Drupal 8/9: Form with "Add more" ajax button

CategoriesSuggestionSettingsForm.php

<?php

namespace Drupal\gurei\Form;

use Drupal\Component\Utility\NestedArray;
use Drupal\Core\Cache\Cache;
use Drupal\Core\Entity\EntityTypeManagerInterface;
use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\State\StateInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Class CategoriesSuggestionSettingsForm.
 */
class CategoriesSuggestionSettingsForm extends FormBase {

  const CATEGORIES_SUGGESTION_KEY = 'gurei_categories_suggestion';

  /**
   * Categories.
   *
   * @var array
   */
  protected $categories;

  /**
   * Term storage.
   *
   * @var \Drupal\taxonomy\TermStorageInterface
   */
  protected $termStorage;

  /**
   * CategoriesSuggestionSettingsForm constructor.
   */
  public function __construct(private StateInterface $state, EntityTypeManagerInterface $entityTypeManager) {
    $this->termStorage = $this->entityTypeManager->getStorage('taxonomy_term');
    $this->categories = $this->state->get(static::CATEGORIES_SUGGESTION_KEY);
    $this->categories[] = '';
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('state'),
      $container->get('entity_type.manager')
    );
  }

  /**
   * {@inheritDoc}
   */
  public function getFormId() {
    return 'gurei_categories_suggestion_form';
  }

  /**
   * {@inheritDoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    if (empty($form_state->get('categories'))) {
      $form_state->set('categories', $this->categories);
    }
    else {
      $this->categories = $form_state->get('categories');
    }
    $container_id = 'search-categories-suggestion';
    $form['container'] = [
      '#type' => 'fieldset',
      '#title' => $this->t('Categories'),
      '#tree' => TRUE,
      '#attributes' => [
        'id' => $container_id,
      ],
    ];
    foreach ($this->categories as $delta => $category_id) {
      $term = NULL;
      if (!empty($category_id)) {
        $term = $this->termStorage->load($category_id);
      }
      $form['container']['category'][$delta] = [
        '#type' => 'entity_autocomplete',
        '#default_value' => $term,
        '#attributes' => [
          'class' => ['container-inline'],
        ],
        '#placeholder' => $this->t('Search by category name'),
        '#target_type' => 'taxonomy_term',
        '#selection_settings' => [
          'target_bundles' => ['product_category'],
        ],
      ];
    }

    $form['container']['category_add'] = [
      '#type' => 'submit',
      '#value' => $this->t('Add more'),
      '#submit' => [[self::class, 'addItem']],
      '#ajax' => [
        'wrapper' => $container_id,
        'callback' => [self::class, 'ajaxRefresh'],
      ],
    ];

    $form['actions']['submit'] = [
      '#type' => 'submit',
      '#value' => $this->t('Save'),
      '#attributes' => [
        'class' => [
          'button--primary',
        ],
      ],
    ];
    return $form;
  }

  /**
   * Ajax callback.
   */
  public static function ajaxRefresh(array &$form, FormStateInterface $form_state) {
    $parents = $form_state->getTriggeringElement()['#parents'];
    array_pop($parents);
    return NestedArray::getValue($form, $parents);
  }

  /**
   * Add new item on the form.
   */
  public function addItem(array $form, FormStateInterface $form_state) {
    $values = $form_state->getValues();
    $categories = $values['container']['category'];
    $categories[] = '';
    $form_state->set('categories', $categories);
    $form_state->setRebuild();
  }

  /**
   * {@inheritDoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $values = $form_state->getValues();
    $categories_to_save = [];
    foreach ($values['container']['category'] as $category_id) {
      if ($category_id) {
        $categories_to_save[] = $category_id;
      }
    }
    $this->state->set(static::CATEGORIES_SUGGESTION_KEY, $categories_to_save);
    Cache::invalidateTags(['gurei.categories_suggestions']);
  }

}