class TestFieldEmptyFormatter

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;
    }

}