Im struggling to understand why map and combine works differ kotlinlang #coroutines

Im struggling to understand why .map and .combine works differently with .stateIn in my tests The test testing .map works as expected, the one testing the combine does not.

class MyTest() : FunSpec({

    val dispatcher = TestCoroutineDispatcher()
    listeners(MainCoroutineListener(dispatcher), MtLogListener())

    class MyViewModel : ViewModel(){

        val x = MutableStateFlow(5)
        val y = MutableStateFlow(2)

        val doubleX = x.map {
            it * 2
        }.stateIn(
            viewModelScope,
            SharingStarted.Eagerly,
            null
        )

        val combinedFlow = x.combine(y){ xValue, yValue ->
            xValue * yValue
        }.stateIn(
            viewModelScope,
            SharingStarted.Eagerly,
            null
        )
    }

    val viewModel = MyViewModel()

    test("stateflow mapped"){
        viewModel.doubleX.value shouldBe 10
        viewModel.x.value = 9
        viewModel.doubleX.value shouldBe 18
    }

    test("combined"){
        viewModel.x.value = 10
        viewModel.y.value = 3
        viewModel.combinedFlow.value shouldBe 30
    }
}){
    override fun isolationMode(): IsolationMode = IsolationMode.InstancePerLeaf
}

Nick Allen

02/08/2022, 10:02 PM

TestCoroutineDispatcher

will immediately run top-level launched“ coroutines, but not nested coroutines (this is to avoid stack overflows).

combine

launches coroutines to collect from each

Flow

map

just calls the transform directly. Here the top-level coroutine is the

stateIn

coroutine collecting from the

combine/map

derived

Flow

. For

map

, that's all there is, no nested launching so it runs completely. For

combine

the nested coroutines to read from the combined `Flow`s are not run immediately. If you tell the dispatcher to runCurrent before checking the value, then it'll work. I'm assuming the Main dispatcher is setup correctly (it's not obvious to me from the snippet).

❤️ 1

🙌 1

3 Views

#coroutines Join Slack