buildForm: how differentiate between page reload and ajax callback?

Question

I'm building a custom form in Drupal 9, which has some ajax callbacks.

During the buildForm I need to load some extra data via a rest call to an external service, which then I put inside a private_tempstore variable.

I'd like to avoid to call the rest end-point during the ajax callbacks, and rely on the stored variable.

However, I can't find a way to differentiate between the "load page" case and the "ajax case". Is it possible?

I've found this answer that seems to generally work:

// Example for brevity only, inject the request_stack service and call 
// getCurrentRequest() on it to get the request object if possible.
$request = \Drupal::request();
$is_ajax = $request->isXmlHttpRequest();

But I'd like to know if there's some helper\solution using the form API.

Answer 1

Since Drupal 8 the form object instantiated with buildForm() is not preserved between the request rendering the form and the first Ajax request. So be prepared that buildForm() is called again and has to produce the exact same result. When you get data from $form_state this is not the data you expect from the first buildForm() because this is never cached. Adding to the complexity the rendered result of the first build is cached, so what you store in the first build elsewhere, for example in tempStore, might be outdated in the Ajax request. The only data which works as expected are form values, which can be hidden if you want to have them in the submitted form and not visible to the user.

TLDR: buildForm() is called more often than you think and you shouldn't put code in it which is expensive to run. Refactor the external API call to a service, with proper caching, so that it doesn't matter how often it is called. Invalidate the service cache the same way as the rendered form, so that neither of them has outdated data.