How do you test code that depends on coroutines using runTest and test dispatchers?

Short answer

Using runTest, which automatically skips over time delays (delay skipping), and replacing Dispatchers.Main with a test dispatcher through Dispatchers.setMain.

Explanation

Without runTest, calling a function containing delay(10_000L) would make the test wait 10 real seconds. The runTest environment controls virtual time and automatically jumps the virtual clock. StandardTestDispatcher provides manual scheduling of tasks through advanceUntilIdle() or advanceTimeBy(), while UnconfinedTestDispatcher executes new tasks eagerly and immediately without waiting.

Code example

@OptIn(ExperimentalCoroutinesApi::class)
class CoroutineTest {
    private val testDispatcher = StandardTestDispatcher()

    @Before
    fun setUp() { Dispatchers.setMain(testDispatcher) }

    @After
    fun tearDown() { Dispatchers.resetMain() }

    @Test
    fun delayedOperation_completesImmediately() = runTest(testDispatcher) {
        val deferred = async { delay(5000L); "Done" }
        advanceUntilIdle() // Jumps through five virtual seconds in a fraction of a millisecond!
        assertEquals("Done", deferred.getCompleted())
    }
}

Common mistakes

• Forgetting to call Dispatchers.setMain at the start of tests that use viewModelScope, causing an IllegalStateException crash because the main thread's looper is missing.

Follow-up questions

• How do you test ViewModel logic using the MockK library and mocks?

Sources

• Android Developers — Testing Kotlin coroutines on Android