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Donovan LaDuke - Developer

Turbine and the combine operator

Featured in Android Weekly Issue 582

When testing a Flow result from a combine operator using the Turbine library, you will find that the test appears to miss emissions when using awaitItem. If you want to understand more about the cause of the issue, check out this issue on the turbine project and this issue on the kotlinx.coroutines project. While there is no sure-fire solution, a workaround that I found was to add a brief delay in the flows being input to the combine operation during the test. This delay call is skipped when called in the runTest block but ensures the test call doesn't miss emissions. A quick example of a working test is below or see the whole example in this gist.

fun `this test succeeds`() = runTest {
    val number = (0..2).asFlow().onEach { delay(1) }
    val intro = listOf("Hello").asFlow()

    val combined = combine(intro, number) { one, two ->
        "$one $two"

    combined.test {
        assertEquals("Hello 0", awaitItem())
        assertEquals("Hello 1", awaitItem())
        assertEquals("Hello 2", awaitItem())

Now you can get back to writing your tests. Until next time, thanks!

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