class MockLayoutBlockDeriver

class MockLayoutBlockDeriver implements DerivativeInterface {

  /**
   * Implements Drupal\Component\Plugin\Derivative\DerivativeInterface::getDerivativeDefinition().
   */
  public function getDerivativeDefinition($derivative_id, array $base_plugin_definition) {
    $derivatives = $this
      ->getDerivativeDefinitions($base_plugin_definition);
    if (isset($derivatives[$derivative_id])) {
      return $derivatives[$derivative_id];
    }
  }

  /**
   * Implements Drupal\Component\Plugin\Derivative\DerivativeInterface::getDerivativeDefinitions().
   */
  public function getDerivativeDefinitions(array $base_plugin_definition) {

    // This isn't strictly necessary, but it helps reduce clutter in
    // DerivativePluginTest::testDerivativeDecorator()'s $expected variable.
    // Since derivative definitions don't need further deriving, we remove this
    // key from the returned definitions.
    unset($base_plugin_definition['derivative']);
    $derivatives = array(
      // Adding a NULL key signifies that the base plugin may also be used in
      // addition to the derivatives. In this case, we allow the administrator
      // to add a generic layout block to the page.
      NULL => $base_plugin_definition,
      // We also allow them to add a customized one. Here, we just mock the
      // customized one, but in a real implementation, this would be fetched
      // from some config() object.
      'foo' => array(
        'label' => t('Layout Foo'),
      ) + $base_plugin_definition,
    );
    return $derivatives;
  }

}