Dispatches events to listeners, and provides ways for listeners to register themselves.
The EventBus allows publish-subscribe-style communication between components without requiring the components to explicitly register with one another (and thus be aware of each other). It is designed exclusively to replace traditional Java in-process event distribution using explicit registration. It is not a general-purpose publish-subscribe system, nor is it intended for interprocess communication.
Receiving Events
To receive events, an object should:
- Expose a public method, known as the event handler, which accepts a single argument of the type of event desired;
- Mark it with a {@link Subscribe} annotation;
- Pass itself to an EventBus instance's {@link #register(Object)} method.
Posting Events
To post an event, simply provide the event object to the {@link #post(Object)} method. The EventBus instance will determine the typeof event and route it to all registered listeners.
Events are routed based on their type — an event will be delivered to any handler for any type to which the event is assignable. This includes implemented interfaces, all superclasses, and all interfaces implemented by superclasses.
When {@code post} is called, all registered handlers for an event are runin sequence, so handlers should be reasonably quick. If an event may trigger an extended process (such as a database load), spawn a thread or queue it for later. (For a convenient way to do this, use an {@link AsyncEventBus}.)
Handler Methods
Event handler methods must accept only one argument: the event.
Handlers should not, in general, throw. If they do, the EventBus will catch and log the exception. This is rarely the right solution for error handling and should not be relied upon; it is intended solely to help find problems during development.
The EventBus guarantees that it will not call a handler method from multiple threads simultaneously, unless the method explicitly allows it by bearing the {@link AllowConcurrentEvents} annotation. If this annotation isnot present, handler methods need not worry about being reentrant, unless also called from outside the EventBus.
Dead Events
If an event is posted, but no registered handlers can accept it, it is considered "dead." To give the system a second chance to handle dead events, they are wrapped in an instance of {@link DeadEvent} and reposted.
If a handler for a supertype of all events (such as Object) is registered, no event will ever be considered dead, and no DeadEvents will be generated. Accordingly, while DeadEvent extends {@link Object}, a handler registered to receive any Object will never receive a DeadEvent.
This class is safe for concurrent use.
@author Cliff Biffle
@since Guava release 10