FieldInstanceDefaults.php

<?php

namespace Drupal\field\Plugin\migrate\process\d7;

use Drupal\migrate\Attribute\MigrateProcess;
use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\Row;

/**
 * Determines the default values for a field instance.
 */
class FieldInstanceDefaults extends ProcessPluginBase {
  
  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    [
      $default_value,
      $widget_settings,
    ] = $value;
    $widget_type = $widget_settings['type'];
    $default_value = $default_value ?: [];
    // In Drupal 7, the default value for email fields is stored in the key
    // 'email' while in Drupal 8 it is stored in the key 'value'.
    if ($widget_type == 'email_textfield' && $default_value) {
      $default_value[0]['value'] = $default_value[0]['email'];
      unset($default_value[0]['email']);
    }
    if ($widget_type == 'link_field' && $default_value) {
      $default_value[0]['uri'] = $default_value[0]['url'];
      $default_value[0]['options'] = [
        'attributes' => [],
      ];
      unset($default_value[0]['url']);
    }
    return $default_value;
  }

}