private void onNext0() {
            currentExposedBodyPartPublisher = null;
            if (demandOfMultipart > 0 && !isComplete()) {
                // BodyPart must be triggered after onSubscribe.
                assert subscription != null;
                // Trigger next body part
                subscription.request(1);
            }
        }
For hiding Subscriber interface from MultipartDecoder.
Subscribing {@code DecodedHttpStreamMessage<BodyPart>} and delegating to external subscriber.
core/src/main/java/com/linecorp/armeria/common/multipart/MultipartDecoder.java