TestController.php

<?php

namespace Drupal\contextual_test\Controller;


/**
 * Test controller to provide a callback for the contextual link.
 */
class TestController {
    
    /**
     * Callback for the contextual link.
     *
     * @return array
     *   Render array.
     */
    public function render() {
        return [
            '#type' => 'markup',
            '#markup' => 'Everything is contextual!',
        ];
    }

}