LayoutTestPlugin.php

<?php

namespace Drupal\layout_test\Plugin\Layout;

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Layout\Attribute\Layout;
use Drupal\Core\Layout\LayoutDefault;
use Drupal\Core\Plugin\PluginFormInterface;
use Drupal\Core\StringTranslation\TranslatableMarkup;

/**
 * The plugin that handles the default layout template.
 */
class LayoutTestPlugin extends LayoutDefault implements PluginFormInterface {
    
    /**
     * {@inheritdoc}
     */
    public function defaultConfiguration() {
        return [
            'setting_1' => 'Default',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
        $form['setting_1'] = [
            '#type' => 'textfield',
            '#title' => 'Blah',
            '#default_value' => $this->configuration['setting_1'],
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {
        if ($form_state->getValue('setting_1') === 'Test Validation Error Message') {
            $form_state->setErrorByName('setting_1', 'Validation Error Message');
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
        $this->configuration['setting_1'] = $form_state->getValue('setting_1');
    }

}