<?php

class CustomParagraphsBookmaker extends ProcessPluginBase {

  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    $source = $row->getSource();

    $paragraphs = [];

    for($i = 1; $i <= 7; $i++) {
      if (!empty($source['field_heading_' . $i][0]['value'])) {
        $paragraph = Paragraph::create([
          'type' => 'paragraph',
          'field_textfield' => $source['field_heading_' . $i][0]['value'],
          'field_textfield2' => $source['field_anchor_' . $i][0]['value'],
          'field_textarea' => array('value' => $source['field_body_' . $i][0]['value'], 'format' => 'full_html'),
        ]);
        $paragraph->save();
        $paragraphs[] = array(
          'target_id' => $paragraph->id(),
          'target_revision_id' => $paragraph->getRevisionId(),
        );
      }
    }

    return $paragraphs;
  }
}