Testing Coroutines With runTest — Virtual Time Without Wall Clock

runTest is the only correct entry point for testing suspend functions and viewModelScope-backed code on the JVM. This skill nails down the TestScope contract, the two TestDispatcher flavors, the shared TestCoroutineScheduler virtual clock, and the MainDispatcherRule plumbing that keeps Dispatchers.setMain from leaking between tests. Flow-specific assertions live in ../testing-flows-with-turbine/SKILL.md.

When to use this skill

• The class under test exposes a suspend fun or launches into viewModelScope / lifecycleScope.
• The developer reaches for runBlocking { … } and the test hangs, or a 10-minute delay makes the test slow.
• A ViewModel emits Loading -> Success from a coroutine and the test needs to assert each intermediate state.
• The build emits the deprecation runBlockingTest is deprecated. Use runTest or runTest(... dispatchTimeoutMs = ...) errors.
• The test fails with IllegalStateException: Module with the Main dispatcher is missing because viewModelScope routed work through Dispatchers.Main.
• The developer needs to advance virtual time (advanceTimeBy(5.seconds), advanceUntilIdle()) to trigger a timeout / retry / debounce.

When NOT to use this skill

• The test asserts Flow emissions across time. Use ../testing-flows-with-turbine/SKILL.md (Turbine handles cancellation, hot vs cold, awaitItem/awaitComplete).
• The test exercises Compose's MainTestClock rather than kotlinx.coroutines.test. Use ../../../compose/synchronization/controlling-the-test-clock/SKILL.md.
• The test runs on an emulator/device through AndroidJUnit4. JUnit4 setup itself is ../../runner/configuring-junit4-on-android/SKILL.md.