Exceptions being swallowed during tests / UncaughtExceptionsBeforeTest about kotlinx.coroutines

[dkhalanskyjb]

There's a potential for the documentation to be improved, but yes, it's not enough to replace the Main dispatcher with a TestDispatcher: someone has to actually execute the tasks on that dispatcher (that is, something should emulate the event loop of the main thread), and runTest is the one doing it. It's certainly not enough to just override the main dispatcher, even if your coroutines don't throw exceptions: some tasks spawned on the main thread just won't run.

In theory, you can use a TestDispatcher with a TestCoroutineScheduler separately from runTest, but this is a very advanced use case that requires a deep understanding, so their docs certainly should mention runTest.

[SnyersK]

Would there be a way to detect if a test is not using runTest when required, and throw an exception, so it's immediately clear that this is missing?

(Or an existing test needs to be updated, after a change)

I don't know if the TestDispatcher can perhaps check if it's running in a TestScope, for example?

[dkhalanskyjb]

Unfortunately, no: injecting a TestDispatcher before a TestScope exists or runTest is run is an important pattern. Dependency injection is typically initialized before the test is started.

[SnyersK]

Ah yes, I understand now.
Classes create their own / use another CoroutineScope.
There's no link between the TestScope and the coroutines launched from it's scope.

I've found that apparently there's a Detekt rule to check if tests which interact with coroutines make use of runTest.
Will need to try this out.

https://github.com/detekt/detekt/blob/f8bf79b24baebd10fda01ffe2ef586cf8bc1fff1/detekt-core/src/main/resources/default-detekt-config.yml#L175-L178

[SnyersK]

It's safe to assume that, as a general rule of thumb:

all of the tests in a test class which replaces the Main scheduler; and injects the TestDispatcher for all of the other dispatchers; should use runTest, to avoid running into potential (future) issues, even if the test itself is not directly interacting with a coroutine / using the time methods, right?

So even if a test just calls a method which is not launching any coroutine at all, it would be safer to just use runTest regardless?