Entity::toUrl

public Entity::toUrl($rel = 'canonical', array $options = [])

links = {
  "canonical" = "/node/{node}",
  "edit-form" = "/node/{node}/edit",
  "version-history" = "/node/{node}/revisions"
}

uri_callback = "comment_uri",

public function toUrl($rel = 'canonical', array $options = []) {
  if ($this->id() === NULL) {
    throw new EntityMalformedException(sprintf('The "%s" entity cannot have a URI as it does not have an ID', $this->getEntityTypeId()));
  }

  // The links array might contain URI templates set in annotations.
  $link_templates = $this->linkTemplates();

  // Links pointing to the current revision point to the actual entity. So
  // instead of using the 'revision' link, use the 'canonical' link.
  if ($rel === 'revision' && $this instanceof RevisionableInterface && $this->isDefaultRevision()) {
    $rel = 'canonical';
  }

  if (isset($link_templates[$rel])) {
    $route_parameters = $this->urlRouteParameters($rel);
    $route_name = "entity.{$this->entityTypeId}." . str_replace(array('-', 'drupal:'), array('_', ''), $rel);
    $uri = new Url($route_name, $route_parameters);
  }
  else {
    $bundle = $this->bundle();
    // A bundle-specific callback takes precedence over the generic one for
    // the entity type.
    $bundles = $this->entityManager()->getBundleInfo($this->getEntityTypeId());
    if (isset($bundles[$bundle]['uri_callback'])) {
      $uri_callback = $bundles[$bundle]['uri_callback'];
    }
    elseif ($entity_uri_callback = $this->getEntityType()->getUriCallback()) {
      $uri_callback = $entity_uri_callback;
    }

    // Invoke the callback to get the URI. If there is no callback, use the
    // default URI format.
    if (isset($uri_callback) && is_callable($uri_callback)) {
      $uri = call_user_func($uri_callback, $this);
    }
    else {
      throw new UndefinedLinkTemplateException("No link template '$rel' found for the '{$this->getEntityTypeId()}' entity type");
    }
  }

  // Pass the entity data through as options, so that alter functions do not
  // need to look up this entity again.
  $uri
  ->setOption('entity_type', $this->getEntityTypeId())
    ->setOption('entity', $this);

  // Display links by default based on the current language.
  // Link relations that do not require an existing entity should not be
  // affected by this entity's language, however.
  if (!in_array($rel, ['collection', 'add-page', 'add-form'], TRUE)) {
    $options += ['language' => $this->language()];
  }

  $uri_options = $uri->getOptions();
  $uri_options += $options;

  return $uri->setOptions($uri_options);
}