@Override
        public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {
            if (evt instanceof Http2ConnectionPrefaceAndSettingsFrameWrittenEvent) {
                ctx.flush();
            }
            ctx.fireUserEventTriggered(evt);
        }
Class that capture Prior-Knowledge method's Preface frame written Events
triggered on this channel, and Flush Channel for send preface frame to remote
realy
remoting/remoting-http/src/main/java/com/alipay/sofa/rpc/transport/http/Http2ClientInitializer.java