Custom.php

<?php

namespace Drupal\views\Plugin\views\field;

use Drupal\Component\Utility\Xss;
use Drupal\Core\Form\FormStateInterface;
use Drupal\views\Render\ViewsRenderPipelineMarkup;
use Drupal\views\ResultRow;

/**
 * A handler to provide a field that is completely custom by the administrator.
 *
 * @ingroup views_field_handlers
 *
 * @ViewsField("custom")
 */
class Custom extends FieldPluginBase {
    
    /**
     * {@inheritdoc}
     */
    public function usesGroupBy() {
        return FALSE;
    }
    
    /**
     * {@inheritdoc}
     */
    public function query() {
        // do nothing -- to override the parent query.
    }
    
    /**
     * {@inheritdoc}
     */
    protected function defineOptions() {
        $options = parent::defineOptions();
        // Override the alter text option to always alter the text.
        $options['alter']['contains']['alter_text'] = [
            'default' => TRUE,
        ];
        $options['hide_alter_empty'] = [
            'default' => FALSE,
        ];
        return $options;
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildOptionsForm(&$form, FormStateInterface $form_state) {
        parent::buildOptionsForm($form, $form_state);
        // Remove the checkbox
        unset($form['alter']['alter_text']);
        unset($form['alter']['text']['#states']);
        unset($form['alter']['help']['#states']);
        $form['#pre_render'][] = [
            $this,
            'preRenderCustomForm',
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public static function trustedCallbacks() {
        $callbacks = parent::trustedCallbacks();
        $callbacks[] = 'preRenderCustomForm';
        return $callbacks;
    }
    
    /**
     * {@inheritdoc}
     */
    public function render(ResultRow $values) {
        // Return the text, so the code never thinks the value is empty.
        return ViewsRenderPipelineMarkup::create(Xss::filterAdmin($this->options['alter']['text']));
    }
    
    /**
     * Prerender function to move the textarea to the top of a form.
     *
     * @param array $form
     *   The form build array.
     *
     * @return array
     *   The modified form build array.
     */
    public function preRenderCustomForm($form) {
        $form['text'] = $form['alter']['text'];
        $form['help'] = $form['alter']['help'];
        unset($form['alter']['text']);
        unset($form['alter']['help']);
        return $form;
    }

}