Android Testing

Android Testing

Android-specific testing on a test-first foundation. This reference focuses on the test-first discipline plus the Compose-test and KMP traps that are easy to get wrong, not the basics of the three tiers, fakes over mocks, runTest, or Given-When-Then naming — except where a name breaks one target's compile.

Test-first (the foundation)

• No production code without a failing test first. Write the test, watch it fail (RED), write the minimal code to pass (GREEN), then refactor. A behaviour you never watched fail isn't proven.
• A bug isn't fixed until a test that was red because of the bug is green.

This layers on top of any dedicated TDD discipline skill (superpowers:test-driven-development, ace:test-driven-development) but requires none. For bootstrapping the test stack from scratch (test DI, JUnit/Robolectric/Roborazzi/Paparazzi selection, the instrumented runner, Compose Preview Screenshot Testing, UI Automator, Jacoco), see Google's official testing-setup skill (android skills add testing-setup).

commonTest names must survive Kotlin/Native, not just the JVM

commonTest is compiled once per declared target, so a green jvmTest proves nothing about the iOS compile. Backticks themselves are fine on every target — they are escaping syntax, not part of the name — but Kotlin/Native's frontend rejects 24 characters inside them: . ; , ( ) [ ] { } / < > : \ $ & ~ * ? # | § % @ (FirNativeIdentifierChecker — an unconditional error since Kotlin 1.7; the set also reserves the IR mangler's own characters). Kotlin/JVM forbids only . ; [ ] / < > : \. The comma is the gap between the two lists; a space is in neither, so a Given-When-Then convention with commas fails every native target while CI stays green:

// commonTest — compiles for jvm; fails compileTestKotlinIosSimulatorArm64 with
//   error: name contains illegal characters: ","
@Test fun `given an empty cart, when an item is added, then the total updates`() { … }

// commonTest — same sentence, commas dropped. Backticks and spaces compile everywhere.
@Test fun `given an empty cart when an item is added then the total updates`() { … }

Two rules follow. Keep , . ( ) : ? out of every commonTest declaration name — the check covers classes, properties, type aliases, value parameters and enum entries, not just funs. And verify a KMP test with allTests or <target>Test, never jvmTest, which depends only on compileTestKotlinJvm; a device-only target has no test task at all, so compile its test binary explicitly (linkDebugTestIosArm64).

Don't confuse this with the Android rule Kotlin's coding conventions cite: D8 rejects spaces below minSdk 30, which bites instrumented tests only — and never commas, which the DEX SimpleName grammar has never allowed at any API level.

Compose tests now default to StandardTestDispatcher

createComposeRule() / runComposeUiTest {} default to StandardTestDispatcher (matching kotlinx.coroutines.test.runTest) — there is no separate "v2" package. It's gated by androidx.compose.ui.test.ComposeUiTestFlags.isStandardTestDispatcherSupportEnabled (defaults to true), so the regular androidx.compose.ui.test.junit4.createComposeRule already uses it. To pin a scheduler, pass it through: createComposeRule(effectContext = StandardTestDispatcher()). Under this default, a LaunchedEffect that previously ran eagerly (the old UnconfinedTestDispatcher behaviour) may need an explicit mainClock.advanceTimeBy(0) / runCurrent() to drain queued work; set the flag false only to temporarily restore the legacy behaviour.

Two-schedulers trap (the one coroutine-test gotcha worth stating): a MainDispatcherRule's TestDispatcher and the dispatcher runTest { } creates have separate TestCoroutineSchedulers. Pass mainRule.dispatcher into runTest(mainRule.dispatcher) so Dispatchers.Main and the test body share one — otherwise advanceUntilIdle() flushes only one and assertions race the ViewModel.