android/guava/src/com/google/common/eventbus/package-info.java

/*
 * 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 <em>not</em> a general-purpose publish-subscribe system, nor is it intended
 * for interprocess communication.
 *
 * <p>See the Guava User Guide article on <a
 * href="https://github.com/google/guava/wiki/EventBusExplained">{@code EventBus}</a>.
 *
 * <h2>One-Minute Guide</h2>
 *
 * <p>Converting an existing EventListener-based system to use the EventBus is easy.
 *
 * <h3>For Listeners</h3>
 *
 * <p>To listen for a specific flavor of event (say, a CustomerChangeEvent)...
 *
 * <ul>
 *   <li><strong>...in traditional Java events:</strong> implement an interface defined with the
 *       event &mdash; such as CustomerChangeEventListener.
 *   <li><strong>...with EventBus:</strong> create a method that accepts CustomerChangeEvent as its
 *       sole argument, and mark it with the {@link com.google.common.eventbus.Subscribe}
 *       annotation.
 * </ul>
 *
 * <p>To register your listener methods with the event producers...
 *
 * <ul>
 *   <li><strong>...in traditional Java events:</strong> 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.
 *   <li><strong>...with EventBus:</strong> 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.
 * </ul>
 *
 * <p>To listen for a common event supertype (such as EventObject or Object)...
 *
 * <ul>
 *   <li><strong>...in traditional Java events:</strong> not easy.
 *   <li><strong>...with EventBus:</strong> events are automatically dispatched to listeners of any
 *       supertype, allowing listeners for interface types or "wildcard listeners" for Object.
 * </ul>
 *
 * <p>To listen for and detect events that were dispatched without listeners...
 *
 * <ul>
 *   <li><strong>...in traditional Java events:</strong> add code to each event-dispatching method
 *       (perhaps using AOP).
 *   <li><strong>...with EventBus:</strong> 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.)
 * </ul>
 *
 * <h3>For Producers</h3>
 *
 * <p>To keep track of listeners to your events...
 *
 * <ul>
 *   <li><strong>...in traditional Java events:</strong> write code to manage a list of listeners to
 *       your object, including synchronization, or use a utility class like EventListenerList.
 *   <li><strong>...with EventBus:</strong> EventBus does this for you.
 * </ul>
 *
 * <p>To dispatch an event to listeners...
 *
 * <ul>
 *   <li><strong>...in traditional Java events:</strong> write a method to dispatch events to each
 *       event listener, including error isolation and (if desired) asynchronicity.
 *   <li><strong>...with EventBus:</strong> pass the event object to an EventBus's {@link
 *       com.google.common.eventbus.EventBus#post(Object)} method.
 * </ul>
 *
 * <h2>Glossary</h2>
 *
 * <p>The EventBus system and code use the following terms to discuss event distribution:
 *
 * <dl>
 *   <dt>Event
 *   <dd>Any object that may be <em>posted</em> to a bus.
 *   <dt>Subscribing
 *   <dd>The act of registering a <em>listener</em> with an EventBus, so that its <em>subscriber
 *       methods</em> will receive events.
 *   <dt>Listener
 *   <dd>An object that wishes to receive events, by exposing <em>subscriber methods</em>.
 *   <dt>Subscriber method
 *   <dd>A public method that the EventBus should use to deliver <em>posted</em> events. Subscriber
 *       methods are marked by the {@link com.google.common.eventbus.Subscribe} annotation.
 *   <dt>Posting an event
 *   <dd>Making the event available to any <em>listeners</em> through the EventBus.
 * </dl>
 *
 * <h2>FAQ</h2>
 *
 * <h3>Why must I create my own Event Bus, rather than using a singleton?</h3>
 *
 * <p>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.
 *
 * <p>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).
 *
 * <p>In short, the EventBus is not a singleton because we'd rather not make that decision for you.
 * Use it how you like.
 *
 * <h3>Why use an annotation to mark subscriber methods, rather than requiring the listener to
 * implement an interface?</h3>
 *
 * <p>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.
 *
 * <p>Traditional Java Events use a listener interface which typically sports only a handful of
 * methods -- typically one. This has a number of disadvantages:
 *
 * <ul>
 *   <li>Any one class can only implement a single response to a given event.
 *   <li>Listener interface methods may conflict.
 *   <li>The method must be named after the event (e.g. {@code handleChangeEvent}), rather than its
 *       purpose (e.g. {@code recordChangeInJournal}).
 *   <li>Each event usually has its own interface, without a common parent interface for a family of
 *       events (e.g. all UI events).
 * </ul>
 *
 * <p>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.
 *
 * <p>Compare these two cases:
 *
 * <pre>{@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());
 *   }
 * }
 * }</pre>
 *
 * <p>The intent is actually clearer in the second case: there's less noise code, and the event
 * subscriber has a clear and meaningful name.
 *
 * <h3>What about a generic {@code Subscriber<T>} interface?</h3>
 *
 * <p>Some have proposed a generic {@code Subscriber<T>} interface for EventBus listeners. This runs
 * into issues with Java's use of type erasure, not to mention problems in usability.
 *
 * <p>Let's say the interface looked something like the following:
 *
 * <pre>{@code
 * interface Subscriber<T> {
 *   void handleEvent(T event);
 * }
 * }</pre>
 *
 * <p>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!
 *
 * <h3>Doesn't EventBus destroy static typing and eliminate automated refactoring support?</h3>
 *
 * <p>Some have freaked out about EventBus's {@code register(Object)} and {@code post(Object)}
 * methods' use of the {@code Object} type.
 *
 * <p>{@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)}).
 *
 * <p>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).
 *
 * <p>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.
 *
 * <h3>What happens if I {@code register} a listener without any subscriber methods?</h3>
 *
 * <p>Nothing at all.
 *
 * <p>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 <i>every</i> created object to an EventBus's {@code register(Object)} method.
 *
 * <p>This way, any object created by the container/factory/environment can hook into the system's
 * event model simply by exposing subscriber methods.
 *
 * <h3>What Event Bus problems can be detected at compile time?</h3>
 *
 * <p>Any problem that can be unambiguously detected by Java's type system. For example, defining a
 * subscriber method for a nonexistent event type.
 *
 * <h3>What Event Bus problems can be detected immediately at registration?</h3>
 *
 * <p>Immediately upon invoking {@code register(Object)}, the listener being registered is checked
 * for the <i>well-formedness</i> of its subscriber methods. Specifically, any methods marked with
 * {@code @Subscribe} must take only a single argument.
 *
 * <p>Any violations of this rule will cause an {@code IllegalArgumentException} to be thrown.
 *
 * <p>(This check could be moved to compile-time using APT, a solution we're researching.)
 *
 * <h3>What Event Bus problems may only be detected later, at runtime?</h3>
 *
 * <p>If a component posts events with no registered listeners, it <i>may</i> indicate an error
 * (typically an indication that you missed a {@code @Subscribe} annotation, or that the listening
 * component is not loaded).
 *
 * <p>(Note that this is <i>not necessarily</i> 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.)
 *
 * <p>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.
 *
 * <h3>How do I test event listeners and their subscriber methods?</h3>
 *
 * <p>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;