The Form API requires all form classes to implement the
\Drupal\Core\Form\FormInterface. The interface defines a
validation method. The
validateForm method is invoked once a form has been submitted and provides a way to validate the data and halt the processing of the data if required. The form state object provides methods for marking specific fields as having the error, providing a user experience tool to alert your users specifically to the problem input.
This recipe will be based on the custom module and form created in the Creating a form section of this chapter. We will be validating the length of the submitted field.
This recipe will be using the module and custom form created in the first Creating a form ...