hook_form_alter bypasses weight system

I actually stubled upon this issue when writing my own module and adding some fields programativally to fieldset created by form_group module. I am unable to set weight of field programatically on a field which is wrapped in fieldset by admin UI.

I can actually solve this by simple JS on client-side but I wanted to solve this in backend. What is the proposed resolution in this case? Can my form alter hook run after the field_group module form alter hook?

@lubwn - two suggestions: either use hook_module_implements_alter(), to get your hook_form_alter to act after field_group. Or add a #pre_render or #after_build callback to the form in your hook_form_alter. Those can then change things in the form array, after the form_alter stage.

either use hook_module_implements_alter(), to get your hook_form_alter to act after field_group.

This actually is not possible without modifying module weights, because field_group_module_implements_alter is run after custom implementations. So even if you do implement it, field group's will run later, causing field_group_form_alter to run last.

@mrweiner you should add a #pre_render to your form_alter and add a class implementing Drupal\Core\Security\TrustedCallbackInterface

user_module.module

<?php

use Drupal\user_module\YourCustomFormAlterBuilder;

function user_module_form_FORM_ID_alter(&$form, $form_state) {
  $form['#pre_render'][] = [YourCustomFormAlterBuilder::class, 'preRender'];
}

Add something like this in your modules src/ folder.

YourCustomFormAlterBuilder.php

<?php

namespace Drupal\user_module;

use Drupal\Core\Security\TrustedCallbackInterface;

class YourCustomFormAlterBuilder implements TrustedCallbackInterface {

  public static function trustedCallbacks() {
    return ['preRender'];
  }

  public static function preRender($form) {
    // Alter the form here.
    return $form;
}

Thanks #8 did the trick, but needed a tweak for the .module code

<?php

use Drupal\user_module\YourCustomFormAlterBuilder;

function user_module_form_FORM_ID_alter(&$form, $form_state) {
  $form['#pre_render'][] = [new YourCustomFormAlterBuilder, 'preRender'];
}

Great solution in #8. Maybe it should be added to the documentation?