@Override
    public long demand() {
        final OuterSubscriber<T> outerSubscriber = this.outerSubscriber;
        if (outerSubscriber == null) {
            return 0;
        }

        final StreamMessage<? extends T> currentPublisher = outerSubscriber.currentStreamMessage;
        if (currentPublisher == null) {
            return 0;
        }

        return currentPublisher.demand();
    }
Relay items in order from a {@link StreamMessage} of {@link StreamMessage}s as a single {@link StreamMessage}
source.
core/src/main/java/com/linecorp/armeria/common/stream/ConcatPublisherStreamMessage.java