drupal8-migrations/docroot/modules/custom/sample_migration/src/Form/TitleBodyParagraphForm.php

<?php

namespace Drupal\sample_migration\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Class TitleBodyParagraphForm.
 */
class TitleBodyParagraphForm extends FormBase {

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'title_body_paragraph_form';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Submit'),
    ];

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    foreach ($form_state->getValues() as $key => $value) {
      // @TODO: Validate fields.
    }
    parent::validateForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $batch = [
      'title' => $this->t('Bringing over Title Body Field Collections...'),
      'operations' => [
        [
          'sample_migration_title_body_paragraphs',
          [

          ]
        ]
      ],
      'init_message' => $this->t('Loading nodes...'),
      'progress_message' => $this->t('Processed @current out of @total'),
      'finished' => $this->t('sample_migration_title_body_paragraphs_finished')
    ];

    batch_set($batch);
  }

}