ViewModel Unit Testing Guide - Turbine, MockK & TestDispatcher

Unit Testing ViewModels with Turbine and MockK

Write comprehensive ViewModel tests using modern Kotlin testing tools.

MainDispatcherRule Setup

@get:Rule
val mainDispatcherRule = MainDispatcherRule()

@Test
fun testUserLoading() = runTest {
    // Automatically uses TestDispatchers
}

MockK coEvery/coVerify

val mockRepository = mockk<UserRepository>()
coEvery { mockRepository.fetchUser(any()) } returns User(1, "Alice")

val viewModel = UserViewModel(mockRepository)

coVerify { mockRepository.fetchUser("123") }

Turbine Flow Testing

@Test
fun testUiStateFlow() = runTest {
    turbineScope {
        val uiStateTurbine = viewModel.uiState.testIn(backgroundScope)

        assertEquals(UiState.Loading, uiStateTurbine.awaitItem())
        assertEquals(UiState.Success(user), uiStateTurbine.awaitItem())
        uiStateTurbine.ensureAllEventsConsumed()
    }
}

StateFlow State Transitions

@Test
fun testStateTransitions() = runTest {
    viewModel.loadUser("123")

    advanceTimeBy(100) // Simulate time passage

    assertEquals(User(1, "Alice"), viewModel.user.value)
}

SavedStateHandle Testing

val savedStateHandle = SavedStateHandle(mapOf("userId" to "123"))
val viewModel = UserViewModel(
    repository = mockRepository,
    savedStateHandle = savedStateHandle
)

Test Structure

1. Setup mocks with coEvery
2. Create ViewModel with dependencies
3. Trigger action
4. Verify state changes with Turbine
5. Verify interactions with coVerify

Common Assertions

• awaitItem(): Get next emitted value
• awaitComplete(): Wait for completion
• ensureAllEventsConsumed(): Verify no leftover events
• skipItems(n): Skip n events

Best Practices

• Test state transitions, not implementation
• Use Turbine for Flow/StateFlow testing
• Mock external dependencies
• Test error cases and edge cases

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