Kotlin coroutine unit test fails with "Module with the Main dispatcher had failed to initialize"

Question

While running unit test for kotlin suspend method which uses withContext(Dispatchers.Main) the test method fails with below exception:

My coroutine lib versions are kotlinx-coroutines-core:1.1.1 and kotlinx-coroutines-android:1.1.1

Example:

suspend fun methodToTest() {
        withContext(Dispatchers.Main) {
           doSomethingOnMainThread()
                val data = withContext(Dispatchers.IO) {
                    doSomethingOnIOThread()
                }
        }
    }

Also, when I remove the withContext(Dispatchers.Main) it works fine.

java.lang.IllegalStateException: Module with the Main dispatcher had failed to initialize. For tests Dispatchers.setMain from kotlinx-coroutines-test module can be used

at kotlinx.coroutines.internal.MissingMainCoroutineDispatcher.missing(MainDispatchers.kt:79)
at kotlinx.coroutines.internal.MissingMainCoroutineDispatcher.isDispatchNeeded(MainDispatchers.kt:54)
at kotlinx.coroutines.DispatchedKt.resumeCancellable(Dispatched.kt:373)
at kotlinx.coroutines.intrinsics.CancellableKt.startCoroutineCancellable(Cancellable.kt:25)
at kotlinx.coroutines.BuildersKt__Builders_commonKt.withContext(Builders.common.kt:152)
at kotlinx.coroutines.BuildersKt.withContext(Unknown Source)

Answer 1

When running tests e.g for ViewModel that launch coroutines you are most likely to fall into the following exception

java.lang.IllegalStateException: Module with the Main dispatcher had failed to initialize. For tests, Dispatchers.setMain from kotlinx-coroutines-test module can be used

The reason behind this is the lack of Looper.getMainLooper() on the testing environment which is present on a real application. To fix this you need to swap the Main dispatcher with TestCoroutineDispatcher

Make sure you have a coroutine-test dependency on your Gradle file

"org.jetbrains.kotlinx:kotlinx-coroutines-test:$coroutine_version"

SOLUTION 1 - Not scalable

Define the following on your test class -> Annotate your class with @ExperimentalCoroutinesApi

val dispatcher = TestCoroutineDispatcher()

@Before
fun setup() {
    Dispatchers.setMain(dispatcher)
}

@After
fun tearDown() {
    Dispatchers.resetMain()
}

Note: You can also pass Dispatchers.Main as constructor dependency for your repositories as CoroutineDispatcher in case you have one. It is recommended not to hardcode your dispatchers on repositories/viewmodels etc WATCH-THIS PLEASEEEEEEEE

Why not scalable: You will need to copy and paste the same code on each test class

SOLUTION 2 - Scalable [Use This - It is used by Google]

In this solution, you create the custom rule. Add a utility class on your test package

    @ExperimentalCoroutinesApi
    class MainCoroutineRule(
        private val dispatcher: TestCoroutineDispatcher = TestCoroutineDispatcher()
    ) : TestWatcher(), TestCoroutineScope by TestCoroutineScope(dispatcher) {
        override fun starting(description: Description?) {
            super.starting(description)
            Dispatchers.setMain(dispatcher)
        }
    
        override fun finished(description: Description?) {
            super.finished(description)
            cleanupTestCoroutines()
            Dispatchers.resetMain()
        }
    }

If you want explanations on the utility class above, refer to this CODE-LAB

On your test class just add this the following lines and you will be good to go:

    @get:Rule
    val coroutineRule = MainCoroutineRule()

I think you can see why this is scalable if you have a lot of test classes.

SOLUTION 3 [I hope you don't reach here]

You can also use Dispatchers.Unconfined LINK

A coroutine dispatcher that is not confined to any specific thread. It executes the initial continuation of a coroutine in the current call-frame and lets the coroutine resume in whatever thread that is used by the corresponding suspending function, without mandating any specific threading policy. Nested coroutines launched in this dispatcher form an event-loop to avoid stack overflows.

You can add it as follows

    @Before
    fun setup() {
        Dispatchers.setMain(Dispatchers.Unconfined)
    }
    
    @After
    fun tearDown() {
        Dispatchers.resetMain()
    }

Happy coding . . . .