Every request that is sent to Matomo (formerly Piwik)'s reporting side (as opposed to Matomo's tracking side) is sent to the index.php file in Matomo's root directory. This file creates an instance of the FrontController and uses it to dispatch the current request.
The FrontController looks for the module and action query parameters. If action is missing, it takes the default value "index". Matomo then invokes the matching controller method:
Piwik\Plugins\<module>\Controller::<action>
Examples:
module=MyPlugin&action=hello will invoke:
Piwik\Plugins\MyPlugin\Controller::hello()
module=MyPlugin will invoke:
Piwik\Plugins\MyPlugin\Controller::index()
Controller methods have one thing to do: return a string response (or anything that can be cast to a string). Such string could contain HTML, JSON, …
As a plugin developer you can do this in any way you'd like, Matomo won't stop you, but the convention used by the rest of Matomo is to create a Matomo View, query APIs to request any needed data and then render the view. For example:
class Controller extends \Piwik\Plugin\Controller
{
public function index()
{
$view = new View("@MyPlugin\index.twig");
$view->data = \Piwik\Plugins\MyPlugin\API::getInstance()->getData();
return $view->render();
}
}
Read on to learn more about the individual components in this workflow.