TestFieldEmptyFormatter.php

<?php

namespace Drupal\field_test\Plugin\Field\FieldFormatter;

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

/**
 * Plugin implementation of the 'field_empty_test' formatter.
 *
 * @FieldFormatter(
 *   id = "field_empty_test",
 *   label = @Translation("Field empty test"),
 *   field_types = {
 *     "test_field",
 *   },
 *   weight = -5
 * )
 */
class TestFieldEmptyFormatter extends FormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        return [
            'test_empty_string' => '**EMPTY FIELD**',
        ] + parent::defaultSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        if ($items->isEmpty()) {
            // For fields with no value, just add the configured "empty" value.
            $elements[0] = [
                '#markup' => $this->getSetting('test_empty_string'),
            ];
        }
        else {
            foreach ($items as $delta => $item) {
                // This formatter only needs to output raw for testing.
                $elements[$delta] = [
                    '#markup' => $item->value,
                ];
            }
        }
        return $elements;
    }

}