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.
     */
    protected abstract 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);
    }

}