Drupal 8 Form API #ajax.callback

ajax_callback.md

'#ajax' => ['callback' is weird.

Looking at examples from Drupal core, there seems to be a bunch of different ways you can call it...

• 'callback' => 'some_global_function' is the simplest and always seems to work, but the function it calls has to be defined in a .module file, not a class.

• 'callback' => [$this, 'someFunction'] works sometimes, but you get a 'cannot serialize database service' error if any of $this' properties (or anything they reference) involves a database connection.

• 'callback' => [static::class, 'someFunction'] works, but calls someFunction() in a static context, meaning you can't use $this->someService->doSomething().

  This is equivalent to, and has the same problems as:

  • 'callback' => [get_class($this), 'someFunction'],
  • 'callback' => '\Fully\Qualified\Class::someFunction',
  • 'callback' => get_called_class() . '::someFunction', and
  • 'callback' => __CLASS__ . '::someFunction'.

• 'callback' => '::someFunction' is supposed to work in a dynamic context, but it only works on objects that implement FormInterface — so if your hook_form_alter() calls another class to do some of its work and that class wants to define an AJAX callback, it cannot (at least, not out-of-the-box)