Kotlinx.coroutines: Test module shall provide ability to replace Dispatchers.IO and Dispatchers.Default

[elizarov]

This seems to be as useful for predictable tests as replacing Dispatchers.Main. See discussion in #242 and #890.

[elizarov]

Note, that since Dispatchers.IO is conceptually a "special view" of Dispatchers.Default (sharing the same thread pool) we might as well just provide an ability to replace Dispatchers.Default with a dispatcher that has an API for creating such views. It would work nicely when such an API is made public (see #261)

[yigit]

I think that fact (using same pool) is not very clear to average developer so I would prefer having them separate APIs to make them match the existing public API.
Unless of course kotlin provides a TestRule that does all of the requests in (#242)

something like:

@RunWith(AndroidJunit::class)
class Test {
    @Rule
    @JvmField
    val dispatchers = CustomDispatchersRule()
}

where CustomDispatchersRule
a) automatically registers to espresso (if possible), or at least implements IdlingResource interface (that is a separate artifact so taking that dependency seems to be OK)
b) provides await methods (dispatchers.awaitIdle(time : long, unit : TimeUnit)).

It could be handy for this class to receive a scheduled executor for further customization though i'm not sure if there is a clear use case for that (vs one that it can create).

[elizarov]

@yigit A test rule is definitely on the radar.

[hameno]

Any progress here? Wondering how I should test my code using withContext(Dispatchers.Default) while using runBlockingTest

[AOrobator]

For now you can probably just inject your Dispatcher so that you can swap it out for TestCoroutineDispatcher in the test

[CDRussell]

I wrote about using TestCoroutineDispatcher if it's of use to you:

https://craigrussell.io/2019/11/unit-testing-coroutine-suspend-functions-using-testcoroutinedispatcher/

[hameno]

@CDRussell Thanks, actually stumbled upon your blog and implemented that solution 👍

[shanling]

Wondering if there is any update on this issue? Just curious on timelines, if there are any.