Symfony Event Dispatcher is a library that implements the observer pattern, enabling event-driven communication between application components. It provides a central dispatch mechanism where components can broadcast named events and register listeners to react to them, decoupling event producers from consumers.
The library supports any PHP callable as an event listener and organizes listeners in a priority-ordered queue for controlled execution. Listeners can stop event propagation to prevent subsequent handlers from running, and the subscriber-based registration system allows grouping multiple event-to-listener mappings into reusable classes that self-register with the dispatcher.
Custom events can be dispatched with data payloads, and listeners can be attached to specific events with configurable priority. The system supports the event subscriber pattern for organizing multiple event handlers into single classes.