Java – When observable is observed on AndroidSchedulers.mainThread(), Android JUnit tests block indefinitely

I’m writing a simple test that’s equivalent to:

Test fun testObservable() {
    val returnedObservable = Observable.create(object : Observable.OnSubscribe<String> {
        override fun call(t: Subscriber<in String>) {
            t.onNext("hello")
            t.onCompleted()
        }

}).subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())

val result = returnedObservable.toBlocking().first()
    assertEquals("hello", result)
}

When .observeOn(AndroidSchedulers.mainThread()) appears, the test blocks indefinitely on returnedObservable.toBlocking().first().

Is there a way to convert the observable to return the result?

returnedObservable is returned from a method call where .subscribeOn and .observeOn are already applied, so removing them is not an option.

I guess this is the error mentioned here: https://github.com/ReactiveX/RxAndroid/issues/50

By the way, why don’t you use RxKotlin?

Your example would look much better :

    val returnedObservable = observable<String> { subscriber ->
        subscriber.onNext("hello")
        subscriber.onCompleted()
    }
    .subscribeOn(Schedules.io())
    .observeOn(AndroidSchedulers.mainThread())