combine(flow1, flow2) does not work instantly

Hi. I am writing a mini-framework with reactive properties. One of the core concepts in this framework is a computed property. This property recalculates its value automatically whenever values of some other properties are changed.

For example:

    var appleCount by state(0)
    var bananaCount by state(0)
    val fruitCount by computed(::appleCount, ::bananaCount) { a, b -> a + b }

    fun changeFruits() {
        appleCount = 2
        bananaCount = 3
        // fruitCount is 5 here
    }

My framework uses MutableStateFlow and combine under the hood. The problem is that it is not always works as expected. The computed value is not updated instantly in some cases. For example:

    fun changeFruits() {
        appleCount = 2
        bananaCount = 3
        appleCount = 20
        bananaCount = 30
        // fruitCount is 5 here, but 50 is expected
    }

I wrote a test to reproduce the problem:

    @Test
    fun `flows are combined instantly`() = runBlockingTest {
        val flow1 = MutableStateFlow(0)
        val flow2 = MutableStateFlow(0)
        val resultFlow = MutableStateFlow(0)

        val job = launch {
            combine(flow1, flow2) { v1, v2 ->
                v1 + v2
            }.collect {
                resultFlow.value = it
            }
        }

        flow1.value = 2
        flow2.value = 3
        flow1.value = 20
        flow2.value = 30

        assertEquals(50, resultFlow.value)
        job.cancel()
    }

In the real code I use Main.immediate dispatcher. That’s why I expect an instant recalculation.

Take a look, please. If it is an expected behavior, how can I solve my problem?

[elizarov]

There is no such guarantee. In simple cases it will, indeed, work seemingly synchronously. In complex nested cases, when you mutate a value inside another mutation (which has been already immedaitely-dispatched) it will not work this way.

[aartikov]

Thanks 👍