TestThemeNegotiator.php

<?php

namespace Drupal\menu_test\Theme;

use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Theme\ThemeNegotiatorInterface;

/**
 * Tests the theme negotiation functionality.
 *
 * Retrieves the theme key of the theme to use for the current request based on
 * the theme name provided in the URL.
 */
class TestThemeNegotiator implements ThemeNegotiatorInterface {
    
    /**
     * {@inheritdoc}
     */
    public function applies(RouteMatchInterface $route_match) {
        return (bool) $route_match->getParameter('inherited');
    }
    
    /**
     * {@inheritdoc}
     */
    public function determineActiveTheme(RouteMatchInterface $route_match) {
        $argument = $route_match->getParameter('inherited');
        // Test using the variable administrative theme.
        if ($argument == 'use-admin-theme') {
            return \Drupal::config('system.theme')->get('admin');
        }
        elseif ($argument == 'use-test-theme') {
            return 'test_theme';
        }
        elseif ($argument == 'use-fake-theme') {
            return 'fake_theme';
        }
        // For any other value of the URL argument, do not return anything. This
        // allows us to test that returning nothing from a theme negotiation
        // causes the page to correctly fall back on using the main site theme.
    }

}