Architecture & Standards

Testing — Android / KMP

Goal: fewer bugs at low cost. Test the logic that breaks, skip the trivial.

1. What to test (priority order)

Must test (high value, cheap)	Skip (low value)
Reducers — pure `(state, intent) → state`, every branch	Composable pixels (use previews instead)
Use cases — business rules, success + failure paths	Getters / data class equality
Mappers / validators — every edge case	Framework/DI wiring
Conflict resolution & offline queue logic	Third-party libs
Repository impls — local read/write + enqueue (with fakes)

Reducers are pure → testing them is trivial and catches most state bugs. Aim 100% branch coverage on reducers, ~90% on use cases.

2. Patterns

Reducer (no coroutines needed — it's pure)

@Test
fun `QueryChanged filters visible items`() {
    val vm = CartViewModel(getTotal = FakeGetTotal(), dispatchers = TestDispatchers())
    vm.onIntent(CartIntent.Loaded(listOf(apple, banana)))

    vm.onIntent(CartIntent.QueryChanged("app"))

    assertThat(vm.state.value.visibleItems).containsExactly(apple)
}

Use case (runTest + fake repo)

@Test
fun `returns Failure when repo errors`() = runTest {
    val useCase = GetCartTotalUseCase(FakeCartRepository(error = AppError.Storage))
    val result = useCase(cartId)
    assertThat(result).isInstanceOf(AppResult.Failure::class.java)
}

Flow (Turbine)

repo.observeItems().test {
    assertThat(awaitItem()).isEqualTo(AppResult.Success(emptyList()))
    cancelAndIgnoreRemainingEvents()
}

3. Rules

Fakes over mocks. Hand-write Fake{Interface} in commonTest/.../fake/; a fake must honor the real contract (LSP).
AAA structure: Arrange / Act / Assert, one logical assertion per test.
Names: subject condition expected in backticks.
Deterministic: inject DispatcherProvider (test = UnconfinedTestDispatcher); inject a Clock — never System.currentTimeMillis() in code under test.
One behaviour per test. A failing test name should point at the bug.

4. Coverage targets (suggested, CI-enforced)

Area	Line	Branch
Reducers	95%	100%
Use cases	90%	85%
Mappers / validators	100%	100%
Crypto / security-critical	100%	100%
Everything else	70%	—

Stack: kotlin-test / JUnit, Turbine (Flow), Truth or kotlin-test assertions.