` tag
* which instructs the browser to start prefetching DNS
* records for every domain listed in the `domains` attribute
* of this ViewHelper. Prefetching starts as soon as the browser
* becomes aware of the tag - to optimise even further, you may
* wish to control the output buffer's size to deliver your site
* HTML in chunks, the first chunk being the one containing this
* ViewHelper.
*
* Note that the web server daemon may send headers which prevent
* this prefetching and that these headers can be added in many
* ways. If prefetching does not work, you will need to inspect
* the HTTP headers returned from the actual environment. Or you
* may prefer to simply add `force="TRUE"` to this tag - but
* beware that this will affect the entire document's behaviour,
* not just for this particular set of domain prefetches. Once
* force-enabled this setting cannot be disabled (unless done so
* by manually adding an additional meta header tag as examplified
* by the `build()` method.
*
* ### Example usage:
*
* ```
*
* ```
*
* See: https://developer.mozilla.org/en-US/docs/Controlling_DNS_prefetching
*/
class PrefetchViewHelper extends AbstractAssetViewHelper
{
protected string $type = 'link';
public function initializeArguments(): void
{
parent::initializeArguments();
$this->registerArgument(
'domains',
'mixed',
'Domain DNS names to prefetch. By default will add all sys_domain record DNS names',
true
);
$this->registerArgument(
'protocol',
'string',
'Optional value of protocol as inserted in the resulting HREF value. If you experience problems with ' .
'a non-protocol link, try enforcing http/https here'
);
$this->registerArgument(
'protocolSeparator',
'string',
'If you do not enforce a particular protocol and wish to remove the double slashes from the hostname ' .
'(your browser may not understand this!), set this attribute to an empty value (not-zero)',
false,
'//'
);
$this->registerArgument(
'force',
'boolean',
'If TRUE, adds an additional meta header tag which forces prefetching to be enabled even if otherwise ' .
'requested by the http daemon',
false,
false
);
}
/**
* @return void
*/
public function render()
{
$this->arguments['standalone'] = true;
$this->arguments['movable'] = false;
$this->tagBuilder->forceClosingTag(false);
$this->tagBuilder->addAttribute('rel', 'dns-prefetch');
$this->tagBuilder->addAttribute('href', '');
$this->tagBuilder->setTagName('link');
$this->finalize();
}
public function build(): string
{
/** @var array|string $domains */
$domains = $this->arguments['domains'];
if (!is_array($domains)) {
$domains = GeneralUtility::trimExplode(',', $domains, true);
}
$headerCode = '';
if ($this->arguments['force']) {
$headerCode .= '' . LF;
}
foreach ($domains as $domain) {
$this->tagBuilder->removeAttribute('href');
$this->tagBuilder->addAttribute(
'href',
$this->arguments['protocol'] . $this->arguments['protocolSeparator'] . $domain
);
$headerCode .= $this->tagBuilder->render() . LF;
}
return $headerCode;
}
}