com.google.common.eventbus.package-info Maven / Gradle / Ivy
/*
* Copyright (C) 2007 The Guava Authors
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
* in compliance with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software distributed under the License
* is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
* or implied. See the License for the specific language governing permissions and limitations under
* the License.
*/
/**
* 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.
*
* See the Guava User Guide article on {@code EventBus}.
*
*
One-Minute Guide
*
* Converting an existing EventListener-based system to use the EventBus is easy.
*
*
For Listeners
*
* To listen for a specific flavor of event (say, a CustomerChangeEvent)...
*
*
* - ...in traditional Java events: implement an interface defined with the
* event — such as CustomerChangeEventListener.
*
- ...with EventBus: create a method that accepts CustomerChangeEvent as its
* sole argument, and mark it with the {@link com.google.common.eventbus.Subscribe}
* annotation.
*
*
* To register your listener methods with the event producers...
*
*
* - ...in traditional Java events: pass your object to each producer's {@code
* registerCustomerChangeEventListener} method. These methods are rarely defined in common
* interfaces, so in addition to knowing every possible producer, you must also know its type.
*
- ...with EventBus: pass your object to the {@link
* com.google.common.eventbus.EventBus#register(Object)} method on an EventBus. You'll need to
* make sure that your object shares an EventBus instance with the event producers.
*
*
* To listen for a common event supertype (such as EventObject or Object)...
*
*
* - ...in traditional Java events: not easy.
*
- ...with EventBus: events are automatically dispatched to listeners of any
* supertype, allowing listeners for interface types or "wildcard listeners" for Object.
*
*
* To listen for and detect events that were dispatched without listeners...
*
*
* - ...in traditional Java events: add code to each event-dispatching method
* (perhaps using AOP).
*
- ...with EventBus: subscribe to {@link
* com.google.common.eventbus.DeadEvent}. The EventBus will notify you of any events that were
* posted but not delivered. (Handy for debugging.)
*
*
* For Producers
*
* To keep track of listeners to your events...
*
*
* - ...in traditional Java events: write code to manage a list of listeners to
* your object, including synchronization, or use a utility class like EventListenerList.
*
- ...with EventBus: EventBus does this for you.
*
*
* To dispatch an event to listeners...
*
*
* - ...in traditional Java events: write a method to dispatch events to each
* event listener, including error isolation and (if desired) asynchronicity.
*
- ...with EventBus: pass the event object to an EventBus's {@link
* com.google.common.eventbus.EventBus#post(Object)} method.
*
*
* Glossary
*
* The EventBus system and code use the following terms to discuss event distribution:
*
*
* - Event
*
- Any object that may be posted to a bus.
*
- Subscribing
*
- The act of registering a listener with an EventBus, so that its subscriber
* methods will receive events.
*
- Listener
*
- An object that wishes to receive events, by exposing subscriber methods.
*
- Subscriber method
*
- A public method that the EventBus should use to deliver posted events. Subscriber
* methods are marked by the {@link com.google.common.eventbus.Subscribe} annotation.
*
- Posting an event
*
- Making the event available to any listeners through the EventBus.
*
*
* FAQ
*
* Why must I create my own Event Bus, rather than using a singleton?
*
* The Event Bus doesn't specify how you use it; there's nothing stopping your application from
* having separate EventBus instances for each component, or using separate instances to separate
* events by context or topic. This also makes it trivial to set up and tear down EventBus objects
* in your tests.
*
*
Of course, if you'd like to have a process-wide EventBus singleton, there's nothing stopping
* you from doing it that way. Simply have your container (such as Guice) create the EventBus as a
* singleton at global scope (or stash it in a static field, if you're into that sort of thing).
*
*
In short, the EventBus is not a singleton because we'd rather not make that decision for you.
* Use it how you like.
*
*
Why use an annotation to mark subscriber methods, rather than requiring the listener to
* implement an interface?
*
* We feel that the Event Bus's {@code @Subscribe} annotation conveys your intentions just as
* explicitly as implementing an interface (or perhaps more so), while leaving you free to place
* event subscriber methods wherever you wish and give them intention-revealing names.
*
*
Traditional Java Events use a listener interface which typically sports only a handful of
* methods -- typically one. This has a number of disadvantages:
*
*
* - Any one class can only implement a single response to a given event.
*
- Listener interface methods may conflict.
*
- The method must be named after the event (e.g. {@code handleChangeEvent}), rather than its
* purpose (e.g. {@code recordChangeInJournal}).
