FieldLayoutTestHooks.php

<?php

declare (strict_types=1);
namespace Drupal\field_layout_test\Hook;

use Drupal\Core\Hook\Attribute\Hook;

/**
 * Hook implementations for field_layout_test.
 */
class FieldLayoutTestHooks {
  
  /**
   * Implements hook_layout_alter().
   */
  public function layoutAlter(&$definitions) : void {
    /** @var \Drupal\Core\Layout\LayoutDefinition[] $definitions */
    if (\Drupal::state()->get('field_layout_test.alter_regions') && isset($definitions['layout_onecol'])) {
      $definitions['layout_onecol']->setRegions([
        'foo' => [
          'label' => 'Foo',
        ],
      ]);
    }
  }

}