Altering the #options property of a checkboxes field in an ajax callback function doesn't reflect the changes.

I'm on 8.5.3, facing the same issue exactly. @gnuget did you ever find a solution?

Yep same here, D8.5.3.

Same here D8.5

Same here I've just run into this too. It looks like each options is in its own section in the array e.g:
$form['test'][0]
$form['test'][1]

So it looks like setting the options has no effect because the form is now using these renderable elements. e.g.:
$form['test'][0] = ['#type' => 'checkbox'..... ]

So looks something needs to be triggered to rebuild those options but unfortunately I cant see what. And manually recreating them isn't really an option as all the labels and input attributes are all lost if you try and recreate them with:
$form['test'][0] = ['#type' => 'checkbox'..... ]

I was able to get this to work by fighting really hard. Here's a work-around, which I think makes it pretty clear that this needs a fix. I'm doing this in a custom ajax callback ($form['my_field']['#ajax'] = `_my_custom_callback`:

use Drupal\Core\Render\Element;
use Drupal\Core\Render\Element\Checkboxes;

function _my_custom_callback(array &$form, FormStateInterface $form_state) {
  $response = new AjaxResponse();
  $renderer = \Drupal::service('renderer');
  foreach (Element::children($form['my_field']) as $i) {
     unset($form['my_field'][$i]);
  }
  Checkboxes::processCheckboxes($form['my_field'], $form_state, $form);
  $response->addCommand(new ReplaceCommand("#my_field_id", $renderer->render($form['my_field'])));
  return $response;
}

I discovered that with the above code, my options were indeed renewed to match what I had put into #options, but if they had been checked--either when the form was initially built or by the user in the time since then--those defaults were lost once the ajax callback had rebuilt the field. Using FormBuilder->doBuildForm() instead of Checkboxes::processCheckboxes()addressed that.

use Drupal\Core\Render\Element;
use Drupal\Core\Render\Element\Checkboxes;

function _my_custom_callback(array &$form, FormStateInterface $form_state) {
  $response = new AjaxResponse();
  $renderer = \Drupal::service('renderer');
  foreach (Element::children($form['my_field']) as $i) {
     unset($form['my_field'][$i]);
  }
  \Drupal::formBuilder()->doBuildForm($form['#form_id'], $field, $form_state);
  $response->addCommand(new ReplaceCommand("#my_field_id", $renderer->render($form['my_field'])));
  return $response;
}

@ #12

May you please post your full code as an example ? :)
When I am trying to use your code, getValue() on the checkbox field is empty.

Hey @sgurlt - sure! Hope this helps.

use Drupal\Core\Render\Element;
use Drupal\Core\Render\Element\Checkboxes;


/**
 * Implements hook_form_alter().
 */
function mymodule_form_alter(array &$form, FormStateInterface $form_state, $form_id) {
  if ($form_id == 'the_form_id_i_care_about') {
    // This is a multi-value field with a "check boxes/radio buttons" widget.
    $form['my_field']['widget']['#ajax'] = [
      'callback' => '_my_custom_callback',
    ];
  }
}

function _my_custom_callback(array &$form, FormStateInterface $form_state) {
  $response = new AjaxResponse();
  $renderer = \Drupal::service('renderer');
  foreach (Element::children($form['my_field']) as $i) {
     unset($form['my_field'][$i]);
  }
  \Drupal::formBuilder()->doBuildForm($form['#form_id'], $field, $form_state);
  $response->addCommand(new ReplaceCommand("#my_field_id", $renderer->render($form['my_field'])));
  return $response;
}

I think you might be going about this the wrong way.

My experience with D8 ajax forms has lead me to stick with several strict rules. Two of them are:

The only good place to build or modify the form is in the buildForm method. Don't do that anywhere else.
The ajax callback function should do nothing but return the form or a part of it. Don't do anything else in the callback.

Violating those is the path to pain and frustration :)

For the example discussed here, you can achieve what you want by doing something like this in buildForm.

    if ($form_state->getValue('target') === 'option2') {
      $options = ['test3' => 'Test 3', 'test4' => 'Test4'];
    }
    else {
      $options = ['test1' => 'Test 1', 'test2' => 'Test2'];
    }

    $form['test'] = [
      '#type' => 'checkboxes',
      '#title' => $this->t('Options'),
      '#options' => $options,
      '#prefix' => '<div id="edit-options">',
      '#suffix' => '</div>',
    ];

That works with a triggering element (i.e., target here) which is not a submit button. When the ajax happens, validateForm is called and then the form is rebuilt so that inserted code knows the new value of target.

To add some unsolicited advice:
I tend to use submit buttons as the triggering element where possible (they can be styled to look like links). That's obviously not always possible like in this case but when a submit button is used a custom submit callback should be added with #submit.

In the custom submit callback, my technique is to set flags or whatever in $form_state->set(), and then call $form_state->setRebuild(). The data you set using $form_state->set() is available in buildForm with $form_state->get(). Use that to dynamically modify the form.

I ran into the same issue today, trying to change options on a checkboxes element in an Ajax callback. @tetranz, your advice rings a bell, and I think it was the same situation in D7, but I'd forgotten how it worked. For me the uncertainty came from not knowing that it bounces back into the buildForm method while processing the Ajax call. Sure enough, when stepping with the debugger I can see it doing that. The one thing I would add to your example is that in the Ajax callback method you would just return $form['test']. In my testing that worked for rebuilding the checkboxes form element.

I'm having a similar problem except that it is a dropdown select instead of checkboxes. Drupal 9.5.9, PHP 8.0. I have one select field that has an AJAX callback which restricts the allowed options of another dropdown field. It will work once, showing the message that it is running and then updating the values of the second field. But then if I try to change the first field again, it will show the message that it is running and afterward the options of the second field have not changed at all.

I tried adding a couple of the options mentioned above that did not help:

\Drupal::formBuilder()->doBuildForm($form['#form_id'], $field, $form_state);

and

\Drupal\Core\Render\Element\Select::processSelect($element,$form_state,$form);

Here's some more context of the code:

class ReservoBookingForm extends ContentEntityForm {

  /** AJAX fired function to determine allowed values of end time */
  public function updateEndTimes(array &$form, FormStateInterface $form_state) {
    $element = $form['booking_end_time'];
    $options = [different complicated function to get valid options];
    $element['widget']['#options'] = $options;
    $element['widget']['#empty_option'] = [different function that returns different text whether there are any remaining options or not]
    /* Note: empty option requires patch for a bug discussed here: https://www.drupal.org/project/drupal/issues/3180011 */ 
    return $element;

  }

  public function buildForm(array $form, FormStateInterface $form_state) {

    $form['booking_start_time']['widget']['#ajax'] = [
      'wrapper' => 'edit-booking-end-time-wrapper',
      'callback' => [$this, 'updateEndTimes'],
      'event' => 'change',
      'progress' => [
        'type' => 'throbber',
        'message' => $this->t('Checking available end times'),
      ],
    ];

    return $form;

  }

}