Drupal 8: CRUD admin form (AJAX add, delete)

ExampleCrudAdminForm.php

<?php

namespace Drupal\example\Form;

use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Form\ConfigFormBase;
use Drupal\Core\Form\FormStateInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\HttpFoundation\RequestStack;

/**
 * Class ExampleCrudAdminForm.
 */
class ExampleCrudAdminForm extends ConfigFormBase {

  /**
   * Property: Number of items in form
   *
   * @var integer
   */
  protected $itemTotal = 1;

  /**
   * Temporary config, to be used by the Remove button.
   *
   * @var array
   */
  protected $tempConfig = [];

  /**
   * Item id to remove.
   *
   * @var integer
   */
  protected $itemToRemove;

  /**
   * Form state
   *
   * @var FormStateInterface
   */
  protected $formState = NULL;

  /**
   * Request stack
   *
   * @var RequestStack
   */
  protected $request;

  /**
   * Class constructor.
   */
  public function __construct(ConfigFactoryInterface $config_factory, RequestStack $RequestStack) {
    parent::__construct($config_factory);
    $this->tempConfig = $this->config('example.config')->get('items');
    $this->request = $RequestStack;
  }

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


  /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return [
      'example.config',
    ];
  }

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

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    if (is_null($this->formState)) {
      $this->formState = $form_state;
    }
    // Get config.
    $items = $this->tempConfig;
    // Make sure to use tree.
    $form['#tree'] = TRUE;
    // Disable caching on this form.
    $form_state->setCached(FALSE);
    // Get number of items already loaded into config.
    if (!empty($items) && !$form_state->get('ajax_pressed')) {
      $this->itemTotal = count($items) > 0 ? count($items) : 1;
    }
    // Build items container.
    $form['items'] = [
      '#type' => 'table',
      '#header' => [
          $this->t('Text'),
          $this->t('Select'),
          '',
      ],
      '#empty' => $this->t('No privileges.'),
      '#tableselect' => FALSE,
      '#attributes' => ['id' => 'items-container'],
    ];
    for ($i = 0; $i < $this->itemTotal; $i++) {
      $form['items'][$i] = [
        '#type'       => 'fieldset',
      ];
      // Path.
      $form['items'][$i]['text'] = [
        '#type'       => 'textfield',
        '#attributes' => ['placeholder' => $this->t('Path to rewrite')],
        '#size'       => 50,
        '#required' => TRUE,
        '#default_value' => empty($items[$i]['text']) ? '' : $items[$i]['text'],
      ];
      // View.
      $form['items'][$i]['select'] = [
        '#type'       => 'select',
        '#options' => [
          'First',
          'Second',
          'Third',
        ],
        '#default_value' => empty($items[$i]['select']) ? null : $items[$i]['select'],
        '#required' => TRUE,
      ];
      // Remove button.
      $form['items'][$i]['remove_item_' . $i] =[
        '#type'                    => 'submit',
        '#name'                    => 'remove_' . $i,
        '#value'                   => $this->t('Remove'),
        '#submit'                  => ['::removeItem'],
        // Since we are removing an item, don't validate until later.
        '#limit_validation_errors' => [],
        '#ajax'                    => [
          'callback' => [$this, 'ajaxCallback'],
          'wrapper'  => 'items-container',
        ],
      ];
    }
    // Add item button.
    $form['items']['actions'] = [
      '#type' => 'actions',
      'add_item' => [
        '#type'   => 'submit',
        '#value'  => $this->t('Add a new item'),
        '#submit' => ['::addItem'],
        '#ajax'   => [
          'callback' => [$this, 'ajaxCallback'],
          'wrapper'  => 'items-container',
        ],
      ]
    ];

    return parent::buildForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
  }

  /**
   * Implements callback for Ajax event
   *
   * @param array $form
   *   From render array.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   Current state of form.
   *
   * @return array
   *   Container section of the form.
   */
  public function ajaxCallback($form, $form_state) {
    // Set new values if remove was pressed.
    if ($this->getCurrentRequestVariable('remove_pressed')) {
      // Get input values;
      $values = $form_state->getUserInput();
      // Remove the removed item;
      unset($values['items'][$this->itemToRemove]);
      $values['items'] = array_combine(range(0, count($values['items']) - 1), array_values($values['items']));
      // Set new values;
      for ($i = 0; $i < $this->itemTotal; $i++) {
        $form['items'][$i]['text']['#value'] = empty($values['items'][$i]['text']) ? '' : $values['items'][$i]['text'];
        $form['items'][$i][]['#value'] = empty($values['items'][$i]['select']) ? '' : $values['items'][$i]['select'];
      }
    }
    // This is Request-specific variable necessary because a 'removePressed'
    // class property would persist between requests because of form caching.
    $this->setCurrentRequestVariable('remove_pressed', FALSE);
    return $form['items'];
  }

  /**
   * Adds an item to form
   *
   * @param array $form
   * @param FormStateInterface $form_state
   */
  public function addItem(array &$form, FormStateInterface $form_state) {
    $form_state->set('ajax_pressed', TRUE);
    $this->itemTotal++;
    $form_state->setRebuild();
  }

  /**
   * Removes an item from form
   *
   * @param array $form
   * @param FormStateInterface $form_state
   */
  public function removeItem(array &$form, FormStateInterface $form_state) {
    $form_state->set('ajax_pressed', TRUE);
    // This is Request-specific variable necessary because a 'removePressed'
    // class property would persist between requests because of form caching.
    $this->setCurrentRequestVariable('remove_pressed', TRUE);
    $this->itemTotal--;
    // Get triggering item id;
    $triggering_element = $form_state->getTriggeringElement();
    preg_match_all('!\d+!', $triggering_element['#name'], $matches);
    $item_id = (int) $matches[0][0];
    $this->itemToRemove = $item_id;
    // Remove item from config, reindex at 1, and set tempConfig to it.
    unset($this->tempConfig[$item_id]);
    $this->tempConfig = array_combine(range(0, count($this->tempConfig) - 1), array_values($this->tempConfig));
    // Rebuild form;
    $form_state->setRebuild();
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    parent::submitForm($form, $form_state);
    // add to config
    $items_values =  $form_state->getValue('items');
    unset($items_values['actions']);
    foreach ($items_values as $key => &$value) {
      unset($value['remove_item_' . $key]);
    }
    $this->config('example.config')
      ->set('items', $items_values)
      ->save();
  }

  /**
   * Set volatile variable, specific to current request time
   *
   * @param string $name
   * @param mixed $value
   */
  protected function setCurrentRequestVariable($name, $value) {
    $vars_identifier = sha1($this->request->getCurrentRequest()->server->get('REQUEST_TIME'));
    $vars = $this->formState->get($vars_identifier) ? $this->formState->get($vars_identifier) : [];
    $vars[$name] = $value;
    $this->formState->set($vars_identifier, $vars);
  }

  /**
   * Get volatile variable, specific to current request time
   *
   * @param mixed|null $name
   */
  protected function getCurrentRequestVariable($name) {
    $vars_identifier = sha1($this->request->getCurrentRequest()->server->get('REQUEST_TIME'));
    if (($vars = $this->formState->get($vars_identifier)) && isset($vars[$name])) {
      return $vars[$name];
    }
    return NULL;
  }

}

good example. see this article and sample code