I'm putting together a fairly complex form with Drupal 8 (currently building on beta 7). In my validation function, I need to be able to detect whether a field, which was populated with default values, has changed.
In Drupal 7, this would have been pretty straightforward. I could dig into the form_state array and find the appropriate default_value entry and compare it to the submitted value.
But now that form_state is an encapsulated object, I don't see an obvious way to get the default value.
I know I can kluge this if I need to, by including hidden fields in the form itself with the default values. But I know that the information is in the form_state object: if I print_r form_state I can see both the old and new values--but not in any way I can figure out how to access using the methods listed in FormStateInterface.
Bottom line: in Drupal 8, how do I test whether the submitted value of a form element is the same as its default value?