code: public function onConfig(Event $event)
      {
          $cfg = $event['config'];
          if (!isset($cfg['debug'])) {
              // no debug config values have changed
              return;
          }
          $cfg = $cfg['debug'];
          if (isset($cfg['file'])) {
              $this->debug->addPlugin($this->debug->output->file);
          }
          if (isset($cfg['onBootstrap'])) {
              if (!$this->debug->parentInstance) {
                  // we're initializing
                  $this->debug->eventManager->subscribe('debug.bootstrap', $cfg['onBootstrap']);
              } else {
                  // boostrap has already occured, so go ahead and call
                  \call_user_func($cfg['onBootstrap'], new Event($this->debug));
              }
          }
          if (isset($cfg['onLog'])) {
              /*
                  Replace - not append - subscriber set via setCfg
              */
              if (isset($this->cfg['onLog'])) {
                  $this->debug->eventManager->unsubscribe('debug.log', $this->cfg['onLog']);
              }
              $this->debug->eventManager->subscribe('debug.log', $cfg['onLog']);
          }
          if (!static::$profilingEnabled) {
              $cfg = $this->debug->getCfg('debug/*');
              if ($cfg['enableProfiling'] && $cfg['collect']) {
                  static::$profilingEnabled = true;
                  $pathsExclude = array(
                      __DIR__,
                  );
                  FileStreamWrapper::register($pathsExclude);
              }
          }
      }
docs: debug.config subscriber
  
  @param Event $event event instance
  
  @return void