Coroutine Testing in Kotlin - runTest, Turbine, and TestDispatchers (v2)

#android #kotlin #compose #tutorial

Testing coroutines requires special handling to manage time and control execution flow. Learn how to use runTest, Turbine, and TestDispatcher for robust async tests.

Basic Coroutine Testing with runTest

The runTest builder automatically advances virtual time and manages dispatcher scope:

@Test
fun testFlowEmission() = runTest {
    val flow = flowOf(1, 2, 3)
    val results = mutableListOf<Int>()

    flow.collect { results.add(it) }

    assertEquals(listOf(1, 2, 3), results)
}

Testing Delayed Emissions with advanceTimeBy

Control virtual time progression to test delays:

@Test
fun testDelayedEmission() = runTest {
    val flow = flow {
        emit(1)
        delay(1000)
        emit(2)
    }

    val results = mutableListOf<Int>()
    launch {
        flow.collect { results.add(it) }
    }

    advanceTimeBy(500)
    assertEquals(listOf(1), results)

    advanceUntilIdle()
    assertEquals(listOf(1, 2), results)
}

Advanced Testing with Turbine

Turbine simplifies Flow testing with expressive assertions:

@Test
fun testFlowWithTurbine() = runTest {
    val viewModel = MyViewModel()

    viewModel.uiState.test {
        awaitItem().apply {
            assertEquals("Loading", status)
        }
        advanceUntilIdle()
        awaitItem().apply {
            assertEquals("Success", status)
        }
        expectNoEvents()
    }
}

Custom TestDispatcher Setup

Create scoped test dispatchers for complex scenarios:

val testDispatcher = StandardTestDispatcher()
val testScope = TestScope(testDispatcher)

@Test
fun testWithCustomDispatcher() {
    testScope.runTest {
        val job = launch {
            delay(100)
            println("Done")
        }
        advanceUntilIdle()
        job.join()
    }
}