Kotlin Testing

This file extends common/testing.md with Kotlin and Android/KMP-specific content.

Test Framework

kotlin.test for multiplatform (KMP) — @Test, assertEquals, assertTrue
JUnit 4/5 for Android-specific tests
Turbine for testing Flows and StateFlow
kotlinx-coroutines-test for coroutine testing (runTest, TestDispatcher)

ViewModel Testing with Turbine

@Test
fun `loading state emitted then data`() = runTest {
    val repo = FakeItemRepository()
    repo.addItem(testItem)
    val viewModel = ItemListViewModel(GetItemsUseCase(repo))

    viewModel.state.test {
        assertEquals(ItemListState(), awaitItem())     // initial state
        viewModel.onEvent(ItemListEvent.Load)
        assertTrue(awaitItem().isLoading)               // loading
        assertEquals(listOf(testItem), awaitItem().items) // loaded
    }
}

Fakes Over Mocks

Prefer hand-written fakes over mocking frameworks:

class FakeItemRepository : ItemRepository {
    private val items = mutableListOf<Item>()
    var fetchError: Throwable? = null

    override suspend fun getAll(): Result<List<Item>> {
        fetchError?.let { return Result.failure(it) }
        return Result.success(items.toList())
    }

    override fun observeAll(): Flow<List<Item>> = flowOf(items.toList())

    fun addItem(item: Item) { items.add(item) }
}

Coroutine Testing

@Test
fun `parallel operations complete`() = runTest {
    val repo = FakeRepository()
    val result = loadDashboard(repo)
    advanceUntilIdle()
    assertNotNull(result.items)
    assertNotNull(result.stats)
}

Use runTest — it auto-advances virtual time and provides TestScope.

Ktor MockEngine

val mockEngine = MockEngine { request ->
    when (request.url.encodedPath) {
        "/api/items" -> respond(
            content = Json.encodeToString(testItems),
            headers = headersOf(HttpHeaders.ContentType, ContentType.Application.Json.toString())
        )
        else -> respondError(HttpStatusCode.NotFound)
    }
}

val client = HttpClient(mockEngine) {
    install(ContentNegotiation) { json() }
}

Room/SQLDelight Testing

Room: Use Room.inMemoryDatabaseBuilder() for in-memory testing
SQLDelight: Use JdbcSqliteDriver(JdbcSqliteDriver.IN_MEMORY) for JVM tests

@Test
fun `insert and query items`() = runTest {
    val driver = JdbcSqliteDriver(JdbcSqliteDriver.IN_MEMORY)
    Database.Schema.create(driver)
    val db = Database(driver)

    db.itemQueries.insert("1", "Sample Item", "description")
    val items = db.itemQueries.getAll().executeAsList()
    assertEquals(1, items.size)
}

Test Naming

Use backtick-quoted descriptive names:

@Test
fun `search with empty query returns all items`() = runTest { }

@Test
fun `delete item emits updated list without deleted item`() = runTest { }

Test Organization

src/
├── commonTest/kotlin/     # Shared tests (ViewModel, UseCase, Repository)
├── androidUnitTest/kotlin/ # Android unit tests (JUnit)
├── androidInstrumentedTest/kotlin/  # Instrumented tests (Room, UI)
└── iosTest/kotlin/        # iOS-specific tests

Minimum test coverage: ViewModel + UseCase for every feature.

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