FlowCollector<T>
): Unit

Accepts the given collector and emits values into it. This method should never be implemented or used directly.

Extension Functions

broadcastIn

fun <T> Flow<T>.broadcastIn(
    scope: CoroutineScope,
    start: CoroutineStart = CoroutineStart.LAZY
): BroadcastChannel<T>

Creates a broadcast coroutine that collects the given flow.

buffer

fun <T> Flow<T>.buffer(capacity: Int = BUFFERED): Flow<T>

Buffers flow emissions via channel of a specified capacity and runs collector in a separate coroutine.

cancellable

fun <T> Flow<T>.cancellable(): Flow<T>

Returns a flow which checks cancellation status on each emission and throws the corresponding cancellation cause if flow collector was cancelled. Note that flow builder is cancellable by default.

catch

fun <T> Flow<T>.catch(
    action: suspend FlowCollector<T>.(cause: Throwable) -> Unit
): Flow<T>

Catches exceptions in the flow completion and calls a specified action with the caught exception. This operator is transparent to exceptions that occur in downstream flow and does not catch exceptions that are thrown to cancel the flow.

collect

suspend fun Flow<*>.collect(): Unit

Terminal flow operator that collects the given flow but ignores all emitted values. If any exception occurs during collect or in the provided flow, this exception is rethrown from this method.

suspend fun <T> Flow<T>.collect(
    action: suspend (value: T) -> Unit
): Unit

Terminal flow operator that collects the given flow with a provided action. If any exception occurs during collect or in the provided flow, this exception is rethrown from this method.

collectIndexed

suspend fun <T> Flow<T>.collectIndexed(
    action: suspend (index: Int, value: T) -> Unit
): Unit

Terminal flow operator that collects the given flow with a provided action that takes the index of an element (zero-based) and the element. If any exception occurs during collect or in the provided flow, this exception is rethrown from this method.

collectLatest

suspend fun <T> Flow<T>.collectLatest(
    action: suspend (value: T) -> Unit
): Unit

Terminal flow operator that collects the given flow with a provided action. The crucial difference from collect is that when the original flow emits a new value, action block for previous value is cancelled.

combine

fun <T1, T2, R> Flow<T1>.combine(
    flow: Flow<T2>,
    transform: suspend (a: T1, b: T2) -> R
): Flow<R>

Returns a Flow whose values are generated with transform function by combining the most recently emitted values by each flow.

combineLatest

fun <T1, T2, R> Flow<T1>.combineLatest(
    other: Flow<T2>,
    transform: suspend (T1, T2) -> R
): Flow<R>
fun <T1, T2, T3, R> Flow<T1>.combineLatest(
    other: Flow<T2>,
    other2: Flow<T3>,
    transform: suspend (T1, T2, T3) -> R
): <ERROR CLASS>
fun <T1, T2, T3, T4, R> Flow<T1>.combineLatest(
    other: Flow<T2>,
    other2: Flow<T3>,
    other3: Flow<T4>,
    transform: suspend (T1, T2, T3, T4) -> R
): <ERROR CLASS>
fun <T1, T2, T3, T4, T5, R> Flow<T1>.combineLatest(
    other: Flow<T2>,
    other2: Flow<T3>,
    other3: Flow<T4>,
    other4: Flow<T5>,
    transform: suspend (T1, T2, T3, T4, T5) -> R
): Flow<R>

combineTransform

fun <T1, T2, R> Flow<T1>.combineTransform(
    flow: Flow<T2>,
    transform: suspend FlowCollector<R>.(a: T1, b: T2) -> Unit
): Flow<R>

Returns a Flow whose values are generated by transform function that process the most recently emitted values by each flow.

conflate

fun <T> StateFlow<T>.conflate(): Flow<T>

Returns this. Applying conflate operator to StateFlow has no effect. See StateFlow documentation on Operator Fusion.

