class EntityTestForm

class EntityTestForm extends ContentEntityForm {

  /**
   * {@inheritdoc}
   */
  protected function prepareEntity() {
    if (empty($this->entity->name->value)) {

      // Assign a random name to new EntityTest entities, to avoid repetition in
      // tests.
      $random = new Random();
      $this->entity->name->value = $random
        ->name();
    }
  }

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

    // @todo: Is there a better way to check if an entity type is revisionable?
    if ($entity
      ->getEntityType()
      ->hasKey('revision') && !$entity
      ->isNew()) {
      $form['revision'] = [
        '#type' => 'checkbox',
        '#title' => t('Create new revision'),
        '#default_value' => $entity
          ->isNewRevision(),
      ];
    }
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function save(array $form, FormStateInterface $form_state) {
    try {
      $entity = $this->entity;

      // Save as a new revision if requested to do so.
      if (!$form_state
        ->isValueEmpty('revision')) {
        $entity
          ->setNewRevision();
      }
      $is_new = $entity
        ->isNew();
      $status = $entity
        ->save();
      if ($is_new) {
        $message = t('%entity_type @id has been created.', [
          '@id' => $entity
            ->id(),
          '%entity_type' => $entity
            ->getEntityTypeId(),
        ]);
      }
      else {
        $message = t('%entity_type @id has been updated.', [
          '@id' => $entity
            ->id(),
          '%entity_type' => $entity
            ->getEntityTypeId(),
        ]);
      }
      $this
        ->messenger()
        ->addStatus($message);
      if ($entity
        ->id()) {
        $entity_type = $entity
          ->getEntityTypeId();
        $form_state
          ->setRedirect("entity.{$entity_type}.edit_form", [
          $entity_type => $entity
            ->id(),
        ]);
      }
      else {

        // Error on save.
        $this
          ->messenger()
          ->addError($this
          ->t('The entity could not be saved.'));
        $form_state
          ->setRebuild();
      }
    } catch (\Exception $e) {
      \Drupal::state()
        ->set('entity_test.form.save.exception', get_class($e) . ': ' . $e
        ->getMessage());
    }
    return $status ?? FALSE;
  }

}