atk4\data\Persistence::serializeSaveField PHP Method

    public function serializeSaveField(Field $f, $value)
    {
        // use $f->serialize = [encode_callback, decode_callback]
        if (is_array($f->serialize) && isset($f->serialize[0]) && is_callable($t = $f->serialize[0])) {
            return $t($f, $value, $this);
        }
        // run persistence-specific serialization of field value
        return $this->_serializeSaveField($f, $value);
    }