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*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.

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

Hi, <@U3K0MNQR1> and <@U2E974ELT> , 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).
 
```

<@U72C3TBDH> If you care to contribute this implementation, PR to `kotlinx.coroutines` will be welcome.

<@U2E974ELT> Since this TestCoroutineContext is for JVM only, where in `kotlinx.coroutines` should this class be added?

`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

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

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