Piwik\Plugin\
The base class of all API singletons.
Plugins that want to expose functionality through the Reporting API should create a class that extends this one. Every public method in that class that is not annotated with @ignore will be callable through Matomo's Web API.
Note: If your plugin calculates and stores reports, they should be made available through the API.
Defining an API for a plugin
class API extends \Piwik\Plugin\API
{
    public function myMethod($idSite, $period, $date, $segment = false)
    {
        $dataTable = // ... get some data ...
        return $dataTable;
    }
}
Linking to an API method
<a href="?module=API&method=MyPlugin.myMethod&idSite=1&period=day&date=2013-10-23">Link</a>
The abstract class defines the following methods:
getInstance() — Returns the singleton instance for the derived class.getInstance()Returns the singleton instance for the derived class. If the singleton instance has not been created, this method will create it.
API value.