Example for a paragraph behavior plugin

BackgroundBehavior.php

<?php

namespace Drupal\behaviors\Plugin\paragraphs\Behavior;

use Drupal\Core\Entity\Display\EntityViewDisplayInterface;
use Drupal\Core\Form\FormStateInterface;
use Drupal\paragraphs\Entity\Paragraph;
use Drupal\paragraphs\ParagraphInterface;
use Drupal\paragraphs\ParagraphsBehaviorBase;

/**
 * Provides a paragraphs behaviour plugin for background styles.
 *
 * @ParagraphsBehavior(
 *   id = "background_behavior",
 *   label = @Translation("Background style"),
 *   description = @Translation("Configurable background style"),
 * )
 */
class BackgroundBehavior extends ParagraphsBehaviorBase {

  /**
   * {@inheritdoc}
   */
  public function view(array &$build, Paragraph $paragraph, EntityViewDisplayInterface $display, $view_mode) {
    $style = $paragraph->getBehaviorSetting($this->getPluginId(), 'style', 'bright');
    $build['#attributes']['class'][] = 'paragraph-background-' . $style;
    $build['#attached']['library'][] = 'behaviors/styles';
  }

  /**
   * {@inheritdoc}
   */
  public function buildBehaviorForm(ParagraphInterface $paragraph, array &$form, FormStateInterface $form_state) {
    $form['style'] = [
      '#type' => 'radios',
      '#title' => $this->t('Background style'),
      '#default_value' => $paragraph->getBehaviorSetting($this->getPluginId(), 'style', 'bright'),
      '#options' => [
        'dark' => $this->t('Dark background, bright text'),
        'bright' => $this->t('Bright background, dark text'),
      ],
    ];
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function settingsSummary(Paragraph $paragraph) {
    $style = $paragraph->getBehaviorSetting('background_behavior', 'style', 'bright');
    return [$this->t('Background style: @style', ['@style' => $style])];
  }

}