Form class private member variable null in submitForm when form uses AJAX

I have provided a repro case demonstrating the steps given above.

I think this may be more of a support request. Does https://drupal.stackexchange.com/questions/231436/form-ajax-not-working-... explain what you are seeing?

I don't think so. I did, however, get some insight from Conrad Lara (@cmlara) in the Slack #support channel. Apparently Drupal sometimes (but not always) serializes the instance of the FormBase-derived class and then reconstitutes most of it, invoking submitForm() on this partially-resurrected instance. In the process it drops the private member variables on the floor. "Sometimes" likely means "when AJAX is in play" in this context. As far as Conrad is aware, this behavior isn't documented anywhere. But sure enough, when I replace private with protected in the repro code I posted the form works correctly. I will edit the original issue summary to clarify that it's private member variables which are discarded.

This has been an issue for us as well. Switching all private variables to protected works to preserve the data in those variables during AJAX calls.

There is documentation about the serialization issue: https://www.drupal.org/docs/drupal-apis/services-and-dependency-injectio... →