count

suspend fun <T> Flow<T>.count(): Int

Returns the number of elements in this flow.

suspend fun <T> Flow<T>.count(
    predicate: suspend (T) -> Boolean
): Int

Returns the number of elements matching the given predicate.

debounce

fun <T> Flow<T>.debounce(timeoutMillis: Long): Flow<T>

Returns a flow that mirrors the original flow, but filters out values that are followed by the newer values within the given timeout. The latest value is always emitted.

fun <T> Flow<T>.debounce(timeout: <ERROR CLASS>): Flow<T>

Returns a flow that mirrors the original flow, but filters out values that are followed by the newer values within the given timeout. The latest value is always emitted.

distinctUntilChanged

fun <T> StateFlow<T>.distinctUntilChanged(): Flow<T>

Returns this. Applying distinctUntilChanged operator to StateFlow has no effect. See StateFlow documentation on Operator Fusion.

fun <T> Flow<T>.distinctUntilChanged(
    areEquivalent: (old: T, new: T) -> Boolean
): Flow<T>

Returns flow where all subsequent repetitions of the same value are filtered out, when compared with each other via the provided areEquivalent function.

distinctUntilChangedBy

fun <T, K> Flow<T>.distinctUntilChangedBy(
    keySelector: (T) -> K
): Flow<T>

Returns flow where all subsequent repetitions of the same key are filtered out, where key is extracted with keySelector function.

drop

fun <T> Flow<T>.drop(count: Int): Flow<T>

Returns a flow that ignores first count elements. Throws IllegalArgumentException if count is negative.

dropWhile

fun <T> Flow<T>.dropWhile(
    predicate: suspend (T) -> Boolean
): Flow<T>

Returns a flow containing all elements except first elements that satisfy the given predicate.

filter

fun <T> Flow<T>.filter(
    predicate: suspend (T) -> Boolean
): Flow<T>

Returns a flow containing only values of the original flow that matches the given predicate.

filterIsInstance

fun <R> Flow<*>.filterIsInstance(): Flow<R>

Returns a flow containing only values that are instances of specified type R.

filterNot

fun <T> Flow<T>.filterNot(
    predicate: suspend (T) -> Boolean
): Flow<T>

Returns a flow containing only values of the original flow that do not match the given predicate.

filterNotNull

fun <T : Any> Flow<T?>.filterNotNull(): Flow<T>

Returns a flow containing only values of the original flow that are not null.

first

suspend fun <T> Flow<T>.first(): T

The terminal operator that returns the first element emitted by the flow and then cancels flow’s collection. Throws NoSuchElementException if the flow was empty.

suspend fun <T> Flow<T>.first(
    predicate: suspend (T) -> Boolean
): T

The terminal operator that returns the first element emitted by the flow matching the given predicate and then cancels flow’s collection. Throws NoSuchElementException if the flow has not contained elements matching the predicate.

firstOrNull

suspend fun <T : Any> Flow<T>.firstOrNull(): T?

The terminal operator that returns the first element emitted by the flow and then cancels flow’s collection. Returns null if the flow was empty.

suspend fun <T : Any> Flow<T>.firstOrNull(
    predicate: suspend (T) -> Boolean
): T?

The terminal operator that returns the first element emitted by the flow matching the given predicate and then cancels flow’s collection. Returns null if the flow did not contain an element matching the predicate.

flatMapConcat

fun <T, R> Flow<T>.flatMapConcat(
    transform: suspend (value: T) -> Flow<R>
): Flow<R>

Transforms elements emitted by the original flow by applying transform, that returns another flow, and then concatenating and flattening these flows.

flatMapLatest

fun <T, R> Flow<T>.flatMapLatest(
    transform: suspend (value: T) -> Flow<R>
): Flow<R>

Returns a flow that switches to a new flow produced by transform function every time the original flow emits a value. When the original flow emits a new value, the previous flow produced by transform block is cancelled.

