The Android coroutine testing <docs> say that if y...

# coroutines

[Chuck Stein]

The Android coroutine testing docs say that if you create your own TestScope (e.g. to inject into the class under test), then you must call runTest on that scope, because there can only be one TestScope instance in a test. But it doesn't say why? Doing this causes my runTest block to timeout and fail the test, because the class I injected the TestScope into uses it to shareIn, which never completes. So my question is what are the repercussions if I disregard this rule and instead call runTest(testScope.testScheduler) (which passes), rather than testScope.runTest as the docs suggest (which fails). Or, if I should really be using the recommended way, how do I get it to not time out for my use case?

[Dmitry Khalanskiy [JB]]

See https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-test/kotlinx.coroutines.test/-test-scope/background-scope.html for a proper way to cancel coroutines after the test finishes.

[Chuck Stein]

Thank you! So if I call testScope.runTest for my test case, but my class uses testScope.backgroundScope instead of the base testScope, that doesn't violate the guidelines of "only one TestScope for the whole test"?

Also, can you point me to what the downsides are if one violates this rule?

[Dmitry Khalanskiy [JB]]

backgroundScope is not a TestScope, so the rule is not violated.

For now, the downsides are, you will lose the exceptions that happen in coroutines in a TestScope on which runTest is not called. Further down the road, new downsides may appear. We simply don't support the case of more than one TestScope.