class FormTestInputForgeryForm

class FormTestInputForgeryForm extends FormBase implements TrustedCallbackInterface {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return '_form_test_input_forgery';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        // For testing that a user can't submit a value not matching one of the
        // allowed options.
        $form['checkboxes'] = [
            '#title' => t('Checkboxes'),
            '#type' => 'checkboxes',
            '#options' => [
                'one' => 'One',
                'two' => 'Two',
            ],
        ];
        $form['submit'] = [
            '#type' => 'submit',
            '#value' => t('Submit'),
        ];
        $form['#post_render'][] = [
            static::class,
            'postRender',
        ];
        return $form;
    }
    
    /**
     * Alters the rendered form to simulate input forgery.
     *
     * It's necessary to alter the rendered form here because Mink does not
     * support manipulating the DOM tree.
     *
     * @param string $rendered_form
     *   The rendered form.
     *
     * @return string
     *   The modified rendered form.
     *
     * @see \Drupal\Tests\system\Functional\Form\FormTest::testInputForgery()
     */
    public static function postRender($rendered_form) {
        return str_replace('value="two"', 'value="FORGERY"', $rendered_form);
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        return new JsonResponse($form_state->getValues());
    }
    
    /**
     * {@inheritdoc}
     */
    public static function trustedCallbacks() {
        return [
            'postRender',
        ];
    }

}