RxJava 2 overriding IO scheduler in unit test

Question

I'm trying to test the following RxKotlin/RxJava 2 code:

validate(data)
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .flatMap { ... }

I'm attempting to override the schedulers as follows:

// Runs before each test suite
RxJavaPlugins.setInitIoSchedulerHandler { Schedulers.trampoline() }
RxAndroidPlugins.setInitMainThreadSchedulerHandler { Schedulers.trampoline() }

However, I get the following error when running the test:

java.lang.ExceptionInInitializerError
...
Caused by: java.lang.NullPointerException: Scheduler Callable result can't be null
    at io.reactivex.internal.functions.ObjectHelper.requireNonNull(ObjectHelper.java:39)
    at io.reactivex.plugins.RxJavaPlugins.applyRequireNonNull(RxJavaPlugins.java:1317)
    at io.reactivex.plugins.RxJavaPlugins.initIoScheduler(RxJavaPlugins.java:306)
    at io.reactivex.schedulers.Schedulers.<clinit>(Schedulers.java:84)

Has anyone experienced this problem?

---

The test worked fine when using RxKotlin/RxJava 1 and the following scheduler overrides:

RxAndroidPlugins.getInstance().registerSchedulersHook(object : RxAndroidSchedulersHook() {
    override fun getMainThreadScheduler() = Schedulers.immediate()
})

RxJavaPlugins.getInstance().registerSchedulersHook(object : RxJavaSchedulersHook() {
    override fun getIOScheduler() = Schedulers.immediate()
})

Answer 1

I suggest you take a different approach and add a layer of abstraction to your schedulers. This guy has a nice article about it.

It would look something like this in Kotlin

interface SchedulerProvider {
    fun ui(): Scheduler
    fun computation(): Scheduler
    fun trampoline(): Scheduler
    fun newThread(): Scheduler
    fun io(): Scheduler 
}

And then you override that with your own implementation of SchedulerProvider:

class AppSchedulerProvider : SchedulerProvider {
    override fun ui(): Scheduler {
        return AndroidSchedulers.mainThread()
    }

    override fun computation(): Scheduler {
        return Schedulers.computation()
    }

    override fun trampoline(): Scheduler {
        return Schedulers.trampoline()
    }

    override fun newThread(): Scheduler {
        return Schedulers.newThread()
    }

    override fun io(): Scheduler {
        return Schedulers.io()
    }
}

And one for testing classes:

class TestSchedulerProvider : SchedulerProvider {
    override fun ui(): Scheduler {
        return Schedulers.trampoline()
    }

    override fun computation(): Scheduler {
        return Schedulers.trampoline()
    }

    override fun trampoline(): Scheduler {
        return Schedulers.trampoline()
    }

    override fun newThread(): Scheduler {
        return Schedulers.trampoline()
    }

    override fun io(): Scheduler {
        return Schedulers.trampoline()
    }
}

Your code would look like this where you call RxJava:

mCompositeDisposable.add(mDataManager.getQuote()
        .subscribeOn(mSchedulerProvider.io())
        .observeOn(mSchedulerProvider.ui())
        .subscribe(Consumer<Quote> {
...

And you'll just override your implementation of SchedulerProvider based on where you test it. Here's a sample project for reference, I am linking the test file that would use the testable-version of SchedulerProvider: https://github.com/Obaied/DingerQuotes/blob/master/app/src/test/java/com/obaied/dingerquotes/QuotePresenterTest.kt#L31

Answer 2

Figured it out! It had to do with the fact that in this code:

validate(data)
    .subscribeOn(Schedulers.io())
    .observeOn(AndroidSchedulers.mainThread())
    .flatMap { ... }

validate(data) was returning an Observable, which was emitting the following: emitter.onNext(null). Since RxJava 2 no longer accepts null values, flatMap was not getting called. I changed validate to return a Completable and updated the scheduler override to the following:

RxJavaPlugins.setIoSchedulerHandler { Schedulers.trampoline() }

Now the tests pass!

Answer 3

As an alternative to proposed solutions, this has been working fine for a while in my projects. You can use it in your test classes like this:

@get:Rule
val immediateSchedulersRule = ImmediateSchedulersRule()

And the class looks like this:

class ImmediateSchedulersRule : ExternalResource() {

    val immediateScheduler: Scheduler = object : Scheduler() {

        override fun createWorker() = ExecutorScheduler.ExecutorWorker(Executor { it.run() })

        // This prevents errors when scheduling a delay
        override fun scheduleDirect(run: Runnable, delay: Long, unit: TimeUnit): Disposable {
            return super.scheduleDirect(run, 0, unit)
        }

    }

    override fun before() {
        RxJavaPlugins.setIoSchedulerHandler { immediateScheduler }
        RxJavaPlugins.setComputationSchedulerHandler { immediateScheduler }
        RxJavaPlugins.setNewThreadSchedulerHandler { immediateScheduler }

        RxAndroidPlugins.setInitMainThreadSchedulerHandler { immediateScheduler }
        RxAndroidPlugins.setMainThreadSchedulerHandler { immediateScheduler }
    }

    override fun after() {
        RxJavaPlugins.reset()
    }

}

You can find a way to migrate from TestRule to ExternalResource here and get more info on testing RxJava 2 here.