Q: How to test time-dependent code with coroutines? There is a very nice `TestScheduler` in Rx, maki...

# coroutines

[dsgryazin]

Q: How to test time-dependent code with coroutines?
There is a very nice TestScheduler in Rx, making time shifting in test a breeze.
I'd expect a special CoroutineDispatcher for testing reasons.
I've looked through the tests in coroutines-core repository, but did not find examples there. Moreover it's full of realtime delays delay(100).
The case is testing UI-sensitive code, where delays can be impermissibly long.

[elizarov]

There is one internally but nothing public/stable at the moment.

[streetsofboston]

Hi, @dsgryazin and @elizarov , I wrote something like that for our projects' unit-tests:
https://gist.github.com/streetsofboston/6ea225c61566e6d349883082fbb9f020

This [CoroutineContext] dispatcher can be used to simulate virtual time to speed up
 code, especially tests, that deal with delays and timeouts.

 Specify an instance of this context when calling the *non-blocking* [kotlinx.coroutines.experimental.launch]
 or [kotlinx.coroutines.experimental.async] and then advance time or trigger the actions
 to make the co-routines execute.

 This works much like the *TestScheduler* in RxJava, which allows to speed up tests that deal
 with non-blocking Rx chains that contain delays or timeouts.

 This dispatcher can also handle *blocking* coroutines that are started by
 [kotlinx.coroutines.experimental.runBlocking]. This dispatcher's virtual time will be automatically
 advanced based based on the delayed actions within the coroutine(s).

[elizarov]

@streetsofboston If you care to contribute this implementation, PR to kotlinx.coroutines will be welcome.

[streetsofboston]

@elizarov Since this TestCoroutineContext is for JVM only, where in kotlinx.coroutines should this class be added?

[elizarov]

kotlinx-coroutines-core module is for JVM. The package does not matter much, as different modules can have slightly varying APIs in the same packages. Moreover, I don't see a big problem to port this class to other platforms in the future, too

[streetsofboston]

Thanks! I’ll put it in a package with the name test (like the TestScheduler on Rx Java 2).

[elizarov]

ok

[streetsofboston]

I put in the PR: https://github.com/Kotlin/kotlinx.coroutines/pull/297