RxJava：“java.lang.IllegalStateException：只允许一个订阅者！”

Question

我正在使用 RxJava 计算 Android 中某些传感器数据的标准化自动相关性。奇怪的是，我的代码抛出了一个异常（“java.lang.IllegalStateException：只允许一个订阅者！”），我不确定该怎么做：我知道 GroupedObservables 在订阅我的多个订阅者时可能会抛出这个异常，但是我不认为我在任何地方都在使用这样的东西。

您可以在下面找到（最有可能）触发异常的方法：

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;
        }
    });
}

这是我得到的堆栈跟踪：

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)

我认为我在这里没有做任何奇怪的事情：一些 zips、reduce、scans 和 flatmaps。

我是否遗漏了一些完全明显的东西，我在这里打破了一些隐藏的规则还是 RxJava 中的一个错误？谢谢！

PS。如果缺少某些代码让您无法得出结论，请询问，我会发布！

Answer 1

在 RxJava 中，运算符 groupBy 和 window 返回一个只能订阅一次的 observable，如果订阅，它们会将累积的内容重播给唯一的订阅者并切换到“热”模式。

这是在返回一个完全热的 observable 和风险缺失值之间的权衡，或者返回一个允许任何订阅者但无限期保留累积内容的无限重放 observable。

中间立场，即单个订阅者，先冷后热 observable 被认为是最不令人惊讶的行为，并且让开发人员可以选择应用更多的运算符并选择两个极端之间的任何点：

source.window(1, TimeUnit.SECONDS)
    .map(w -> w.publish())
    .doOnNext(w -> w.connect())
    .subscribe(...)

source.window(1, TimeUnit.SECONDS)
    .map(w -> w.cache())
    .subscribe(...)