Provide access to TestCoroutineScope's dispatcher

[ebrowne72]

I have a class where I inject both a CoroutineScope (such as MainScope) and a background dispatcher (such as Dispatchers.Default). I have a method which launches a coroutine in the scope and then uses withContext to switch to the background dispatcher, where it calls delay().

class ClassUnderTest(private val mainScope: CoroutineScope, private val bgDispatcher: CoroutineDispatcher {
    fun doSomething() {
        mainScope.launch {
            val data = withContext(bgDispatcher) {
                delay(1000)
                42
            }
            doSomethingWithData(data)
        }
    }
}

private val testBgDispatcher = TestCoroutineDispatcher()
private val testScope = TestCoroutineScope()
private lateinit var subject: ClassUnderTest

@BeforeEach
fun setUp() {
    subject = ClassUnderTest(testScope, testBgDispatcher)
}

@Test
fun testSomething() {
    subject.doSomething()
    testScope.advanceUntilIdle()
}

When I try to test this function I get an UncompletedCoroutinesError because the delay() doesn't advance when I call testScope.advanceUntilIdle().

As a workaround I could call testDispatcher.advanceUntilIdle(). A better workaround is to pass my TestCoroutineDispatcher to the TestCoroutineScope's constructor:

private val testBgDispatcher = TestCoroutineDispatcher()
private val testScope = TestCoroutineScope(testBgDispatcher)

However, I think the best solution would be to expose the TestCoroutineScope's dispatcher, so I can retrieve it and inject it in my class.

subject = ClassUnderTest(testScope, testScope.dispatcher)