Basic Drupal8 Custom Form

NycTestForm.php

<?php

namespace Drupal\nyc_common\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

use Drupal\nyc_map\Utils\NycLocationServices;
use Drupal\nyc_map\Utils\NycGeometryUtility;
use Drupal\Core\Ajax\AjaxResponse;
use Drupal\Core\Ajax\OpenModalDialogCommand;
use Drupal\nyc_map\Ajax\CustomCategoryJsCommand;
use Drupal\nyc_map\Ajax\CustomOnStreetJsCommand;
use Drupal\nyc_map\Ajax\CustomCornerBetweenJsCommand;
use Drupal\Core\Ajax\InvokeCommand;
use Drupal\Core\Ajax\ReplaceCommand;
use Drupal\Core\Ajax\AnnounceCommand;
use Drupal\node\Entity\Node;
use Drupal\file\Entity\File;
use Drupal\comment\Entity\Comment;
use Drupal\Component\Utility\SafeMarkup;
use Symfony\Component\HttpFoundation\RedirectResponse;
use Drupal\Core\Link;
use Drupal\Core\Url;


class NycTestForm extends FormBase {
  use NycLocationServices;

  public $text = '';

  public $nid;

  /**   
   * {@inheritdoc}   
   */
  public function getFormId() {
    return 'nyc_test_form';
  }

  /**   
   * {@inheritdoc}   
   */
  public function buildForm(array $form, FormStateInterface $form_state, $nid = NULL) {
     $form['#prefix'] = '<div>';
      $form['name'] = [
          '#prefix' => '',
          '#suffix' => '',
          '#required'=>TRUE,
          '#attributes'=>[
            'aria-label'=>'Name',
            'tabindex'=>0, 
            'class'=>[
              'pull-left',
              'nyc-map-tabbable'
            ], 
            'id' => 'edit-name', 
            'name' => 'name'
          ],
          '#type' => 'textfield',
          '#element_validate' => array(array($this, 'nycTestFormElementValidator')),
          '#title' => t('Your name'),
         // '#default_value' => '',
          '#description' => t('enter your name.')
        ];


    $form['city'] = [
          '#prefix' => '',
          '#suffix' => '',
          '#required'=>TRUE,
          '#attributes'=>[
            'aria-label'=>'city',
            'tabindex'=>0, 
            'class'=>[
              'pull-left',
              'nyc-map-tabbable'
            ], 
            'id' => 'edit-city', 
            'name' => 'city'
          ],
          '#type' => 'textfield',
          '#element_validate' => array(array($this, 'nycTestFormElementValidator2')),
          '#title' => t('Your city'),
         // '#default_value' => '',
          '#description' => t('enter your name.')
        ];



    $form['actions'] = array('#type' => 'actions');
    
    $form['actions']['button'] = [
      '#type' => 'submit',
      '#value' => t('SUBMIT'),
      '#attributes' => [
        'tabindex'=>0,
        'class' => [
          'nyc-map-tabbable'
          //'use-ajax',
        ]
      ]
    ]; 
    $form['#suffix'] = '</div>';
    return $form;
  }

  /**
   * Validate handler
   * @param array
   * @param object
   * @return array
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    //@todo  Custom validation rules if needed
    parent::validateForm($form, $form_state);
    return $form;
  }

  /**   
   * AJAX callback handler that displays any errors or a success message.   
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $name = $form_state->getValue('name');
      $form_values = $form_state->getValues();
      $uid = \Drupal::currentUser()->id(); 
      $name2 = $form_values['name'];
      $msg = 'Thanks for submitting ' .$name. $name2;
      drupal_set_message($msg, 'status');
  }

  public static function nycTestFormElementValidator(&$element, FormStateInterface $form_state, &$complete_form) {
    $ok = true;
    if (!$ok) {
      $form_state->setErrorByName('name', 'Not ok');
    }
  }
  public static function nycTestFormElementValidator2(&$element, FormStateInterface $form_state, &$complete_form) {
    $ok = true;
    if (!$ok) {
      $form_state->setErrorByName('city', 'bad city');
    }
  } 



}