ActionAjaxTest.php

<?php

namespace Drupal\action_form_ajax_test\Plugin\Action;

use Drupal\Core\Access\AccessResult;
use Drupal\Core\Action\ConfigurableActionBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Session\AccountInterface;

/**
 * Plugin used for testing AJAX in action config entity forms.
 *
 * @Action(
 *   id = "action_form_ajax_test",
 *   label = @Translation("action_form_ajax_test"),
 *   type = "system"
 * )
 */
class ActionAjaxTest extends ConfigurableActionBase {
    
    /**
     * {@inheritdoc}
     */
    public function defaultConfiguration() {
        return [
            'party_time' => '',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function access($object, AccountInterface $account = NULL, $return_as_object = FALSE) {
        $result = AccessResult::allowed();
        return $return_as_object ? $result : $result->isAllowed();
    }
    
    /**
     * {@inheritdoc}
     */
    public function execute() {
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
        $having_a_party = $form_state->getValue('having_a_party', !empty($this->configuration['party_time']));
        $form['having_a_party'] = [
            '#type' => 'checkbox',
            '#title' => $this->t('Are we having a party?'),
            '#ajax' => [
                'wrapper' => 'party-container',
                'callback' => [
                    $this,
                    'partyCallback',
                ],
            ],
            '#default_value' => $having_a_party,
        ];
        $form['container'] = [
            '#type' => 'container',
            '#prefix' => '<div id="party-container">',
            '#suffix' => '</div>',
        ];
        if ($having_a_party) {
            $form['container']['party_time'] = [
                '#type' => 'textfield',
                '#title' => $this->t('Party time'),
                '#default_value' => $this->configuration['party_time'],
            ];
        }
        return $form;
    }
    
    /**
     * Callback for party checkbox.
     */
    public function partyCallback(array $form, FormStateInterface $form_state) {
        return $form['container'];
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
        $this->configuration['party_time'] = $form_state->getValue('party_time');
    }

}