This will read the file
/patterns/pattern.html, parse and output it to the client as the
payload of an ResponseTextHtml response.
You can change the directory where your patterns are stored by setting the
directorykey of the Patterns module configuration file.
But this is just the most common way of using a pattern. You might need to simply get the results of a parsed pattern in a variable. For doing so, instead of using the Patterns::out method, you use Patterns::parse like this:
$parsedContents = $e->Patterns->parse("helloworld.html");