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;
    }

}