ThemeEarlyInitializationTest.php

<?php

namespace Drupal\Tests\system\Functional\Theme;

use Drupal\Tests\BrowserTestBase;

/**
 * Tests theme system initialization early in the page request.
 *
 * @group Theme
 */
class ThemeEarlyInitializationTest extends BrowserTestBase {
    
    /**
     * Modules to enable.
     *
     * @var array
     */
    protected static $modules = [
        'theme_test',
    ];
    
    /**
     * {@inheritdoc}
     */
    protected $defaultTheme = 'starterkit_theme';
    
    /**
     * Tests that the theme system can generate output in a request listener.
     */
    public function testRequestListener() {
        $this->drupalGet('theme-test/request-listener');
        // Verify that themed output generated in the request listener appears.
        $this->assertSession()
            ->responseContains('Themed output generated in a KernelEvents::REQUEST listener');
        // Verify that the default theme's CSS still appears even though the theme
        // system was initialized early.
        $this->assertSession()
            ->responseContains('starterkit_theme/css/components/action-links.css');
    }

}