How can I run a certain action when a taxonomy field value is removed or added? [closed]

Question

I have a user_territories field which can have multiple values. I need to run a certain action when this field value is changed from the user entity, not when other fields are changed.

How can I write a condition for that?

I need to perform an action when taxonomy field is deleted and another action when a taxonomy field is added. How can I achieve this?
Is hook_user_presave() the best hook to fulfill this?

Answer 1

Yes, hook_user_presave() is the way to go.

Here's an example for a standard field. You'll need to tweak this a bit for a taxonomy entity reference.

function MYMODULE_user_presave(UserBundle $user): void {
  // If the user is new, then the value hasn't changed.
  if (!$user->isNew()) {
    $field_value_current = $user->get('my_field_name')->value;
    $field_value_previous = $user->original->get('my_field_name')->value;
    if ($field_value_current !== $field_value_previous) {
      // Do something
    }
  }
}