Hi. I'm trying to understand basics of testing hot Flows when using

runTest

. Have a look at the two tests below. The first uses runBlockingTest, the other runTest.

@Test
fun `test shared flow with deferred`() = runBlockingTest {
    val sharedFlow = MutableSharedFlow<Int>(replay = 0)
    val deferred = async { sharedFlow.first() }
    sharedFlow.emit(1)
    assertEquals(1, deferred.await())
}

@Test
fun `test shared flow with deferred - runTest`() = runTest {
    val sharedFlow = MutableSharedFlow<Int>(replay = 0)
    val deferred = async { sharedFlow.first() }
    sharedFlow.emit(1)
    assertEquals(1, deferred.await())
}

Why the second one never finish?

the contents of

async {}

will not have run yet in `runTest`; use

runCurrent()

between it and

.emit()

or

async(start = CoroutineStart.UNDISPATCHED)

may work as well

You can also use

runTest(UnconfinedTestDispatcher())

. https://github.com/Kotlin/kotlinx.coroutines/blob/master/kotlinx-coroutines-test/MIGRATION.md#replace-runblockingtest-with-runtestunconfinedtestdispatcher https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/-unconfined-test-dispatcher.html