LayoutManager.php

<?php

/**
 * @file
 * Definition of Drupal\layout\Plugin\Type\LayoutManager.
 */
namespace Drupal\layout\Plugin\Type;

use Drupal\Component\Plugin\PluginManagerBase;
use Drupal\Component\Plugin\Discovery\DerivativeDiscoveryDecorator;
use Drupal\Component\Plugin\Discovery\ProcessDecorator;
use Drupal\Core\Plugin\Discovery\AnnotatedClassDiscovery;
use Drupal\Component\Plugin\Factory\ReflectionFactory;

/**
 * Layout plugin manager.
 */
class LayoutManager extends PluginManagerBase {
  protected $defaults = array(
    'class' => 'Drupal\\layout\\Plugin\\Layout\\StaticLayout',
  );

  /**
   * Overrides Drupal\Component\Plugin\PluginManagerBase::__construct().
   *
   * @param \Traversable $namespaces
   *   An object that implements \Traversable which contains the root paths
   *   keyed by the corresponding namespace to look for plugin implementations,
   */
  public function __construct(\Traversable $namespaces) {

    // Create layout plugin derivatives from declaratively defined layouts.
    $this->discovery = new AnnotatedClassDiscovery('Layout', $namespaces);
    $this->discovery = new DerivativeDiscoveryDecorator($this->discovery);
    $this->discovery = new ProcessDecorator($this->discovery, array(
      $this,
      'processDefinition',
    ));
    $this->factory = new ReflectionFactory($this->discovery);
  }

}