How to do a custom taxonomy breadcrumb

Question

I'm using Drupal 8.3.0-beta1 and I need to write a custom module to customize breadcrumbs in some cases. On of this cases consists in customizing the breadcrumb in path taxonomy/term/% but with my current code there is a problem:

When I visit taxonomy/term/% the "applies" callback of my custom implementation of BreadcrumbBuilderInterface doesn't get executed. However, it gets called on taxonomy/term/%/edit. Maybe that's the reason why the core breadcrumb builder only returns "Home /" in this case. Do you think this is a core bug or I have done something wrong?

Notice I have another implementation of BreadcrumbBuilderInterface for nodes which works well. This is my code:

custom_breadcrumb.info.yml

name: 'Custom breadcrumbs'
description: 'Custom breadcrumbs implementation'
package: 'Custom'
# core: 8.x
type: module

version: '8.x-1.0'
core: '8.x'
project: 'custom_breadcrumb'
datestamp: 1443649740

custom_breadcrumb.services.yml

services:
  custom_breadcrumb.breadcrumb_blog:
    class: Drupal\custom_breadcrumb\Breadcrumb\BlogBreadcrumbBuilder
    tags:
      - { name: breadcrumb_builder, priority: 101 }

src/Breadcrumb/BlogBreadcrumbBuilder.php

<?php

namespace Drupal\custom_breadcrumb\Breadcrumb;

use Drupal\Core\Breadcrumb\BreadcrumbBuilderInterface;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Routing\LinkGeneratorTrait;
use Drupal\Core\StringTranslation\StringTranslationTrait;
use Drupal\Core\Link;
use Drupal\taxonomy\Entity\Term;

class BlogBreadcrumbBuilder implements BreadcrumbBuilderInterface {
  use LinkGeneratorTrait;
  use StringTranslationTrait;

  /**
   * @inheritdoc
   */
  public function applies(RouteMatchInterface $route_match) {    
    // This breadcrumb apply only for blog taxonomy terms of vocabulary 'categoria_blog'.    
    $parameters = $route_match->getParameters()->all();

    /*
     * CORE BUG? applies callback gets called on "taxonomy/term/%/edit" but not in "taxonomy/term/%"
     * https://www.drupal.org/node/2622840
     */
    if (isset($parameters['taxonomy_term'])) {
      $term = $parameters['taxonomy_term'];

      if ($term->getVocabularyId() == 'categoria_blog') {
        return TRUE;
      }
    }

    return FALSE;
  }

  /**
   * @inheritdoc
   */
  public function build(RouteMatchInterface $route_match) {

    $parameters = $route_match->getParameters()->all();

    if (isset($parameters['taxonomy_term'])) {

      $term = $parameters['taxonomy_term'];

      // Breadcrumbs set up (cache settings are so important!).
      $breadcrumb = new \Drupal\Core\Breadcrumb\Breadcrumb();
      $breadcrumb->addCacheContexts(["url"]);
      $breadcrumb->addCacheTags(["taxonomy_term:{$term->id()}"]);

      $breadcrumb->addLink(Link::createFromRoute($term->getName(), '<none>'));
    }

    // Reverse order for this to work!!!.
    $breadcrumb->addLink(Link::createFromRoute($this->t('Home'), '<front>'));
    return $breadcrumb;
  }
}

Answer 1

The taxonomy module provides a default breadcrumb service and if you have a look at the declaration of this in the taxonomy.services.yml file you will see it has a priority of 1002.

Setting the priority of your breadcrumb service to a value higher than this should get Drupal to use your breadcrumb rather than the default one.

Something like this should do the trick:

custom_breadcrumb.services.yml

services:
  custom_breadcrumb.breadcrumb_blog:
    class: Drupal\custom_breadcrumb\Breadcrumb\BlogBreadcrumbBuilder
    tags:
      - { name: breadcrumb_builder, priority: 10001 }

Answer 2

according to https://www.drupal.org/docs/8/api/services-and-dependency-injection/service-tags

A higher priority will be used over a lower priority.

and since the default implementation of TermBreadcrumbBuilder has a priority of 1002 the priority needs to be set to at least 1003.

services:
custom_breadcrumb.breadcrumb_blog:
class: Drupal\custom_breadcrumb\Breadcrumb\BlogBreadcrumbBuilder
tags:
  - { name: breadcrumb_builder, priority: 10003 }