MainCoroutineRule

MainCoroutineRule.kt

@ExperimentalCoroutinesApi
class MainCoroutineRule(
    val testDispatcher: TestCoroutineDispatcher = TestCoroutineDispatcher()
) : TestWatcher() {

    override fun starting(description: Description?) {
        super.starting(description)
        Dispatchers.setMain(testDispatcher)
    }

    override fun finished(description: Description?) {
        super.finished(description)
        Dispatchers.resetMain()
        testDispatcher.cleanupTestCoroutines()
    }
    
    fun runBlockingTest(
        block: suspend TestCoroutineScope.() -> Unit
    ): Unit = testDispatcher.runBlockingTest(block)
}

TestExample.kt

@ExperimentalCoroutinesApi
@RunWith(AndroidJUnit4::class)
class UseCaseTest {

    @get:Rule
    val rule = MainCoroutineRule()

    lateinit var useCase: UseCase

    @Before
    fun setup() {
        useCase = UseCase(
            dispatcher = rule.testDispatcher
        )
    }
}