class LayoutDefault

class LayoutDefault extends PluginBase implements LayoutInterface, PluginFormInterface, PreviewAwarePluginInterface {
  use ContextAwarePluginAssignmentTrait;
  use ContextAwarePluginTrait;

  /**
   * Whether the plugin is being rendered in preview mode.
   *
   * @var bool
   */
  protected $inPreview = FALSE;

  /**
   * The layout definition.
   *
   * @var \Drupal\Core\Layout\LayoutDefinition
   */
  protected $pluginDefinition;

  /**
   * {@inheritdoc}
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this
      ->setConfiguration($configuration);
  }

  /**
   * {@inheritdoc}
   */
  public function build(array $regions) {

    // Ensure $build only contains defined regions and in the order defined.
    $build = [];
    foreach ($this
      ->getPluginDefinition()
      ->getRegionNames() as $region_name) {
      if (array_key_exists($region_name, $regions)) {
        $build[$region_name] = $regions[$region_name];
      }
    }
    $build['#in_preview'] = $this->inPreview;
    $build['#settings'] = $this
      ->getConfiguration();
    $build['#layout'] = $this->pluginDefinition;
    $build['#theme'] = $this->pluginDefinition
      ->getThemeHook();
    if ($library = $this->pluginDefinition
      ->getLibrary()) {
      $build['#attached']['library'][] = $library;
    }
    return $build;
  }

  /**
   * {@inheritdoc}
   */
  public function getConfiguration() {
    return $this->configuration;
  }

  /**
   * {@inheritdoc}
   */
  public function setConfiguration(array $configuration) {
    $this->configuration = NestedArray::mergeDeep($this
      ->defaultConfiguration(), $configuration);
  }

  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    return [
      'label' => '',
    ];
  }

  /**
   * {@inheritdoc}
   */
  public function calculateDependencies() {
    return [];
  }

  /**
   * {@inheritdoc}
   *
   * @return \Drupal\Core\Layout\LayoutDefinition
   */
  public function getPluginDefinition() {
    return parent::getPluginDefinition();
  }

  /**
   * {@inheritdoc}
   */
  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
    $form['label'] = [
      '#type' => 'textfield',
      '#title' => $this
        ->t('Administrative label'),
      '#default_value' => $this->configuration['label'],
    ];
    $contexts = $form_state
      ->getTemporaryValue('gathered_contexts') ?: [];
    $form['context_mapping'] = $this
      ->addContextAssignmentElement($this, $contexts);
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {
  }

  /**
   * {@inheritdoc}
   */
  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
    $this->configuration['label'] = $form_state
      ->getValue('label');
  }

  /**
   * {@inheritdoc}
   */
  public function setInPreview(bool $in_preview) : void {
    $this->inPreview = $in_preview;
  }

}