drupal-form-validation

Drupal FormBase custom form validation is implemented by overriding the validateForm() method to enforce data integrity and call setErrorByName() to attach targeted errors to specific fields before submission.

Use #element_validate in the Drupal Form API when you need per-field validation logic attached directly to the form element, rather than validating the entire form structure at once within validateForm().

Conditional validation rules are added to an existing Drupal form by implementing hook_form_alter, allowing you to inject custom validators and apply dynamic data integrity checks based on specific runtime conditions.

To set validation errors on nested fields during Drupal form validation, use the setError() and setErrorByName() functions, which support targeting specific elements within the form structure to display precise error messages.

Yes, this form validation approach is specifically designed for the Drupal 11 Form API, encapsulating best practices to reduce invalid submissions and maintenance costs for FormBase forms requiring custom validators and conditional rules.

The best way to reduce repetitive Drupal form validation code is to encapsulate best practices using validateForm(), #element_validate, and hook_form_alter, ensuring reliable data integrity while lowering long-term maintenance costs.