class TestFieldDefaultFormatter

class TestFieldDefaultFormatter extends FormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        return [
            'test_formatter_setting' => 'dummy test string',
        ] + parent::defaultSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $element['test_formatter_setting'] = [
            '#title' => $this->t('Setting'),
            '#type' => 'textfield',
            '#size' => 20,
            '#default_value' => $this->getSetting('test_formatter_setting'),
            '#required' => TRUE,
        ];
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = [];
        $summary[] = $this->t('@setting: @value', [
            '@setting' => 'test_formatter_setting',
            '@value' => $this->getSetting('test_formatter_setting'),
        ]);
        return $summary;
    }
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        foreach ($items as $delta => $item) {
            $elements[$delta] = [
                '#markup' => $this->getSetting('test_formatter_setting') . '|' . $item->value,
            ];
        }
        return $elements;
    }

}