The Symfony2 Event Dispatcher component implements the Observer 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 before it"s actually used. To make this possible, the Symfony2 kernel throws an event - kernel.response. Here"s how it works: * A listener 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 all listeners of the kernel.response event, allowing each of them to make modifications to the Response object. Optional dependency: HttpKernel