subentity-1.x-dev/src/Form/BundleForm.php

<?php

namespace Drupal\subentity\Form;

use Drupal\Core\Entity\BundleEntityFormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * The entity form for subentity types.
 */
class BundleForm extends BundleEntityFormBase {

  /**
   * {@inheritdoc}
   */
  public function form(array $form, FormStateInterface $form_state): array {
    $form = parent::form($form, $form_state);

    $form['label'] = [
      '#type' => 'textfield',
      '#title' => $this->t('Label'),
      '#maxlength' => 255,
      '#default_value' => $this->entity->label(),
      '#description' => $this->t("Label for the bundle."),
      '#required' => TRUE,
    ];

    $form['id'] = [
      '#type' => 'machine_name',
      '#default_value' => $this->entity->id(),
      '#machine_name' => [
        'exists' => get_class($this->entity) . '::load',
      ],
    ];

    return $this->protectBundleIdElement($form);
  }

  /**
   * {@inheritdoc}
   */
  public function save(array $form, FormStateInterface $form_state) {
    $entity = $this->entity;
    $status = parent::save($form, $form_state);
    switch ($status) {
      case SAVED_NEW:
        $this->messenger()->addMessage($this->t('Created the %label bundle.', [
          '%label' => $entity->label(),
        ]));
        break;

      default:
        $this->messenger()->addMessage($this->t('Saved the %label bundle.', [
          '%label' => $entity->label(),
        ]));
    }

    $form_state->setRedirectUrl($entity->toUrl('collection'));
    return $status;
  }

}