@Nullable
    @Override
    public Boolean isResponseStreaming(Type returnType, @Nullable MediaType contentType) {
        final Class<?> clazz = typeToClass(unwrapUnaryAsyncType(returnType));
        if (clazz == null) {
            return null;
        }

        if (contentType != null && contentType.is(MediaType.EVENT_STREAM)) {
            return Publisher.class.isAssignableFrom(clazz) || Stream.class.isAssignableFrom(clazz);
        }

        if (ServerSentEvent.class.isAssignableFrom(clazz)) {
            return false;
        }

        return null;
    }
A response converter implementation which creates an {@link HttpResponse} with
{@code content-type: text/event-stream}. The objects published from a {@link Publisher} or {@link Stream}
would be converted into Server-Sent Events if a {@link ProducesEventStream} annotation is specified
on an annotated service method.
core/src/main/java/com/linecorp/armeria/server/annotation/ServerSentEventResponseConverterFunction.java