Combine two kotlin flows into a single flow that emits the latest value from the two original flows?

Question

If we have two flows defined like this:

val someflow = flow {
    emit("something")
}

and another flow defined like:

val stateFlow = MutableStateFlow("some value")

Is it possible to combine the two flows into a single flow that just emits the last value emitted by either someflow or stateFlow?

The idea is that stateFlow might emit a value at some point in the future, but until that happens I just want whatever value someflow last emitted. In the "combined" flow, I would like to just take the first value emitted by someflow but then be able to observe the rest of the updates on stateFlow.

It seems like this might be accomplished with a combine function, but I just want to emit the latest emitted value between the two flows, I don't care about what the last value of one flow was.

Answer 1

There is a flattenMerge function, which executes flows as a single flow, it might be what you need. For example:

val numbersFlow = flowOf("1", "2", "3", "4").onEach { delay(1000) }
val lettersFlow = flowOf("A", "B", "C").onEach { delay(2000) }
flowOf(numbersFlow, lettersFlow).flattenMerge().collect {
    println("Result $it")
}

Prints:

Result 1
Result A
Result 2
Result 3
Result B
Result 4
Result C

So for your case it would look something like:

flowOf(someFlow, stateFlow).flattenMerge().collect {
    println("Result $it")
}

Answer 2

You can use something like this:

fun <T> Flow<T>.merge(otherFlow: Flow<T>) = flow {
    this@merge.collect { value ->
        emit(value)
    }
    otherFlow.collect { value ->
        emit(value)
    }
}

Note that the resulting flow will only finish when both flows are completed/canceled.

Usage:

someFlow.merge(stateFlow).collect {
    println(it)
}