UndeliverableException while calling onError of ObservableEmitter in RXjava2

Question

I have a method which creates an emitter like below, there are a problem(maybe it is normal behavior) with calling onError in retrofit callback. I got UndeliverableException when try to call onError.

I can solve this by checking subscriber.isDiposed() by I wonder how can call onError coz i need to notify my UI level.

• Addition 1

--> RxJava2CallAdapterFactoryalready implemented   
private static Retrofit.Builder builderSwift = new Retrofit.Builder()
           .baseUrl(URL_SWIFT)
           .addCallAdapterFactory(RxJava2CallAdapterFactory.create())
           .addConverterFactory(GsonConverterFactory.create())
           .addConverterFactory(new ToStringConverterFactory());

--> When i added below code to application class app won't crash
--> but i get java.lang.exception instead of my custom exception  

RxJavaPlugins.setErrorHandler(Functions<Throwable>emptyConsumer());

@Override
    public void onFileUploadError(Throwable e) {
        Log.d(TAG, "onFileUploadError: " + e.getMessage());
    }

public Observable<UploadResponseBean> upload(final UploadRequestBean uploadRequestBean, final File file) {
    return Observable.create(new ObservableOnSubscribe<UploadResponseBean>() {
        @Override
        public void subscribe(@NonNull final ObservableEmitter<UploadResponseBean> subscriber) throws Exception {

                 // ---> There are no problem with subscriber while calling onError        

                // ---> Retrofit2 service request 
                ftsService.upload(token, uploadRequestBean, body).enqueue(new Callback<UploadResponseBean>() {
                    @Override
                    public void onResponse(Call<UploadResponseBean> call, Response<UploadResponseBean> response) {

                        if (response.code() == 200){
                            // --->  calling onNext works properly
                            subscriber.onNext(new UploadResponseBean(response.body().getUrl()));
                        }
                        else{
                            // --->  calling onError throws UndeliverableException
                            subscriber.onError(new NetworkConnectionException(response.message()));                
                        }
                    }

                    @Override
                    public void onFailure(Call call, Throwable t) {
                        subscriber.onError(new NetworkConnectionException(t.getMessage()));
                    }
                });
        }
    });
}

Answer 1

Since version 2.1.1 tryOnError is available:

The emitter API (such as FlowableEmitter, SingleEmitter, etc.) now features a new method, tryOnError that tries to emit the Throwable if the sequence is not cancelled/disposed. Unlike the regular onError, if the downstream is no longer willing to accept events, the method returns false and doesn't signal an UndeliverableException.

https://github.com/ReactiveX/RxJava/blob/2.x/CHANGES.md