@Beta public class EventBus extends Object
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.
To receive events, an object should:
Subscribe annotation;register(Object) method.
To post an event, simply provide the event object to the
post(Object) method. The EventBus instance will determine the type
of event and route it to all registered listeners.
Events are routed based on their type — an event will be delivered to any subscriber for any type to which the event is assignable. This includes implemented interfaces, all superclasses, and all interfaces implemented by superclasses.
When post is called, all registered subscribers for an event are run
in sequence, so subscribers 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 AsyncEventBus.)
Event subscriber methods must accept only one argument: the event.
Subscribers 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 subscriber method from
multiple threads simultaneously, unless the method explicitly allows it by
bearing the AllowConcurrentEvents annotation. If this annotation is
not present, subscriber methods need not worry about being reentrant, unless
also called from outside the EventBus.
If an event is posted, but no registered subscribers 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 DeadEvent and reposted.
If a subscriber 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 Object, a subscriber registered to
receive any Object will never receive a DeadEvent.
This class is safe for concurrent use.
See the Guava User Guide article on
EventBus.
| Constructor and Description |
|---|
EventBus()
Creates a new EventBus named "default".
|
EventBus(String identifier)
Creates a new EventBus with the given
identifier. |
EventBus(SubscriberExceptionHandler exceptionHandler)
Creates a new EventBus with the given
SubscriberExceptionHandler. |
| Modifier and Type | Method and Description |
|---|---|
String |
identifier()
Returns the identifier for this event bus.
|
void |
post(Object event)
Posts an event to all registered subscribers.
|
void |
register(Object object)
Registers all subscriber methods on
object to receive events. |
String |
toString() |
void |
unregister(Object object)
Unregisters all subscriber methods on a registered
object. |
public EventBus()
public EventBus(String identifier)
identifier.identifier - a brief name for this bus, for logging purposes. Should
be a valid Java identifier.public EventBus(SubscriberExceptionHandler exceptionHandler)
SubscriberExceptionHandler.exceptionHandler - Handler for subscriber exceptions.public final String identifier()
public void register(Object object)
object to receive events.object - object whose subscriber methods should be registered.public void unregister(Object object)
object.object - object whose subscriber methods should be unregistered.IllegalArgumentException - if the object was not previously registered.public void post(Object event)
If no subscribers have been subscribed for event's class, and
event is not already a DeadEvent, it will be wrapped in a
DeadEvent and reposted.
event - event to post.Copyright © 2010-2015. All Rights Reserved.