*
- Each event usually has its own interface, without a common parent interface for a family of
* events (e.g. all UI events).
*
*
* The difficulties in implementing this cleanly has given rise to a pattern, particularly common
* in Swing apps, of using tiny anonymous classes to implement event listener interfaces.
*
*
Compare these two cases:
*
*
{@code
* class ChangeRecorder {
* void setCustomer(Customer cust) {
* cust.addChangeListener(new ChangeListener() {
* void customerChanged(ChangeEvent e) {
* recordChange(e.getChange());
* }
* };
* }
* }
*
* // Class is typically registered by the container.
* class EventBusChangeRecorder {
* }{@code @Subscribe void recordCustomerChange(ChangeEvent e) {
* recordChange(e.getChange());
* }
* }
* }
*
* The intent is actually clearer in the second case: there's less noise code, and the event
* subscriber has a clear and meaningful name.
*
*
What about a generic {@code Subscriber} interface?
*
* Some have proposed a generic {@code Subscriber} interface for EventBus listeners. This runs
* into issues with Java's use of type erasure, not to mention problems in usability.
*
* Let's say the interface looked something like the following:
*
*
{@code
* interface Subscriber {
* void handleEvent(T event);
* }
* }
*
* Due to erasure, no single class can implement a generic interface more than once with
* different type parameters. This is a giant step backwards from traditional Java Events, where
* even if {@code actionPerformed} and {@code keyPressed} aren't very meaningful names, at least you
* can implement both methods!
*
*
Doesn't EventBus destroy static typing and eliminate automated refactoring support?
*
* Some have freaked out about EventBus's {@code register(Object)} and {@code post(Object)}
* methods' use of the {@code Object} type.
*
*
{@code Object} is used here for a good reason: the Event Bus library places no restrictions on
* the types of either your event listeners (as in {@code register(Object)}) or the events
* themselves (in {@code post(Object)}).
*
*
Event subscriber methods, on the other hand, must explicitly declare their argument type --
* the type of event desired (or one of its supertypes). Thus, searching for references to an event
* class will instantly find all subscriber methods for that event, and renaming the type will
* affect all subscriber methods within view of your IDE (and any code that creates the event).
*
*
It's true that you can rename your {@code @Subscribed} event subscriber methods at will; Event
* Bus will not stop this or do anything to propagate the rename because, to Event Bus, the names of
* your subscriber methods are irrelevant. Test code that calls the methods directly, of course,
* will be affected by your renaming -- but that's what your refactoring tools are for.
*
*
What happens if I {@code register} a listener without any subscriber methods?
*
* Nothing at all.
*
*
The Event Bus was designed to integrate with containers and module systems, with Guice as the
* prototypical example. In these cases, it's convenient to have the container/factory/environment
* pass every created object to an EventBus's {@code register(Object)} method.
*
*
This way, any object created by the container/factory/environment can hook into the system's
* event model simply by exposing subscriber methods.
*
*
What Event Bus problems can be detected at compile time?
*
* Any problem that can be unambiguously detected by Java's type system. For example, defining a
* subscriber method for a nonexistent event type.
*
*
What Event Bus problems can be detected immediately at registration?
*
* Immediately upon invoking {@code register(Object)}, the listener being registered is checked
* for the well-formedness of its subscriber methods. Specifically, any methods marked with
* {@code @Subscribe} must take only a single argument.
*
*
Any violations of this rule will cause an {@code IllegalArgumentException} to be thrown.
*
*
(This check could be moved to compile-time using APT, a solution we're researching.)
*
*
What Event Bus problems may only be detected later, at runtime?
*
* If a component posts events with no registered listeners, it may indicate an error
* (typically an indication that you missed a {@code @Subscribe} annotation, or that the listening
* component is not loaded).
*
*
(Note that this is not necessarily indicative of a problem. There are many cases where
* an application will deliberately ignore a posted event, particularly if the event is coming from
* code you don't control.)
*
*
To handle such events, register a subscriber method for the {@code DeadEvent} class. Whenever
* EventBus receives an event with no registered subscribers, it will turn it into a {@code
* DeadEvent} and pass it your way -- allowing you to log it or otherwise recover.
*
*
How do I test event listeners and their subscriber methods?
*
* Because subscriber methods on your listener classes are normal methods, you can simply call
* them from your test code to simulate the EventBus.
*/
@CheckReturnValue
@ParametersAreNonnullByDefault
package com.google.common.eventbus;
import com.google.errorprone.annotations.CheckReturnValue;
import javax.annotation.ParametersAreNonnullByDefault;