code: public function init()
      {
          parent::init();
  
          if ($this->normalizer !== false) {
              $this->normalizer = Yii::createObject($this->normalizer);
              if (!$this->normalizer instanceof UrlNormalizer) {
                  throw new InvalidConfigException('`' . get_class($this) . '::normalizer` should be an instance of `' . UrlNormalizer::className() . '` or its DI compatible configuration.');
              }
          }
  
          if (!$this->enablePrettyUrl) {
              return;
          }
          if (is_string($this->cache)) {
              $this->cache = Yii::$app->get($this->cache, false);
          }
          if (empty($this->rules)) {
              return;
          }
          $this->rules = $this->buildRules($this->rules);
      }
docs: Initializes UrlManager.