class ExifFieldWidgetBase

abstract class ExifFieldWidgetBase extends ExifWidgetBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        return [
            'exif_field_separator' => '',
            'exif_field' => 'naming_convention',
        ] + parent::defaultSettings();
    }
    
    /**
     * Ensure field is correctly configured.
     *
     * @param array $element
     *   A form element array containing basic properties for the widget.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     * @param array $form
     *   The form structure where widgets are being attached to.
     */
    public static function validateExifField(array $element, FormStateInterface $form_state, array $form) {
        $elementSettings = $form_state->getValue($element['#parents']);
        if (!$elementSettings) {
            $message = t('you must choose at least one method to retrieve image metadata.');
            $form_state->setErrorByName('exif_field', $message);
        }
    }
    
    /**
     * Ensure field separator is correctly configured.
     *
     * @param array $element
     *   A form element array containing basic properties for the widget.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     */
    public static function validateExifFieldSeparator(array $element, FormStateInterface &$form_state) {
        $elementSettings = $form_state->getValue($element['#parents']);
        if (!empty($elementSettings) && strlen($elementSettings) > 1) {
            $message = t('the separator is only one character long.');
            $form_state->setErrorByName('exif_field_separator', $message);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $element = parent::settingsForm($form, $form_state);
        $exif_fields = $this->retrieveExifFields();
        $default_exif_value = $this->retrieveExifFieldDefaultValue();
        $default_exif_separator_value = $this->retrieveExifFieldDefaultSeparatorValue();
        $element['exif_field'] = [
            '#type' => 'select',
            '#title' => t('exif field data'),
            '#description' => t('choose to retrieve data from the image field referenced with the selected name or by naming convention.'),
            '#options' => array_merge([
                'naming_convention' => 'name of the field is used as the exif field name',
            ], $exif_fields),
            '#default_value' => $default_exif_value,
            '#element_validate' => [
                [
                    get_class($this),
                    'validateExifField',
                ],
            ],
        ];
        $element['exif_field_separator'] = [
            '#type' => 'textfield',
            '#title' => t('exif field separator'),
            '#description' => t('separator used to split values (if field definition support several values). let it empty if you do not want to split values.'),
            '#default_value' => $default_exif_separator_value,
            '#element_validate' => [
                [
                    get_class($this),
                    'validateExifFieldSeparator',
                ],
            ],
        ];
        return $element;
    }
    
    /**
     * List of possible fields.
     *
     * @return array
     *   List of possible exif fields.
     */
    private function retrieveExifFields() {
        $exif = ExifFactory::getExifInterface();
        return $exif->getFieldKeys();
    }
    
    /**
     * Get exif field name associated to this drupal field.
     *
     * If none found, use naming convention.
     *
     * @return string
     *   name of the exif field or string 'naming_convention'.
     */
    private function retrieveExifFieldDefaultValue() {
        $result = $this->getSetting('exif_field');
        if (empty($result)) {
            $result = 'naming_convention';
        }
        return $result;
    }
    
    /**
     * Get separator value from settings.
     *
     * @return string
     *   separator value or empty string.
     */
    private function retrieveExifFieldDefaultSeparatorValue() {
        $result = $this->getSetting('exif_field_separator');
        if (empty($result)) {
            $result = '';
        }
        return $result;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = parent::settingsSummary();
        $exif_field_separator = $this->getSetting('exif_field_separator');
        if (isset($exif_field_separator) && strlen($exif_field_separator) == 1) {
            $exif_field_msg = t("exif value will be split using character separator '@separator'", [
                '@separator' => $exif_field_separator,
            ]);
        }
        else {
            $exif_field_msg = t('exif value will be extracted as one value');
        }
        array_unshift($summary, $exif_field_msg);
        $exif_field = $this->getSetting('exif_field');
        if (isset($exif_field) && $exif_field != 'naming_convention') {
            $exif_field_msg = t("exif data will be extracted from image metadata field '@metadata'", [
                '@metadata' => $exif_field,
            ]);
        }
        else {
            $fieldname = $this->fieldDefinition
                ->getName();
            $exif_field = str_replace("field_", "", $fieldname);
            $exif_field_msg = t("Using naming convention. so the exif data will be extracted from image metadata field '@metadata'", [
                '@metadata' => $exif_field,
            ]);
        }
        array_unshift($summary, $exif_field_msg);
        return $summary;
    }

}