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

# coroutines

[Greg Rynkowski]

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?

[ephemient]

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

[Albert Chang]

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