Hi all :android-wave: , I am trying to test my fun...

# coroutines

[Abhimanyu]

Hi all 👋 ,
I am trying to test my function that throws an exception from a coroutine.

private fun initViewModel() {
  viewModelScope.launch {
    getCurrentAccount() // Throws exception for certain conditions
  }
}

How to write UT for this method?

I can find assertFailsWith , that is mentioned in some blogs.
I am using junit = "4.13.2" .

[ross_a]

Launch the test with coroutine test's runTest, passing in either 'this' or 'backgroundScope' as the 'viewModelScope'

[Abhimanyu]

Hi @ross_a,
Thanks for the response.
I am already using testScope from runTest like this.

Test Code

@Test
fun `when initViewModel is called and currentAccountId is null, then IllegalStateException is thrown`() = runTest(
    timeout = 3.seconds,
    testBody = {
        val standardTestDispatcher = StandardTestDispatcher(
            scheduler = testScheduler,
        )
        val testScope = TestScope(
            context = standardTestDispatcher,
        )
        with(
            receiver = testScope,
        ) {
            // Mocks here
            
            val vm = MyViewModel(
                coroutineScope = coroutineScope,
                // Other dependencies here
            )

            assertThrows(IllegalStateException::class.java) {
                vm.initViewModel()
                advanceUntilIdle()
            }
        }
    },
)

I am overriding the viewModelScope as mentioned here.
I am following this to inject the Coroutine scope in the view model and it overrides the viewModelScope
https://developer.android.com/jetpack/androidx/releases/lifecycle#2.5.0

It is mentioned here that we can provide CoroutineScope as a parameter to ViewModel to override the viewModelScope .

[Dmitry Khalanskiy [JB]]

You will probably need this: https://kotlinlang.org/docs/exception-handling.html#coroutineexceptionhandler
Also, in general, coroutines launched in viewModelScope should not throw any exceptions, as this would crash the Android application.

[Abhimanyu]

In this specific use case, I am expecting the app to crash as this state should never occur.

Thanks I will check the shared doc. thank you color

[Abhimanyu]

Hi,
I tried using CoroutineExceptionHandler .
But, it is returning the following error.

A CoroutineExceptionHandler was passed to TestScope. Please pass it as an argument to a `launch` or `async` block on an already-created scope if uncaught exceptions require special treatment.
java.lang.IllegalArgumentException: A CoroutineExceptionHandler was passed to TestScope. Please pass it as an argument to a `launch` or `async` block on an already-created scope if uncaught exceptions require special treatment.
	at kotlinx.coroutines.test.TestScopeKt.TestScope(TestScope.kt:170)
	at com.makeappssimple.abhimanyu.coroutinesplayground.android.home.HomeScreenViewModelTest$crashApp$1.invokeSuspend(HomeScreenViewModelTest.kt:27)
	at com.makeappssimple.abhimanyu.coroutinesplayground.android.home.HomeScreenViewModelTest$crashApp$1.invoke(HomeScreenViewModelTest.kt)
	at com.makeappssimple.abhimanyu.coroutinesplayground.android.home.HomeScreenViewModelTest$crashApp$1.invoke(HomeScreenViewModelTest.kt)

To fix this, I changed the ViewModel method like this,

fun crashApp(
        context: CoroutineContext = EmptyCoroutineContext,
): Job {
        return viewModelScope.launch(context) {
            println("Throwing exception")
            throw IllegalStateException("Crash App Test")
        }
}

And test method like this,

@Test
fun crashApp() = runTest {
        val standardTestDispatcher: TestDispatcher = StandardTestDispatcher(
            scheduler = testScheduler,
        )
        val testScope = TestScope(
            context = standardTestDispatcher,
        )
        val homeScreenViewModel = HomeScreenViewModel(
            coroutineScope = testScope,
        )

        val coroutineExceptionHandler = CoroutineExceptionHandler { _, throwable ->
            println("Caught exception: $throwable")
        }

        val job = homeScreenViewModel.crashApp(
            context = coroutineExceptionHandler,
        )
        job.join()

        println("Test completed")
}

But, the CoroutineExceptionHandler is not catching the exception.

[Abhimanyu]

Pushed a minified example here for reference,
Test code - https://github.com/Abhimanyu14/coroutines-playground/blob/main/app/src/test/java/c[…]yu/coroutinesplayground/android/home/HomeScreenViewModelTest.kt
ViewModel code - https://github.com/Abhimanyu14/coroutines-playground/blob/main/app/src/main/java/c[…]imanyu/coroutinesplayground/android/home/HomeScreenViewModel.kt

[Dmitry Khalanskiy [JB]]

Where does the viewModelScope come from? If it has a normal Job and not SupervisorJob, then that Job is responsible for processing exceptions, and CoroutineExceptionHandler will not be invoked; CoroutineExceptionHandler is only for the exceptions not handled by structured concurrency.

[Abhimanyu]

viewModelScope is from the ViewModel that I am overriding like this,

@HiltViewModel
public class HomeScreenViewModel @Inject constructor(
    @ApplicationScope coroutineScope: CoroutineScope,
) : ViewModel(
    viewModelScope = coroutineScope,
) {
    public fun crashApp(
        context: CoroutineContext = EmptyCoroutineContext,
    ): Job {
        return viewModelScope.launch(context) {
            println("Throwing exception")
            throw IllegalStateException("Crash App Test")
        }
    }
}

[Dmitry Khalanskiy [JB]]

Ok, please try this:

@Test
fun crashApp() = runTest {
        val standardTestDispatcher: TestDispatcher = StandardTestDispatcher(
            scheduler = testScheduler,
        )
        val coroutineExceptionHandler = CoroutineExceptionHandler { _, throwable ->
            println("Caught exception: $throwable")
        }
        val testScope = CoroutineScope( // <--- not a TestScope!
            standardTestDispatcher + SupervisorJob() + coroutineExceptionHandler // <--- SupervisorJob
        )
        val homeScreenViewModel = HomeScreenViewModel(
            coroutineScope = testScope,
        )
        val job = homeScreenViewModel.crashApp()
        job.join()

        println("Test completed")
}

[Abhimanyu]

This works as expected. 😄 .

Did a couple of other changes as well to see how it behaves.

1. Using TestScope

val testScope = TestScope( // <--- TestScope!
  standardTestDispatcher + coroutineExceptionHandler + SupervisorJob() // <--- SupervisorJob
)

Error

A CoroutineExceptionHandler was passed to TestScope. Please pass it as an argument to a `launch` or `async` block on an already-created scope if uncaught exceptions require special treatment.

2. Without SupervisorJob

val testScope = CoroutineScope( // <--- not a TestScope!
  standardTestDispatcher + coroutineExceptionHandler
)

This also works.

[Abhimanyu]

Is the SupervisorJob recommended if the test works without it as well?

[Dmitry Khalanskiy [JB]]

It's recommended because a non-injected ViewModelScope also has a SupervisorJob, so with a SupervisorJob, you get the behavior that's closer to reality.

[Abhimanyu]

Cool.
Thanks for the help. thank you color