trait RouteEnhancerCollectorTrait in Page Manager 8.4

trait RouteEnhancerCollectorTrait {

  /**
   * @var \Drupal\Core\Routing\EnhancerInterface[]
   */
  protected $enhancers = [];

  /**
   * Cached sorted list of enhancers.
   *
   * @var \Drupal\Core\Routing\EnhancerInterface[]
   */
  protected $sortedEnhancers = [];

  /**
   * Add route enhancers to the router to let them generate information on
   * matched routes.
   *
   * The order of the enhancers is determined by the priority, the higher the
   * value, the earlier the enhancer is run.
   *
   * @param \Drupal\Core\Routing\EnhancerInterface $enhancer
   * @param int $priority
   *
   * @return $this
   */
  public function addRouteEnhancer(EnhancerInterface $enhancer, $priority = 0) {
    if (empty($this->enhancers[$priority])) {
      $this->enhancers[$priority] = [];
    }
    $this->enhancers[$priority][] = $enhancer;
    $this->sortedEnhancers = [];
    return $this;
  }

  /**
   * Sorts the enhancers and flattens them.
   *
   * @return \Drupal\Core\Routing\EnhancerInterface[]
   *   The enhancers ordered by priority.
   */
  protected function getRouteEnhancers() {
    if (empty($this->sortedEnhancers)) {
      $this->sortedEnhancers = $this
        ->sortRouteEnhancers();
    }
    return $this->sortedEnhancers;
  }

  /**
   * Sort enhancers by priority.
   *
   * The highest priority number is the highest priority (reverse sorting).
   *
   * @return \Drupal\Core\Routing\EnhancerInterface[]
   *   The sorted enhancers.
   */
  protected function sortRouteEnhancers() {
    $sortedEnhancers = [];
    krsort($this->enhancers);
    foreach ($this->enhancers as $enhancers) {
      $sortedEnhancers = array_merge($sortedEnhancers, $enhancers);
    }
    return $sortedEnhancers;
  }

}