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;
    }

}