flatMapMerge

fun <T, R> Flow<T>.flatMapMerge(
    concurrency: Int = DEFAULT_CONCURRENCY,
    transform: suspend (value: T) -> Flow<R>
): Flow<R>

Transforms elements emitted by the original flow by applying transform, that returns another flow, and then merging and flattening these flows.

flowOn

fun <T> StateFlow<T>.flowOn(
    context: CoroutineContext
): Flow<T>

Returns this. Applying flowOn operator to StateFlow has no effect. See StateFlow documentation on Operator Fusion.

flowWith

fun <T, R> Flow<T>.flowWith(
    flowContext: CoroutineContext,
    bufferSize: Int = BUFFERED,
    builder: Flow<T>.() -> Flow<R>
): Flow<R>

The operator that changes the context where all transformations applied to the given flow within a builder are executed. This operator is context preserving and does not affect the context of the preceding and subsequent operations.

fold

suspend fun <T, R> Flow<T>.fold(
    initial: R,
    operation: suspend (acc: R, value: T) -> R
): R

Accumulates value starting with initial value and applying operation current accumulator value and each element

launchIn

fun <T> Flow<T>.launchIn(scope: CoroutineScope): Job

Terminal flow operator that launches the collection of the given flow in the scope. It is a shorthand for scope.launch { flow.collect() }.

map

fun <T, R> Flow<T>.map(
    transform: suspend (value: T) -> R
): Flow<R>

Returns a flow containing the results of applying the given transform function to each value of the original flow.

mapLatest

fun <T, R> Flow<T>.mapLatest(
    transform: suspend (value: T) -> R
): Flow<R>

Returns a flow that emits elements from the original flow transformed by transform function. When the original flow emits a new value, computation of the transform block for previous value is cancelled.

mapNotNull

fun <T, R : Any> Flow<T>.mapNotNull(
    transform: suspend (value: T) -> R?
): Flow<R>

Returns a flow that contains only non-null results of applying the given transform function to each value of the original flow.

onCompletion

fun <T> Flow<T>.onCompletion(
    action: suspend FlowCollector<T>.(cause: Throwable?) -> Unit
): Flow<T>

Invokes the given action when the given flow is completed or cancelled, passing the cancellation exception or failure as cause parameter of action.

onEach

fun <T> Flow<T>.onEach(action: suspend (T) -> Unit): Flow<T>

Returns a flow which performs the given action on each value of the original flow.

onEmpty

fun <T> Flow<T>.onEmpty(
    action: suspend FlowCollector<T>.() -> Unit
): Flow<T>

Invokes the given action when this flow completes without emitting any elements. The receiver of the action is FlowCollector, so onEmpty can emit additional elements. For example:

onStart

fun <T> Flow<T>.onStart(
    action: suspend FlowCollector<T>.() -> Unit
): Flow<T>

Invokes the given action when this flow starts to be collected.

produceIn

fun <T> Flow<T>.produceIn(
    scope: CoroutineScope
): ReceiveChannel<T>

Creates a produce coroutine that collects the given flow.

reduce

suspend fun <S, T : S> Flow<T>.reduce(
    operation: suspend (accumulator: S, value: T) -> S
): S

Accumulates value starting with the first element and applying operation to current accumulator value and each element. Throws NoSuchElementException if flow was empty.

retry

fun <T> Flow<T>.retry(
    retries: Long = Long.MAX_VALUE,
    predicate: suspend (cause: Throwable) -> Boolean = { true }
): Flow<T>

Retries collection of the given flow up to retries times when an exception that matches the given predicate occurs in the upstream flow. This operator is transparent to exceptions that occur in downstream flow and does not retry on exceptions that are thrown to cancel the flow.

fun <T> Flow<T>.retry(
    retries: Int = Int.MAX_VALUE,
    predicate: (Throwable) -> Boolean = { true }
): Flow<T>

retryWhen

fun <T> Flow<T>.retryWhen(
    predicate: suspend FlowCollector<T>.(cause: Throwable, attempt: Long) -> Boolean
): Flow<T>

