doc/dev/plugins/breadcrumbs/breadcrumbs.php

<?php
namespace Grav\Plugin;

use Composer\Autoload\ClassLoader;
use Grav\Common\Plugin;
use Grav\Plugin\Breadcrumbs\Breadcrumbs;

class BreadcrumbsPlugin extends Plugin
{
    /**
     * @return array
     */
    public static function getSubscribedEvents()
    {
        return [
            'onPluginsInitialized' => [
                ['autoload', 100001],
                ['onPluginsInitialized', 0]
            ]
        ];
    }

    /**
     * [onPluginsInitialized:100000] Composer autoload.
     *
     * @return ClassLoader
     */
    public function autoload()
    {
        return require __DIR__ . '/vendor/autoload.php';
    }

    /**
     * Initialize configuration
     */
    public function onPluginsInitialized()
    {
        if ($this->isAdmin()) {
            $this->active = false;
            return;
        }

        class_alias(Breadcrumbs::class, 'Grav\\Plugin\\Breadcrumbs');

        $this->enable([
            'onTwigTemplatePaths' => ['onTwigTemplatePaths', 0],
            'onTwigSiteVariables' => ['onTwigSiteVariables', 0]
        ]);
    }

    /**
     * Add current directory to twig lookup paths.
     */
    public function onTwigTemplatePaths()
    {
        $this->grav['twig']->twig_paths[] = __DIR__ . '/templates';
    }

    /**
     * Set needed variables to display breadcrumbs.
     */
    public function onTwigSiteVariables()
    {
        $this->grav['twig']->twig_vars['breadcrumbs'] = new Breadcrumbs($this->config->get('plugins.breadcrumbs'));

        if ($this->config->get('plugins.breadcrumbs.built_in_css')) {
            $this->grav['assets']->add('plugin://breadcrumbs/css/breadcrumbs.css');
        }
    }
}