Recently, I have been working on a site which has a big mutli-step form which is using a lot of custom form elements, custom auto-completes, custom form api states and ajax based sub-forms. It is all built using Drupal form api. The best thing about this form is that all the fields and sub-forms map to a data model. This gives us the benefit of validating the form fields using the Symfony validation component. This is good because we can write all the validation separately and test it using phpunit without bootstrapping Drupal. To convert this data model into form fields we wrote a form factory and connected it all together using the service container module.

Whenever you are working with complex multistep forms and updating a sub-form based on user input then you can't exactly show the form errors in the order in which the fields appeared on the form. In this post I'll explain a process in which we can achieve just that - keeping the form error messages in the same order as the form fields.