class FieldSettings

class FieldSettings extends ProcessPluginBase {
    
    /**
     * {@inheritdoc}
     *
     * Get the field default/mapped settings.
     */
    public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
        // To maintain backwards compatibility, ensure that $value contains at least
        // three elements.
        if (count($value) == 2) {
            $value[] = NULL;
        }
        [
            $field_type,
            $global_settings,
            $original_field_type,
        ] = $value;
        return $this->getSettings($field_type, $global_settings, $original_field_type);
    }
    
    /**
     * Merge the default D8 and specified D6 settings.
     *
     * @param string $field_type
     *   The destination field type.
     * @param array $global_settings
     *   The field settings.
     * @param string $original_field_type
     *   (optional) The original field type before migration.
     *
     * @return array
     *   A valid array of settings.
     */
    public function getSettings($field_type, $global_settings, $original_field_type = NULL) {
        $max_length = $global_settings['max_length'] ?? '';
        $max_length = empty($max_length) ? 255 : $max_length;
        $allowed_values = [];
        if (isset($global_settings['allowed_values'])) {
            $list = explode("\n", $global_settings['allowed_values']);
            $list = array_map('trim', $list);
            $list = array_filter($list, 'strlen');
            switch ($field_type) {
                case 'list_string':
                case 'list_integer':
                case 'list_float':
                    foreach ($list as $value) {
                        $value = explode("|", $value);
                        $allowed_values[$value[0]] = $value[1] ?? $value[0];
                    }
                    break;
                default:
                    $allowed_values = $list;
            }
        }
        $settings = [
            'text' => [
                'max_length' => $max_length,
            ],
            'datetime' => [
                'datetime_type' => 'datetime',
            ],
            'list_string' => [
                'allowed_values' => $allowed_values,
            ],
            'list_integer' => [
                'allowed_values' => $allowed_values,
            ],
            'list_float' => [
                'allowed_values' => $allowed_values,
            ],
            'boolean' => [
                'allowed_values' => $allowed_values,
            ],
        ];
        if ($original_field_type == 'userreference') {
            return [
                'target_type' => 'user',
            ];
        }
        else {
            return $settings[$field_type] ?? [];
        }
    }

}