(External) Drupal #ajax callback methods must be static

Should you encounter an error like 

The specified #ajax callback is empty or not callable form

Then presumably you're using something like this:

'#ajax' => [
  'callback' => '::updateFormAjaxCallback',
  'wrapper' => 'layout-wrapper',
],

or an external class callback:

'#ajax' => [
  'callback' => '\Drupal\homebox\Form\HomeboxPersonalizeForm::updateFormAjaxCallback',
  'wrapper' => 'layout-wrapper',
],

As you could guess from the :: notation, the called method needs to be static!

So this won't work:

public function updateFormAjaxCallback(array &$form, FormStateInterface $form_state): array {
    // Replace the whole layout:
    return $form['layout_wrapper'];
}

But instead, this will work (see the static):

public static function updateFormAjaxCallback(array &$form, FormStateInterface $form_state): array {
    // Replace the whole layout:
    return $form['layout_wrapper'];
  }

Non-static calls with $this

In contrast to that, if you're using

'#ajax' => [
  'callback' => [$this, 'updateFormAjaxCallback'],
  'wrapper' => 'layout-wrapper',
],

here the non-static call will work (as it's being called on the object / instance)!

Other potential reasons

Thanks to @Jaypan:

• The class that the ajax callback lives in must implement Drupal\Core\Security\TrustedCallbackInterface
• The class must also implement the trustedCallbacks() function, returning an array with the name of your callback function

(https://www.drupal.org/forum/support/module-development-and-code-questions/2022-08-26/the-specified-ajax-callback-is-empty-or-not-callable#comment-15278795)

Mind array vs. string

Another risky thing with the callback is array vs. string notation!

When using the shorthand, ensure you're returning a string, not an array:

Doesn't work:

'callback' => ['::updateFormAjaxCallback'],

Works:

'callback' => '::updateFormAjaxCallback',

These are really hard to find bugs ;)