addLink example

function hook_system_breadcrumb_alter(\Drupal\Core\Breadcrumb\Breadcrumb &$breadcrumb, \Drupal\Core\Routing\RouteMatchInterface $route_match, array $context) {
  // Add an item to the end of the breadcrumb.   $breadcrumb->addLink(\Drupal\Core\Link::createFromRoute(t('Text'), 'example_route_name'));
}

/** * @} End of "addtogroup hooks". */

  public function applies(RouteMatchInterface $route_match) {
    return $route_match->getRouteName() == 'help.help_topic';
  }

  /** * {@inheritdoc} */
  public function build(RouteMatchInterface $route_match) {
    $breadcrumb = new Breadcrumb();
    $breadcrumb->addCacheContexts(['url.path.parent']);
    $breadcrumb->addLink(Link::createFromRoute($this->t('Home'), '<front>'));
    $breadcrumb->addLink(Link::createFromRoute($this->t('Administration'), 'system.admin'));
    $breadcrumb->addLink(Link::createFromRoute($this->t('Help'), 'help.main'));

    return $breadcrumb;
  }

}

// Add all parent forums to breadcrumbs.     /** @var \Drupal\taxonomy\TermInterface $term */
    $term = $route_match->getParameter('taxonomy_term');
    $term_id = $term->id();
    $breadcrumb->addCacheableDependency($term);

    $parents = $this->termStorage->loadAllParents($term_id);
    if ($parents) {
      foreach (array_reverse($parents) as $parent) {
        if ($parent->id() != $term_id) {
          $breadcrumb->addCacheableDependency($parent);
          $breadcrumb->addLink(Link::createFromRoute($parent->label(), 'forum.page', [
            'taxonomy_term' => $parent->id(),
          ]));
        }
      }
    }

    return $breadcrumb;
  }

}

  public function applies(RouteMatchInterface $route_match) {
    return $route_match->getRouteName() == 'entity.taxonomy_term.canonical'
      && $route_match->getParameter('taxonomy_term') instanceof TermInterface;
  }

  /** * {@inheritdoc} */
  public function build(RouteMatchInterface $route_match) {
    $breadcrumb = new Breadcrumb();
    $breadcrumb->addLink(Link::createFromRoute($this->t('Home'), '<front>'));
    $term = $route_match->getParameter('taxonomy_term');
    // Breadcrumb needs to have terms cacheable metadata as a cacheable     // dependency even though it is not shown in the breadcrumb because e.g. its     // parent might have changed.     $breadcrumb->addCacheableDependency($term);
    // @todo This overrides any other possible breadcrumb and is a pure     // hard-coded presumption. Make this behavior configurable per     // vocabulary or term.     $parents = $this->entityTypeManager->getStorage('taxonomy_term')->loadAllParents($term->id());
    // Remove current term being accessed.     array_shift($parents);
    

  public function build(RouteMatchInterface $route_match) {
    $breadcrumb = parent::build($route_match);
    $breadcrumb->addCacheContexts(['route']);

    $parents = $this->termStorage->loadAllParents($route_match->getParameter('node')->forum_tid);
    if ($parents) {
      $parents = array_reverse($parents);
      foreach ($parents as $parent) {
        $breadcrumb->addCacheableDependency($parent);
        $breadcrumb->addLink(Link::createFromRoute($parent->label(), 'forum.page',
          [
            'taxonomy_term' => $parent->id(),
          ]
        ));
      }
    }

    return $breadcrumb;
  }

}

        foreach ($recordB->getExtensions() as $key => $value) {
            if ($value['data'] === null) {
                continue;
            }
            $recordA->addExtension($key, $value);
        }

        foreach ($recordB->getLinks() as $key => $value) {
            if (!empty($value)) {
                $recordA->addLink($key, $value);
            }
        }
    }
}

$this->assertEquals($formBuilder, $controller->createFormBuilder('foo'));
    }

    public function testAddLink()
    {
        $request = new Request();
        $link1 = new Link('mercure', 'https://demo.mercure.rocks');
        $link2 = new Link('self', 'https://example.com/foo');

        $controller = $this->createController();
        $controller->addLink($request, $link1);
        $controller->addLink($request, $link2);

        $links = $request->attributes->get('_links')->getLinks();
        $this->assertContains($link1, $links);
        $this->assertContains($link2, $links);
    }

    public function testSendEarlyHints()
    {
        $container = new Container();
        $container->set('web_link.http_header_serializer', new HttpHeaderSerializer());

        

  public function applies(RouteMatchInterface $route_match) {
    return $route_match->getRouteName() == 'comment.reply' && $route_match->getParameter('entity');
  }

  /** * {@inheritdoc} */
  public function build(RouteMatchInterface $route_match) {
    $breadcrumb = new Breadcrumb();
    $breadcrumb->addCacheContexts(['route']);
    $breadcrumb->addLink(Link::createFromRoute($this->t('Home'), '<front>'));

    $entity = $route_match->getParameter('entity');
    $breadcrumb->addLink(new Link($entity->label(), $entity->toUrl()));
    $breadcrumb->addCacheableDependency($entity);

    if (($pid = $route_match->getParameter('pid')) && ($comment = $this->entityTypeManager->getStorage('comment')->load($pid))) {
      /** @var \Drupal\comment\CommentInterface $comment */
      $breadcrumb->addCacheableDependency($comment);
      // Display link to parent comment.       // @todo Clean-up permalink in https://www.drupal.org/node/2198041       $breadcrumb->addLink(new Link($comment->getSubject(), $comment->toUrl()));
    }

protected function serializeEntity(ResponseFields $fields, Entity $entity, EntityDefinition $definition, JsonApiEncodingResult $result, bool $isRelationship = false): void
    {
        if ($result->containsInData($entity->getUniqueIdentifier(), $definition->getEntityName())
            || ($isRelationship && $result->containsInIncluded($entity->getUniqueIdentifier(), $definition->getEntityName()))
        ) {
            return;
        }

        $self = $result->getBaseUrl() . '/' . $this->camelCaseToSnailCase($definition->getEntityName()) . '/' . $entity->getUniqueIdentifier();

        $serialized = clone $this->createSerializedEntity($fields, $definition, $result);
        $serialized->addLink('self', $self);
        $serialized->merge($entity);

        // add included entities         $this->serializeRelationships($fields, $serialized, $entity, $result);

        $this->addExtensions($fields, $serialized, $entity, $result);

        if ($isRelationship) {
            $result->addIncluded($serialized);
        } else {
            $result->addEntity($serialized);
        }