class BreadcrumbManager

class BreadcrumbManager implements ChainBreadcrumbBuilderInterface {
    
    /**
     * The module handler to invoke the alter hook.
     *
     * @var \Drupal\Core\Extension\ModuleHandlerInterface
     */
    protected $moduleHandler;
    
    /**
     * Holds arrays of breadcrumb builders, keyed by priority.
     *
     * @var array
     */
    protected $builders = [];
    
    /**
     * Holds the array of breadcrumb builders sorted by priority.
     *
     * Set to NULL if the array needs to be re-calculated.
     *
     * @var \Drupal\Core\Breadcrumb\BreadcrumbBuilderInterface[]|null
     */
    protected $sortedBuilders;
    
    /**
     * Constructs a \Drupal\Core\Breadcrumb\BreadcrumbManager object.
     *
     * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
     *   The module handler.
     */
    public function __construct(ModuleHandlerInterface $module_handler) {
        $this->moduleHandler = $module_handler;
    }
    
    /**
     * {@inheritdoc}
     */
    public function addBuilder(BreadcrumbBuilderInterface $builder, $priority) {
        $this->builders[$priority][] = $builder;
        // Force the builders to be re-sorted.
        $this->sortedBuilders = NULL;
    }
    
    /**
     * {@inheritdoc}
     */
    public function applies(RouteMatchInterface $route_match) {
        return TRUE;
    }
    
    /**
     * {@inheritdoc}
     */
    public function build(RouteMatchInterface $route_match) {
        $breadcrumb = new Breadcrumb();
        $context = [
            'builder' => NULL,
        ];
        // Call the build method of registered breadcrumb builders,
        // until one of them returns an array.
        foreach ($this->getSortedBuilders() as $builder) {
            if (!$builder->applies($route_match)) {
                // The builder does not apply, so we continue with the other builders.
                continue;
            }
            $breadcrumb = $builder->build($route_match);
            if ($breadcrumb instanceof Breadcrumb) {
                $context['builder'] = $builder;
                break;
            }
            else {
                throw new \UnexpectedValueException('Invalid breadcrumb returned by ' . get_class($builder) . '::build().');
            }
        }
        // Allow modules to alter the breadcrumb.
        $this->moduleHandler
            ->alter('system_breadcrumb', $breadcrumb, $route_match, $context);
        return $breadcrumb;
    }
    
    /**
     * Returns the sorted array of breadcrumb builders.
     *
     * @return \Drupal\Core\Breadcrumb\BreadcrumbBuilderInterface[]
     *   An array of breadcrumb builder objects.
     */
    protected function getSortedBuilders() {
        if (!isset($this->sortedBuilders)) {
            // Sort the builders according to priority.
            krsort($this->builders);
            // Merge nested builders from $this->builders into $this->sortedBuilders.
            $this->sortedBuilders = array_merge(...$this->builders);
        }
        return $this->sortedBuilders;
    }

}