ThemeTestController.php

<?php

namespace Drupal\theme_legacy_test;

use Drupal\Core\Controller\ControllerBase;

/**
 * Controller routines for test routes for legacy theme functions.
 *
 * @todo Remove in https://www.drupal.org/project/drupal/issues/3097889
 */
class ThemeTestController extends ControllerBase {
    
    /**
     * A theme template that overrides a theme function.
     *
     * @return array
     *   Render array containing a theme.
     */
    public function functionTemplateOverridden() {
        return [
            '#theme' => 'theme_test_function_template_override',
        ];
    }
    
    /**
     * Menu callback for testing suggestion alter hooks with theme functions.
     */
    public function functionSuggestionAlter() {
        return [
            '#theme' => 'theme_test_function_suggestions',
        ];
    }
    
    /**
     * Menu callback for testing includes with suggestion alter hooks.
     */
    public function suggestionAlterInclude() {
        return [
            '#theme' => 'theme_test_suggestions_include',
        ];
    }

}