EmbeddedForm.php

<?php

namespace Drupal\field_layout_test\Form;

use Drupal\Core\Entity\Entity\EntityFormDisplay;
use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Form\SubformState;
use Drupal\entity_test\Entity\EntityTest;
use Drupal\field_layout\FieldLayoutBuilder;

/**
 * Provides the EmbeddedForm class.
 *
 * @package Drupal\field_layout_test\Form
 */
class EmbeddedForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'field_layout_test_embedded_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['foo'] = [
            '#type' => 'fieldset',
            '#title' => $this->t('Wrapper'),
            '#tree' => TRUE,
            '#parents' => [
                'foo',
            ],
        ];
        $entity = EntityTest::load(1);
        if ($entity) {
            if ($entity) {
                $display = EntityFormDisplay::collectRenderDisplay($entity, 'default');
                $subform_state = SubformState::createForSubform($form['foo'], $form, $form_state);
                $display->buildForm($entity, $form['foo'], $subform_state);
                \Drupal::classResolver(FieldLayoutBuilder::class)->buildForm($form['foo'], $display, $subform_state);
            }
        }
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        // Do nothing.
    }

}