Description:

The Symfony2 Event Dispatcher component implements the Observer
(http://en.wikipedia.org/wiki/Observer_pattern) pattern in a simple and
effective way to make all these things possible and to make your projects
truly extensible.

Take a simple example from the Symfony2 HttpKernel component. Once a Response
object has been created, it may be useful to allow other elements in the system
to modify it (e.g. add some cache headers) before it's actually used. To make
this possible, the Symfony2 kernel throws an event - kernel.response. Here's
how it works:

* A listener (PHP object) tells a central dispatcher object that it wants to
listen to the kernel.response event;
* At some point, the Symfony2 kernel tells the dispatcher object to dispatch
the kernel.response event, passing with it an Event object that has access to
the Response object;
* The dispatcher notifies (i.e. calls a method on) all listeners of the
kernel.response event, allowing each of them to make modifications to the
Response object.

Optional dependency: HttpKernel