class StringTextfieldWidget

class StringTextfieldWidget extends WidgetBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        return [
            'size' => 60,
            'placeholder' => '',
        ] + parent::defaultSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $element['size'] = [
            '#type' => 'number',
            '#title' => t('Size of textfield'),
            '#default_value' => $this->getSetting('size'),
            '#required' => TRUE,
            '#min' => 1,
        ];
        $element['placeholder'] = [
            '#type' => 'textfield',
            '#title' => t('Placeholder'),
            '#default_value' => $this->getSetting('placeholder'),
            '#description' => t('Text that will be shown inside the field until a value is entered. This hint is usually a sample value or a brief description of the expected format.'),
        ];
        return $element;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = [];
        $summary[] = t('Textfield size: @size', [
            '@size' => $this->getSetting('size'),
        ]);
        $placeholder = $this->getSetting('placeholder');
        if (!empty($placeholder)) {
            $summary[] = t('Placeholder: @placeholder', [
                '@placeholder' => $placeholder,
            ]);
        }
        return $summary;
    }
    
    /**
     * {@inheritdoc}
     */
    public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
        $element['value'] = $element + [
            '#type' => 'textfield',
            '#default_value' => isset($items[$delta]->value) ? $items[$delta]->value : NULL,
            '#size' => $this->getSetting('size'),
            '#placeholder' => $this->getSetting('placeholder'),
            '#maxlength' => $this->getFieldSetting('max_length'),
            '#attributes' => [
                'class' => [
                    'js-text-full',
                    'text-full',
                ],
            ],
        ];
        return $element;
    }

}