To test Coroutines running on Dispatchers.Main you need kotlinx-coroutines-test library.

💡 At the time of writing the kotlinx-coroutines-test library is marked as experimental. May have breaking changes.

Example ​

package com.example.android.kotlincoroutines.main

/* ...imports */

class MainViewModelTest {
	@get:Rule
	val coroutineScope = MainCoroutineScopeRule()

	@get:Rule
	val instantTaskExecutorRule = InstantTaskExecutorRule()

	lateinit var subject: MainViewModel

	@Before
	fun setup() {
		subject = MainViewModel(
			TitleRepository(
				MainNetworkFake("OK"),
				TitleDaoFake("initial")
			)
		)
	}

	@Test
	fun whenMainClicked_updatesTaps() {
		subject.onMainViewClicked()
		Truth.assertThat(subject.taps.getValueForTest()).isEqualTo("0 taps")
		coroutineScope.advanceTimeBy(1000)
		Truth.assertThat(subject.taps.getValueForTest()).isEqualTo("1 taps")
	}
}

Two rules are used to allow us to test MainViewModel in an off-device test.

In the setup method, a new instance of MainViewModel is created using testing fakes of network and database.

For this test, the fakes are only needed to satisfy the dependencies of MainViewModel.

By calling onMainViewClicked, the coroutine will be launched. This test checks that the taps text stays "0 taps" right after onMainViewClicked is called, then 1 second later it gets updated to "1 taps".

This test uses virtual-time to control the execution of the coroutine launched by onMainViewClicked thanks to the MainCoroutineScopeRule. Here we're calling advanceTimeBy(1_000) which will cause the main dispatcher to immediately execute coroutines that are scheduled to resume 1 second later.

This test is fully deterministic. Because it has full control over the execution of coroutines launched on the Dispatchers.Main it doesn't have to wait one second for the value to be set.