code: public static void checkUnWritabilityAndNotify(ChannelHandlerContext context,
                                                     BackPressureHandler backpressureHandler) {
          if (backpressureHandler != null) {
              Channel channel = context.channel();
              if (!channel.isWritable() && channel.isActive()) {
                  backpressureHandler.getBackPressureObservable().notifyUnWritable();
              }
          }
      }
docs: Checks if channel is unWritable and notifies BackPressure observable.
  
  @param context             The context on which to check writablity.
  @param backpressureHandler The back pressure handler with the observable.