Combine two state flows into new state flow - kotlin

Question

I have two state flows. Is it possible to combine them and get new state flow? Logically it should be possible because both state flows have initials values, but as I see combine function returns just Flow and not StateFlow.

Answer 1

You could use the combine operator, and then use the stateIn function for the Flow that results from that.
From the stateIn documentation in the kotlinx coroutines repository:
The stateIn function "Converts a cold Flow into a hot StateFlow that is started in the given coroutine scope, sharing the most recently emitted value from a single running instance of the upstream flow with multiple downstream subscribers."
It's signature, as of the time of writing this, is:
fun <T> Flow<T>.stateIn(
scope: CoroutineScope,
started: SharingStarted,
initialValue: T
): StateFlow<T> (source)
So you should be able to do whatever transformations you need to your Flows, including combining them, and then ultimately use stateIn to convert them back to a StateFlow.
It could look something like this (perhaps creating a Scrabble game point calculator):
val wordFlow = MutableStateFlow("Hi")
val pointFlow = MutableStateFlow(5)
val stateString = wordFlow.combine(pointFlow) { word, points ->
"$word is worth $points points"
}.stateIn(viewModelScope, SharingStarted.Eagerly, "Default is worth 0 points")
stateString would be of type StateFlow<String>, and you have successfully combined the two other StateFlows into one StateFlow.

Answer 2

So far I created function:
fun <T1, T2, R> combineState(
flow1: StateFlow<T1>,
flow2: StateFlow<T2>,
scope: CoroutineScope = GlobalScope,
sharingStarted: SharingStarted = SharingStarted.Eagerly,
transform: (T1, T2) -> R
): StateFlow<R> = combine(flow1, flow2) {
o1, o2 -> transform.invoke(o1, o2)
}.stateIn(scope, sharingStarted, transform.invoke(flow1.value, flow2.value))

Answer 3

Above mentioned solutions are using stateIn() with GlobalScope and policy as Eagerly which means these StateFlows will never stop being observed once created which can lead to the issues.
I already have mentioned details in this blog. Instead, create a separate class which derives a new StateFlow:
private class TransformedStateFlow<T>(
private val getValue: () -> T,
private val flow: Flow<T>
) : StateFlow<T> {
override val replayCache: List<T> get() = listOf(value)
override val value: T get() = getValue()
override suspend fun collect(collector: FlowCollector<T>): Nothing =
coroutineScope { flow.stateIn(this).collect(collector) }
}
/**
* Returns [StateFlow] from [flow] having initial value from calculation of [getValue]
*/
fun <T> stateFlow(
getValue: () -> T,
flow: Flow<T>
): StateFlow<T> = TransformedStateFlow(getValue, flow)
/**
* Combines all [stateFlows] and transforms them into another [StateFlow] with [transform]
*/
inline fun <reified T, R> combineStates(
vararg stateFlows: StateFlow<T>,
crossinline transform: (Array<T>) -> R
): StateFlow<R> = stateFlow(
getValue = { transform(stateFlows.map { it.value }.toTypedArray()) },
flow = combine(*stateFlows) { transform(it) }
)
/**
* Variant of [combineStates] for combining 3 state flows
*/
inline fun <reified T1, reified T2, reified T3, R> combineStates(
flow1: StateFlow<T1>,
flow2: StateFlow<T2>,
flow3: StateFlow<T3>,
crossinline transform: (T1, T2, T3) -> R
) = combineStates(flow1, flow2, flow3) { (t1, t2, t3) ->
transform(
t1 as T1,
t2 as T2,
t3 as T3
)
}
// Other variants for combining N StateFlows
After this, you can implement it in your use case. For example:
private val isLoading = MutableStateFlow(false)
private val loggedInUser = MutableStateFlow<User?>(null)
private val error = MutableStateFlow<String?>(null)
// Combining these states to form a LoginState
val state: StateFlow<LoginState> = combineStates(isLoading, loggedInUser, error) { loading, user, errorMessage ->
LoginState(loading, user, errorMessage)
}
This approach is safe than other mentioned approaches since it'll only listen to the StateFlow updates when actually it's being collected (inside consumer's Coroutine scope)

Answer 4

Similar solution to #Nikola Despotoski, but in a form of an extension function
/**
* Combines two [StateFlow]s into a single [StateFlow]
*/
fun <T1, T2, R> StateFlow<T1>.combineState(
flow2: StateFlow<T2>,
scope: CoroutineScope = GlobalScope,
sharingStarted: SharingStarted = SharingStarted.Eagerly,
transform: (T1, T2) -> R
): StateFlow<R> = combine(this, flow2) { o1, o2 -> transform.invoke(o1, o2) }
.stateIn(scope, sharingStarted, transform.invoke(this.value, flow2.value))

Answer 5

Combine n state flows
#Suppress("CHANGING_ARGUMENTS_EXECUTION_ORDER_FOR_NAMED_VARARGS")
inline fun <reified T, R> combineStateFlow(
vararg flows: StateFlow<T>,
scope: CoroutineScope = GlobalScope,
sharingStarted: SharingStarted = SharingStarted.Eagerly,
crossinline transform: (Array<T>) -> R
): StateFlow<R> = combine(flows = flows) {
transform.invoke(it)
}.stateIn(
scope = scope,
started = sharingStarted,
initialValue = transform.invoke(flows.map {
it.value
}.toTypedArray())
)
Using:
data class A(val a: String)
data class B(val b: Int)
private val test1 = MutableStateFlow(A("a"))
private val test2 = MutableStateFlow(B(2))
#Suppress("CHANGING_ARGUMENTS_EXECUTION_ORDER_FOR_NAMED_VARARGS")
private val _isValidForm = combineStateFlow(
flows = arrayOf(test1, test2),
scope = viewModelScope
) { combinedFlows: Array<Any> ->
combinedFlows.map {
val doSomething = when (it) {
is A -> true
is B -> false
else -> false
}
}
}
Gist

Answer 6

Use combine operator, it takes two flows and a transformation function to combine the results from both flows.
val int = MutableStateFlow(2)
val double = MutableStateFlow(1.8)
int.combine(double){ i, d ->
i + d
}.collect(::println)