Estou usando o RxJava para calcular a correlação automática normalizada em alguns dados do sensor no Android. Estranhamente, meu código gera uma exceção ("java.lang.IllegalStateException: apenas um assinante é permitido!") E não tenho certeza do que fazer: sei que GroupedObservables pode lançar essa exceção quando inscrito em vários assinantes, mas Eu não acho que estou usando uma coisa dessas em qualquer lugar.

Abaixo, você encontra o método que (provavelmente) aciona a exceção:

public Observable<Float> normalizedAutoCorrelation(Observable<Float> observable, final int lag) {
    Observable<Float> laggedObservable = observable.skip(lag);

    Observable<Float> meanObservable = mean(observable, lag);
    Observable<Float> laggedMeanObservable = mean(laggedObservable, lag);

    Observable<Float> standardDeviationObservable = standardDeviation(observable, meanObservable, lag);
    Observable<Float> laggedStandardDeviationObservable = standardDeviation(laggedObservable, laggedMeanObservable, lag);

    Observable<Float> deviation = observable.zipWith(meanObservable, new Func2<Float, Float, Float>() {
        @Override
        public Float call(Float value, Float mean) {
            return value - mean;
        }
    });

    Observable<Float> laggedDeviation = observable.zipWith(laggedMeanObservable, new Func2<Float, Float, Float>() {
        @Override
        public Float call(Float value, Float mean) {
            return value - mean;
        }
    });

    Observable<Float> autoCorrelationPartObservable = deviation.zipWith(laggedDeviation, new Func2<Float, Float, Float>() {
        @Override
        public Float call(Float value, Float laggedValue) {
            return value * laggedValue;
        }
    });

    Observable<Float> autoCorrelationObservable = flatten(autoCorrelationPartObservable.window(lag, 1).scan(new Func2<Observable<Float>, Observable<Float>, Observable<Float>>() {
        @Override
        public Observable<Float> call(Observable<Float> memoObservable, Observable<Float> observable) {
            if(memoObservable == null) return observable;

            return memoObservable.zipWith(observable, new Func2<Float, Float, Float>() {
                @Override
                public Float call(Float memo, Float value) {
                    return memo + value;
                }
            });
        }
    }));

    Observable<Float> normalizationObservable = standardDeviationObservable.zipWith(laggedStandardDeviationObservable, new Func2<Float, Float, Float>() {
        @Override
        public Float call(Float standardDeviation, Float laggedStandardDeviation) {
            return lag * standardDeviation * laggedStandardDeviation;
        }
    });

    return autoCorrelationObservable.zipWith(normalizationObservable, new Func2<Float, Float, Float>() {
        @Override
        public Float call(Float autoCorrelation, Float normalization) {
            return autoCorrelation / normalization;
        }
    });
}

E este é o stacktrace que recebo:

java.lang.IllegalStateException: Only one subscriber allowed!
at rx.internal.operators.BufferUntilSubscriber$OnSubscribeAction.call(BufferUntilSubscriber.java:124)
at rx.internal.operators.BufferUntilSubscriber$OnSubscribeAction.call(BufferUntilSubscriber.java:81)
at rx.Observable.unsafeSubscribe(Observable.java:7304)
at rx.internal.operators.OperatorZip$Zip.start(OperatorZip.java:210)
at rx.internal.operators.OperatorZip$ZipSubscriber.onNext(OperatorZip.java:154)
at rx.internal.operators.OperatorZip$ZipSubscriber.onNext(OperatorZip.java:120)
at rx.internal.util.ScalarSynchronousObservable$1.call(ScalarSynchronousObservable.java:41)
at rx.internal.util.ScalarSynchronousObservable$1.call(ScalarSynchronousObservable.java:30)
at rx.Observable$1.call(Observable.java:145)
at rx.Observable$1.call(Observable.java:137)
at rx.Observable.unsafeSubscribe(Observable.java:7304)
at rx.internal.operators.OperatorMerge$MergeSubscriber.handleNewSource(OperatorMerge.java:188)
at rx.internal.operators.OperatorMerge$MergeSubscriber.onNext(OperatorMerge.java:158)
at rx.internal.operators.OperatorMerge$MergeSubscriber.onNext(OperatorMerge.java:93)
at rx.internal.operators.OperatorMap$1.onNext(OperatorMap.java:55)
at rx.internal.operators.OperatorScan$2.onNext(OperatorScan.java:110)
at rx.internal.operators.OperatorWindowWithSize$InexactSubscriber.onNext(OperatorWindowWithSize.java:173)
at rx.internal.operators.OperatorZip$Zip.tick(OperatorZip.java:255)
at rx.internal.operators.OperatorZip$Zip$InnerSubscriber.onNext(OperatorZip.java:326)
at rx.internal.operators.OperatorZip$Zip.tick(OperatorZip.java:255)
at rx.internal.operators.OperatorZip$Zip$InnerSubscriber.onNext(OperatorZip.java:326)
at rx.internal.operators.OperatorMerge$InnerSubscriber.emit(OperatorMerge.java:635)
at rx.internal.operators.OperatorMerge$InnerSubscriber.onNext(OperatorMerge.java:545)
  at rx.internal.operators.NotificationLite.accept(NotificationLite.java:150)
  at rx.internal.operators.TakeLastQueueProducer.emit(TakeLastQueueProducer.java:98)
  at rx.internal.operators.TakeLastQueueProducer.startEmitting(TakeLastQueueProducer.java:45)
  at rx.internal.operators.OperatorTakeLast$1.onCompleted(OperatorTakeLast.java:59)
  at rx.internal.operators.OperatorScan$2.onCompleted(OperatorScan.java:121)
  at rx.internal.operators.BufferUntilSubscriber.onCompleted(BufferUntilSubscriber.java:161)
  at rx.internal.operators.OperatorWindowWithSize$InexactSubscriber.onNext(OperatorWindowWithSize.java:183)
  at rx.internal.operators.OperatorSkip$1.onNext(OperatorSkip.java:58)
  at rx.internal.operators.OperatorMap$1.onNext(OperatorMap.java:55)
  at rx.subjects.SubjectSubscriptionManager$SubjectObserver.onNext(SubjectSubscriptionManager.java:224)
  at rx.subjects.PublishSubject.onNext(PublishSubject.java:121)
  at com.github.joopaue.smartphonesensing.SensorService$3.onSensorChanged(SensorService.java:102)
  at android.hardware.SystemSensorManager$SensorEventQueue.dispatchSensorEvent(SystemSensorManager.java:418)
  at android.os.MessageQueue.nativePollOnce(Native Method)
  at android.os.MessageQueue.next(MessageQueue.java:138)
  at android.os.Looper.loop(Looper.java:123)
  at android.app.ActivityThread.main(ActivityThread.java:5146)
  at java.lang.reflect.Method.invokeNative(Native Method)
  at java.lang.reflect.Method.invoke(Method.java:515)
  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:732)
  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:566)
  at dalvik.system.NativeStart.main(Native Method)

Acho que não estou fazendo nada de estranho aqui: alguns zips, reduções, digitalizações e mapas planos.

Estou perdendo algo completamente óbvio, existe alguma regra oculta que estou quebrando aqui ou é um bug no RxJava? Obrigado!

PS. Se falta algum código para você tirar suas conclusões, basta perguntar e eu postarei!