What's the proper way to test if a flow collector received a value emitted by a sharedflow? using `...

# coroutines

[Brendan Campbell-hartzell]

What's the proper way to test if a flow collector received a value emitted by a sharedflow? using runTest doesn't seem to wait for a collector running in another scope to fully process the emitted value before returning. Keeping everything in the runTest scope and context just hangs the test because Flow.collect on a sharedflow never returns.

[Trevor Stone]

https://github.com/cashapp/turbine might help?

[Trevor Stone]

https://github.com/cashapp/turbine#hot-flows more specifically

[Brendan Campbell-hartzell]

I found this article: https://jeff-padgett.medium.com/unit-testing-mutablesharedflow-12aec5b6e05

Which was a bit outdated because it used runBlockingTest. I tried it with:

@Test
    fun `does this even work`() {
        val emitter = MutableSharedFlow<String>()
        var emitted: String? = null
        runTest {
            val job = launch {
                emitter.collect { emitted = it }
            }
            emitter.emit("new value")

            job.cancel()
        }
        assertEquals("new value", emitted)
    }

But this test failed.

Upon further investigation, replacing runTest with runTest(UnconfinedTestDispatcher()) (to get similar behavior as runBlockingTest) is what made it pass. Fingers crossed this remains consistent.

[Trevor Stone]

That test will fail most likely due to the job being canceled before the block inside of launch is called

[Trevor Stone]

you could test by adding a delay in between the emit and the cancel to verify

[Brendan Campbell-hartzell]

That's what was happening with runTest on its own, but the UnconfinedTestDispatcher "eagerly enters all launch calls," so it actually behaves as I'd like it to.

[Trevor Stone]

launch(start = CoroutineStart.UNDISPATCHED)

could also work more how you’re expecting

[Brendan Campbell-hartzell]

True, though, my actual use case is a bit more complicated than the simplified example. The launch call is happing in production code, so I'm getting it to pass by providing the production code with a coroutine scope that will behave the way I need 🙂