Specifying conditional form elements

The Form API provides a mechanism for defining form element states. These states are mapped to JavaScript interactions that can control whether an element is required, visible, and more. In this example, we will demonstrate a form that has a disabled submit button until a checkbox is checked.

How to do it…

1. Create a file named ApprovalRequiredForm.php in the src/Form directory for your module to hold the ApprovalRequiredForm form class.
2. We will define the ApprovalRequiredForm class with a form ID of mymodule_approval_form:

   <?php

   namespace Drupal\mymodule\Form;

   use Drupal\Core\Form\FormBase;

   use Drupal\Core\Form\FormStateInterface;

   class ApprovalRequiredForm extends FormBase {

     public function getFormId() {

       return 'mymodule_approval_form';

     }

     public function buildForm(array $form,

       FormStateInterface $form_state) {

       return...