migrateDebug.

<?php

namespace Drupal\rl_migrates\Plugin\migrate\process;

use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\Row;
use Drupal\migrate\MigrateSkipRowException;

/**
 * Debug are us.
 *
 * @code
 * process:
 *   field_xxx:
 *     plugin: rl_debug
 *     addSource: 1
 *     forceSkip: 1
 * @endcode
 *
 * @MigrateProcessPlugin(
 *   id = "rl_debug",
 * )
 */
class RlDebug extends ProcessPluginBase {

  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    $debugOutput = [];
    $debugOutput[] = 'rl_debug';
    $debugOutput[] = $value;
    if (!empty($this->configuration['addSource'])) {
      $debugOutput[] = $row->getSource();
    }
    drush_print_r($debugOutput);
    if (!empty($this->configuration['forceSkip'])) {
      throw new MigrateSkipRowException('rl_debug forceSkip');
    }
    return $value;
  }

}