BasicStringFormatter.php

<?php

namespace Drupal\Core\Field\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\FormatterBase;
use Drupal\Core\Field\FieldItemListInterface;

/**
 * Plugin implementation of the 'basic_string' formatter.
 *
 * @FieldFormatter(
 *   id = "basic_string",
 *   label = @Translation("Plain text"),
 *   field_types = {
 *     "string_long",
 *     "email"
 *   }
 * )
 */
class BasicStringFormatter extends FormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        foreach ($items as $delta => $item) {
            // The text value has no text format assigned to it, so the user input
            // should equal the output, including newlines.
            $elements[$delta] = [
                '#type' => 'inline_template',
                '#template' => '{{ value|nl2br }}',
                '#context' => [
                    'value' => $item->value,
                ],
            ];
        }
        return $elements;
    }

}