EventBus

EventBus is a software design pattern and component that facilitates publish-subscribe messaging within an application. It enables decoupled communication between producers, known as publishers, and consumers, known as subscribers, by routing events to interested listeners without requiring direct references.

An event is typically an object or message describing something that occurred. Components register as listeners for specific event types; when a publisher posts an event to the bus, the bus delivers the event to all registered subscribers for that type. Depending on the implementation, dispatching can be synchronous (in the publisher's thread) or asynchronous (via a thread pool or dedicated event loop). Some systems support multiple handlers, prioritization, and sticky events that are delivered to late subscribers.

Common implementations include Google Guava's EventBus and GreenRobot's EventBus for Android. These libraries typically provide registration and unregistration APIs, and a convention for defining handler methods (for example, detailing which event type they handle).

EventBus patterns are used to decouple components in user interfaces, modular architectures, and plugin systems, where components should not depend on concrete collaborators. They are also used in server-side applications to decouple internal modules.

Drawbacks include potential debugging complexity, difficulty tracing event flows, and risk of memory leaks if subscribers are not unregistered; ordering and delivery guarantees may be weak or configurable. In distributed systems, in-process EventBus concepts are extended using external message brokers for cross-process or cross-service communication.

Related concepts include the publish-subscribe pattern, message brokers, and reactive streams.