Retries collection of the given flow when an exception occurs in the upstream flow and the predicate returns true. The predicate also receives an attempt number as parameter, starting from zero on the initial call. This operator is transparent to exceptions that occur in downstream flow and does not retry on exceptions that are thrown to cancel the flow.

sample

fun <T> Flow<T>.sample(periodMillis: Long): Flow<T>

Returns a flow that emits only the latest value emitted by the original flow during the given sampling period.

fun <T> Flow<T>.sample(period: <ERROR CLASS>): Flow<T>

Returns a flow that emits only the latest value emitted by the original flow during the given sampling period.

scan

fun <T, R> Flow<T>.scan(
    initial: R,
    operation: suspend (accumulator: R, value: T) -> R
): Flow<R>

Folds the given flow with operation, emitting every intermediate result, including initial value. Note that initial value should be immutable (or should not be mutated) as it is shared between different collectors. For example:

scanReduce

fun <T> Flow<T>.scanReduce(
    operation: suspend (accumulator: T, value: T) -> T
): Flow<T>

Reduces the given flow with operation, emitting every intermediate result, including initial value. The first element is taken as initial value for operation accumulator. This operator has a sibling with initial value – scan.

single

suspend fun <T> Flow<T>.single(): T

The terminal operator, that awaits for one and only one value to be published. Throws NoSuchElementException for empty flow and IllegalStateException for flow that contains more than one element.

singleOrNull

suspend fun <T : Any> Flow<T>.singleOrNull(): T?

The terminal operator, that awaits for one and only one value to be published. Throws IllegalStateException for flow that contains more than one element.

switchMap

fun <T, R> Flow<T>.switchMap(
    transform: suspend (value: T) -> Flow<R>
): Flow<R>

take

fun <T> Flow<T>.take(count: Int): Flow<T>

Returns a flow that contains first count elements. When count elements are consumed, the original flow is cancelled. Throws IllegalArgumentException if count is not positive.

takeWhile

fun <T> Flow<T>.takeWhile(
    predicate: suspend (T) -> Boolean
): Flow<T>

Returns a flow that contains first elements satisfying the given predicate.

toCollection

suspend fun <T, C : MutableCollection<in T>> Flow<T>.toCollection(
    destination: C
): C

Collects given flow into a destination

toList

suspend fun <T> Flow<T>.toList(
    destination: MutableList<T> = ArrayList()
): List<T>

Collects given flow into a destination

toSet

suspend fun <T> Flow<T>.toSet(
    destination: MutableSet<T> = LinkedHashSet()
): Set<T>

Collects given flow into a destination

transform

fun <T, R> Flow<T>.transform(
    transform: suspend FlowCollector<R>.(value: T) -> Unit
): Flow<R>

Applies transform function to each value of the given flow.

transformLatest

fun <T, R> Flow<T>.transformLatest(
    transform: suspend FlowCollector<R>.(value: T) -> Unit
): Flow<R>

Returns a flow that produces element by transform function every time the original flow emits a value. When the original flow emits a new value, the previous transform block is cancelled, thus the name transformLatest.

withIndex

fun <T> Flow<T>.withIndex(): Flow<IndexedValue<T>>

Returns a flow that wraps each element into IndexedValue, containing value and its index (starting from zero).

zip

fun <T1, T2, R> Flow<T1>.zip(
    other: Flow<T2>,
    transform: suspend (T1, T2) -> R
): Flow<R>

Zips values from the current flow (this) with other flow using provided transform function applied to each pair of values. The resulting flow completes as soon as one of the flows completes and cancel is called on the remaining flow.

Inheritors

MutableStateFlow

interface MutableStateFlow<T> : StateFlow<T>

A mutable StateFlow that provides a setter for value.