atk4\data\Persistence::typecastSaveField PHP Method

    public function typecastSaveField(Field $f, $value)
    {
        // use $f->typecast = [typecast_save_callback, typecast_load_callback]
        if (is_array($f->typecast) && isset($f->typecast[0]) && is_callable($t = $f->typecast[0])) {
            return $t($value, $f, $this);
        }
        // normalize value
        $value = $f->normalize($value);
        // we respect null values
        if ($value === null) {
            return;
        }
        // run persistence-specific typecasting of field value
        return $this->_typecastSaveField($f, $value);
    }