Since Symfony 2.4, the current Request is accessible from the RequestStack.
Before that, we passed the current Request instance across our Events, allowing to return custom responses depending on the Request.
Now, developers just have to inject the request_stack service in their listeners and retrieve the current request as needed.
This release only depreciate the practice to inject and retrieve the request by adding a default null value to all arguments type-hinted as Request, keeping BC almost intact while triggering deprecation notices from constructors and getters, before we totally remove the practice in the v2.0 major release.