class EntityAutocomplete

class EntityAutocomplete implements FormInterface, ContainerInjectionInterface {
    use StringTranslationTrait;
    use MessengerTrait;
    
    /**
     * The entity type manager service.
     *
     * We need this for the submit handler.
     *
     * @var \Drupal\Core\Entity\EntityTypeManagerInterface
     */
    protected $entityTypeManager;
    
    /**
     * Container injection factory.
     *
     * @param \Symfony\Component\DependencyInjection\ContainerInterface $container
     *   The service discovery container.
     *
     * @return self
     *   The form object.
     */
    public static function create(ContainerInterface $container) {
        $form = new static($container->get('entity_type.manager'));
        $form->setStringTranslation($container->get('string_translation'));
        $form->setMessenger($container->get('messenger'));
        return $form;
    }
    
    /**
     * Constructor.
     *
     * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
     *   The entity type manager service.
     */
    public function __construct(EntityTypeManagerInterface $entity_type_manager) {
        $this->entityTypeManager = $entity_type_manager;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'ajax_example_autocomplete_user';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['info'] = [
            '#markup' => '<div>' . $this->t("This example uses the <code>entity_autocomplete</code> form element to select users. You'll need a few users on your system for it to make sense.") . '</div>',
        ];
        // Here we use the delightful entity_autocomplete form element. It allows us
        // to consistently select entities. See https://www.drupal.org/node/2418529.
        $form['users'] = [
            // A type of entity_autocomplete lets Drupal know it should autocomplete
            // entities.
'#type' => 'entity_autocomplete',
            // We can specify entity types to autocomplete.
'#target_type' => 'user',
            // Specifying #tags as TRUE allows for multiple selections, separated by
            // commas.
'#tags' => TRUE,
            '#title' => $this->t('Choose a user (Separate with commas)'),
        ];
        $form['actions'] = [
            '#type' => 'actions',
        ];
        $form['actions']['submit'] = [
            '#type' => 'submit',
            '#value' => $this->t('Submit'),
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     *
     * Here we validate and signal an error if there are no users selected.
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        $state_users = $form_state->getValue('users');
        if (empty($state_users)) {
            $form_state->setErrorByName('users', 'There were no users selected.');
        }
    }
    
    /**
     * {@inheritdoc}
     *
     * On submit, show the user the names of the users they selected.
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        $state_users = $form_state->getValue('users');
        $users = [];
        foreach ($state_users as $state_user) {
            $uid = $state_user['target_id'];
            $users[] = $this->entityTypeManager
                ->getStorage('user')
                ->load($uid)
                ->getDisplayName();
        }
        $this->messenger()
            ->addMessage('These are your users: ' . implode(' ', $users));
    }

}