code: public function processOutput(QtiRunnerServiceContext $context)
      {
          $messages = [];
          $channels = $this->getOption(self::OPTION_CHANNELS);
          if (is_array($channels[self::CHANNEL_TYPE_OUTPUT])) {
              foreach ($channels[self::CHANNEL_TYPE_OUTPUT] as $outputChannelName => $outputChannelClass) {
                  $channel = $this->getChannel($outputChannelName, self::CHANNEL_TYPE_OUTPUT);
                  $message = $this->processChannel($channel, $context);
                  if ($message !== null) {
                      $messages[] = [
                          'channel' => $channel->getName(),
                          'message' => $message,
                      ];
                  }
              }
          }
          return $messages;
      }
docs: Builds the list of output messages
  @param QtiRunnerServiceContext $context - Needs the current runner context
  @return array - Returns a list of output, each one is represented by a channel's name that is a string and a message that can be any type