code: public function validateAttribute($model, $attribute)
      {
          $value = $model->$attribute;
          $result = $this->validateValue($value);
          if (!empty($result)) {
              $this->addError($model, $attribute, $result[0], $result[1]);
          } elseif ($this->defaultScheme !== null && strpos($value, '://') === false) {
              $model->$attribute = $this->defaultScheme . '://' . $value;
          }
      }
docs: {@inheritdoc}