Custom AJAX command to trigger Drupal.attachBehaviors() from PHP

Sometimes developing complex forms with AJAX there is a necessity to force triggering Drupal behaviors. Drupal.attachBehaviors() is used to be called after AJAX call but sometimes in specific cases it doesn't work. For such cases there is a solution – creation of custom AJAX command.

First thin you need to do is to create a PHP class for new AJAX command in Drupal\my_module\Ajax namespace:

<?php

namespace Drupal\my_module\Ajax;

use Drupal\Core\Ajax\CommandInterface;

/**
 * An AJAX command to trigger Drupal.attachBehaviors().
 */
class TriggerAttachBehaviors implements CommandInterface {

  /**
   * {@inheritdoc}
   */
  public function render() {
    return [
      'command' => 'triggerAttachBehaviors',
    ];
  }

}

Then you need to create js file in your module folder e.g.:  assets/js/attach-behaviors-command.js:

(function (Drupal) {

  /**
   * Ajax command for trigger Drupal.attachBehaviors().
   */
  Drupal.AjaxCommands.prototype.triggerAttachBehaviors = function (ajax, response, status) {
    Drupal.attachBehaviors();
  };

})(Drupal);

Javascript file has to be added to some library to be added to the page, like

my_module.custom_js:
  js:
    assets/js/attach-behaviors-command.js: { }
  dependencies:
    - core/drupal
    - core/drupal.ajax

Feel free to attach this library any way you know. After that it will be possible to use that custom AJAX command inside AJAX callbacks the following way:
 

  /**
   * {@inheritdoc}
   */
  public function ajaxFormCallback(array &$form, FormStateInterface $form_state, $reset_secondary = FALSE) {
    $response = parent::ajaxFormCallback($form, $form_state, $reset_secondary);

    // Any needed logic.

    // Triggering behaviors.
    $response->addCommand(new TriggerAttachBehaviors());

    // The rest of the logic.
    return $response;
  }

That's it!