class MultiWidthLayoutBase

abstract class MultiWidthLayoutBase extends LayoutDefault implements PluginFormInterface {
  
  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    $configuration = parent::defaultConfiguration();
    return $configuration + [
      'column_widths' => $this->getDefaultWidth(),
    ];
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
    $form['column_widths'] = [
      '#type' => 'select',
      '#title' => $this->t('Column widths'),
      '#default_value' => $this->configuration['column_widths'],
      '#options' => $this->getWidthOptions(),
      '#description' => $this->t('Choose the column widths for this layout.'),
    ];
    return parent::buildConfigurationForm($form, $form_state);
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
    parent::submitConfigurationForm($form, $form_state);
    $this->configuration['column_widths'] = $form_state->getValue('column_widths');
  }
  
  /**
   * {@inheritdoc}
   */
  public function build(array $regions) {
    $build = parent::build($regions);
    $build['#attributes']['class'] = [
      'layout',
      $this->getPluginDefinition()
        ->getTemplate(),
      $this->getPluginDefinition()
        ->getTemplate() . '--' . $this->configuration['column_widths'],
    ];
    return $build;
  }
  
  /**
   * Gets the width options for the configuration form.
   *
   * The first option will be used as the default 'column_widths' configuration
   * value.
   *
   * @return string[]
   *   The width options array where the keys are strings that will be added to
   *   the CSS classes and the values are the human readable labels.
   */
  abstract protected function getWidthOptions();
  
  /**
   * Provides a default value for the width options.
   *
   * @return string
   *   A key from the array returned by ::getWidthOptions().
   */
  protected function getDefaultWidth() {
    // Return the first available key from the list of options.
    $width_classes = array_keys($this->getWidthOptions());
    return array_shift($width_classes);
  }

}