class NestedEntityTestForm

class NestedEntityTestForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'field_test_entity_nested_form';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state, EntityInterface $entity_1 = NULL, EntityInterface $entity_2 = NULL) {
        // First entity.
        $form_state->set('entity_1', $entity_1);
        $form_display_1 = EntityFormDisplay::collectRenderDisplay($entity_1, 'default');
        $form_state->set('form_display_1', $form_display_1);
        $form_display_1->buildForm($entity_1, $form, $form_state);
        // Second entity.
        $form_state->set('entity_2', $entity_2);
        $form_display_2 = EntityFormDisplay::collectRenderDisplay($entity_2, 'default');
        $form_state->set('form_display_2', $form_display_2);
        $form['entity_2'] = [
            '#type' => 'details',
            '#title' => t('Second entity'),
            '#tree' => TRUE,
            '#parents' => [
                'entity_2',
            ],
            '#weight' => 50,
            '#attributes' => [
                'class' => [
                    'entity-2',
                ],
            ],
        ];
        $form_display_2->buildForm($entity_2, $form['entity_2'], $form_state);
        if ($entity_2 instanceof EntityChangedInterface) {
            // Changed must be sent to the client, for later overwrite error checking.
            // @see \Drupal\Tests\field\Functional\NestedFormTest::testNestedEntityFormEntityLevelValidation()
            $form['entity_2']['changed'] = [
                '#type' => 'hidden',
                '#default_value' => $entity_1->getChangedTime(),
            ];
        }
        $form['save'] = [
            '#type' => 'submit',
            '#value' => t('Save'),
            '#weight' => 100,
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        $entity_1 = $form_state->get('entity_1');
        
        /** @var \Drupal\Core\Entity\Display\EntityFormDisplayInterface $form_display_1 */
        $form_display_1 = $form_state->get('form_display_1');
        $form_display_1->extractFormValues($entity_1, $form, $form_state);
        $form_display_1->validateFormValues($entity_1, $form, $form_state);
        $entity_2 = $form_state->get('entity_2');
        
        /** @var \Drupal\Core\Entity\Display\EntityFormDisplayInterface $form_display_2 */
        $form_display_2 = $form_state->get('form_display_2');
        $extracted = $form_display_2->extractFormValues($entity_2, $form['entity_2'], $form_state);
        // Extract the values of fields that are not rendered through widgets, by
        // simply copying from top-level form values. This leaves the fields that
        // are not being edited within this form untouched.
        // @see \Drupal\Tests\field\Functional\NestedFormTest::testNestedEntityFormEntityLevelValidation()
        foreach ($form_state->getValues()['entity_2'] as $name => $values) {
            if ($entity_2->hasField($name) && !isset($extracted[$name])) {
                $entity_2->set($name, $values);
            }
        }
        $form_display_2->validateFormValues($entity_2, $form['entity_2'], $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        
        /** @var \Drupal\Core\Entity\EntityInterface $entity_1 */
        $entity_1 = $form_state->get('entity_1');
        $entity_1->save();
        
        /** @var \Drupal\Core\Entity\EntityInterface $entity_2 */
        $entity_2 = $form_state->get('entity_2');
        $entity_2->save();
        $this->messenger()
            ->addStatus($this->t('test_entities @id_1 and @id_2 have been updated.', [
            '@id_1' => $entity_1->id(),
            '@id_2' => $entity_2->id(),
        ]));
    }

}