How unit test multithreaded Android RxJava

You can write your own ThreadFactory

ThreadFactory custom = new CustomThreadFactory();
ExecutorService executorService = Executors.newCachedThreadPool(custom); //or use newSingleThreadExecutor(..)
Scheduler customScheduler = Schedulers.from(executorService);

now you can use this scheduler and not block the main queue plus getting called when a new thread is needed:

class CustomThreadFactory implements ThreadFactory {
  public Thread lastT;
  public int newThreadCounter = 0;
  @Override
  public Thread newThread(Runnable r) {
    newThreadCounter++;
    System.out.println("newThread called");
    Thread lastT = new Thread(r); //or CustomThread(r)
    return lastT;
  }
}

You can even go further and instrument the new Thread -

 class CustomThread extends Thread {
    public CustomThread(Runnable r) {
      super(r);
    }
    @Override
    public void run() {
      System.out.printf("About to run!");
      super.run();
    }
  }
}

I suggest you to use RxUtil

1). Provide default implementation of RxUtil through the constructor or DI

2). When you create your observable, instead of applying schedulers directly:

    .subscribeOn(Schedulers.newThread())
    .observeOn(AndroidSchedulers.mainThread())

Use RxUtil:

.compose(rxUtil.applySchedulers())

Example:

https://github.com/DAYTeam/GoEnglish/blob/master/app/src/main/java/ru/goenglish/goenglish/services/impl/ScheduleServiceImpl.java#L38-L44

3). In unit tests, instead of default implementation of RxUtil, create mocked version:

public class UnitTestRxUtil implements RxUtil {
    @Override
    public <T> Observable.Transformer<T, T> applySchedulers() {
        return observable -> observable.subscribeOn(Schedulers.immediate())
                .observeOn(Schedulers.immediate());
    }
}

link: https://github.com/DAYTeam/GoEnglish/blob/master/app/src/test/java/ru/goenglish/goenglish/util/UnitTestRxUtil.java

4). Pass this implementation through the constructor, or DI.
Example (constructor): https://github.com/DAYTeam/GoEnglish/blob/master/app/src/test/java/ru/goenglish/goenglish/ScheduleServiceImplTest.java#L45

As a result, all the tests, will be executed in one thread, and in the application, it will be executed on different executors