Cache DcaExtractor result

[Toflar]

I discovered a huge performance issue with the DcaExtractor while doing some performance analysis for Isotope with xdebug. Contao calls $objDca = new \DcaExtractor(static::$strTable); every single time a model is created (see Model::__construct()). In my case I had 1300 model instances which results in 1300 instances of DcaExtractor. Each and everyone including the 'system/cache/sql/' . $strTable . '.php'; and thus accessing the file system 1300 times. In fact, it would be enough to do this one time per table.

I suggest introducing a DcaExtractor::getForTable($strTable) and using this one wherever possible:

public static function getForTable($strTable)
{
    if (static::$arrInstances[$strTable] === null)
    {
        static::$arrInstances[$strTable] = new static($strTable);
    }

    return static::$arrInstances[$strTable];
}

I was able to speed up an Isotope product list generation by 6%.

[leofeyer]

Changed in 800ed2d. Unfortunately, I cannot protect the constructor to force people to use the getInstance() method, because it would break backwards compatibility.

[Toflar]

I know, that's fine :)

[leofeyer]

Changed it in Contao 4 though: contao/contao-library@4336580f10967f19a3ff78affe10a4c28fce7926

[leofeyer]

The change log entry and update notice is here: contao/contao@94ed57e