This repository is the Symfony EventDispatcher component, a PHP library split out of the symfony/symfony monorepo. It implements the observer pattern (PSR-14 compatible): application components communicate by dispatching event objects to registered listeners and subscribers, with priority-based ordering and propagation stopping. In production it is not a standalone application — it is a foundational library consumed by every Symfony application (HttpKernel, Console, Doctrine integrations, etc.) and usable standalone via Composer. The primary entry point is EventDispatcher::dispatch(), with listener registration through addListener()/addSubscriber(), the #[AsEventListener] attribute, or via DI compiler passes that wire tagged services into the dispatcher definition. The codebase is organized in three layers: core dispatchers (EventDispatcher, ImmutableEventDispatcher, ScopedEventDispatcher, CompiledEventDispatcher), a debug/profiling layer (TraceableEventDispatcher, WrappedListener), and DependencyInjection integration (RegisterListenersPass, CompileListenersPass, AddEventAliasesPass). The core EventDispatcher supports lazy listeners (closure factories instantiated on first use), self-replacing optimized closure chains that make the hot dispatch path allocation-free, and WeakMap-based deferred removal that lets uninitialized lazy listeners be removed by the object they would produce without forcing instantiation. The 8.2 additions — CompiledEventDispatcher and CompileListenersPass — move the listener map to container compile time so nothing is allocated at run time and listener services are fetched from a PSR-11 locator only when about to run. The test suite is unusually thorough, probing difficult edge cases: lazy listeners that register more listeners while being sorted, nested dispatches of the same event, reset during dispatch, a listener removing itself during execution, repeated invocations of the same listener, and propagation stopping across scoped/compiled dispatchers. ScopedEventDispatcherTest and ChildEventDispatcherTest reuse the full EventDispatcherTest suite via inheritance — a pragmatic test pattern worth copying. Maintenance is active (Symfony 8.2 branch, zero open issues); the code targets PHP 8.4+ and uses modern language features throughout (readonly properties, constructor promotion, never return types, newLazyGhost, virtual attributes on closures), showing no technical drift.