Allow nested event dispatch. Yet another thing which I should one day try and PR to Guava
This commit is contained in:
md_5
parent
834ac24b38
commit
0afefa8f61
330
api/src/main/java/com/google/common/eventbus/EventBus.java
Normal file
330
api/src/main/java/com/google/common/eventbus/EventBus.java
Normal file
@ -0,0 +1,330 @@
|
||||
/*
|
||||
* 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.
|
||||
*/
|
||||
|
||||
package com.google.common.eventbus;
|
||||
|
||||
import static com.google.common.base.Preconditions.checkNotNull;
|
||||
|
||||
import com.google.common.annotations.Beta;
|
||||
import com.google.common.annotations.VisibleForTesting;
|
||||
import com.google.common.base.Throwables;
|
||||
import com.google.common.cache.CacheBuilder;
|
||||
import com.google.common.cache.CacheLoader;
|
||||
import com.google.common.cache.LoadingCache;
|
||||
import com.google.common.collect.HashMultimap;
|
||||
import com.google.common.collect.Multimap;
|
||||
import com.google.common.collect.SetMultimap;
|
||||
import com.google.common.reflect.TypeToken;
|
||||
import com.google.common.util.concurrent.UncheckedExecutionException;
|
||||
|
||||
import java.lang.reflect.InvocationTargetException;
|
||||
import java.util.Collection;
|
||||
import java.util.LinkedList;
|
||||
import java.util.Map.Entry;
|
||||
import java.util.Queue;
|
||||
import java.util.Set;
|
||||
import java.util.concurrent.locks.ReadWriteLock;
|
||||
import java.util.concurrent.locks.ReentrantReadWriteLock;
|
||||
import java.util.logging.Level;
|
||||
import java.util.logging.Logger;
|
||||
|
||||
/**
|
||||
* Dispatches events to listeners, and provides ways for listeners to register
|
||||
* themselves.
|
||||
*
|
||||
* <p>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.
|
||||
*
|
||||
* <h2>Receiving Events</h2>
|
||||
* To receive events, an object should:<ol>
|
||||
* <li>Expose a public method, known as the <i>event handler</i>, which accepts
|
||||
* a single argument of the type of event desired;</li>
|
||||
* <li>Mark it with a {@link Subscribe} annotation;</li>
|
||||
* <li>Pass itself to an EventBus instance's {@link #register(Object)} method.
|
||||
* </li>
|
||||
* </ol>
|
||||
*
|
||||
* <h2>Posting Events</h2>
|
||||
* To post an event, simply provide the event object to the
|
||||
* {@link #post(Object)} method. The EventBus instance will determine the type
|
||||
* of event and route it to all registered listeners.
|
||||
*
|
||||
* <p>Events are routed based on their type — an event will be delivered
|
||||
* to any handler for any type to which the event is <em>assignable.</em> This
|
||||
* includes implemented interfaces, all superclasses, and all interfaces
|
||||
* implemented by superclasses.
|
||||
*
|
||||
* <p>When {@code post} is called, all registered handlers for an event are run
|
||||
* in 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}.)
|
||||
*
|
||||
* <h2>Handler Methods</h2>
|
||||
* Event handler methods must accept only one argument: the event.
|
||||
*
|
||||
* <p>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.
|
||||
*
|
||||
* <p>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 is
|
||||
* not present, handler methods need not worry about being reentrant, unless
|
||||
* also called from outside the EventBus.
|
||||
*
|
||||
* <h2>Dead Events</h2>
|
||||
* 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.
|
||||
*
|
||||
* <p>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.
|
||||
*
|
||||
* <p>This class is safe for concurrent use.
|
||||
*
|
||||
* <p>See the Guava User Guide article on <a href=
|
||||
* "http://code.google.com/p/guava-libraries/wiki/EventBusExplained">
|
||||
* {@code EventBus}</a>.
|
||||
*
|
||||
* @author Cliff Biffle
|
||||
* @since 10.0
|
||||
*/
|
||||
@Beta
|
||||
public class EventBus {
|
||||
|
||||
/**
|
||||
* A thread-safe cache for flattenHierarchy(). The Class class is immutable. This cache is shared
|
||||
* across all EventBus instances, which greatly improves performance if multiple such instances
|
||||
* are created and objects of the same class are posted on all of them.
|
||||
*/
|
||||
private static final LoadingCache<Class<?>, Set<Class<?>>> flattenHierarchyCache =
|
||||
CacheBuilder.newBuilder()
|
||||
.weakKeys()
|
||||
.build(new CacheLoader<Class<?>, Set<Class<?>>>() {
|
||||
@SuppressWarnings({"unchecked", "rawtypes"}) // safe cast
|
||||
@Override
|
||||
public Set<Class<?>> load(Class<?> concreteClass) {
|
||||
return (Set) TypeToken.of(concreteClass).getTypes().rawTypes();
|
||||
}
|
||||
});
|
||||
|
||||
/**
|
||||
* All registered event handlers, indexed by event type.
|
||||
*
|
||||
* <p>This SetMultimap is NOT safe for concurrent use; all access should be
|
||||
* made after acquiring a read or write lock via {@link #handlersByTypeLock}.
|
||||
*/
|
||||
private final SetMultimap<Class<?>, EventHandler> handlersByType =
|
||||
HashMultimap.create();
|
||||
private final ReadWriteLock handlersByTypeLock = new ReentrantReadWriteLock();
|
||||
|
||||
/**
|
||||
* Logger for event dispatch failures. Named by the fully-qualified name of
|
||||
* this class, followed by the identifier provided at construction.
|
||||
*/
|
||||
private final Logger logger;
|
||||
|
||||
/**
|
||||
* Strategy for finding handler methods in registered objects. Currently,
|
||||
* only the {@link AnnotatedHandlerFinder} is supported, but this is
|
||||
* encapsulated for future expansion.
|
||||
*/
|
||||
private final HandlerFindingStrategy finder = new AnnotatedHandlerFinder();
|
||||
|
||||
/** queues of events for the current thread to dispatch */
|
||||
private final ThreadLocal<Queue<EventWithHandler>> eventsToDispatch =
|
||||
new ThreadLocal<Queue<EventWithHandler>>() {
|
||||
@Override protected Queue<EventWithHandler> initialValue() {
|
||||
return new LinkedList<EventWithHandler>();
|
||||
}
|
||||
};
|
||||
|
||||
/**
|
||||
* Creates a new EventBus named "default".
|
||||
*/
|
||||
public EventBus() {
|
||||
this("default");
|
||||
}
|
||||
|
||||
/**
|
||||
* Creates a new EventBus with the given {@code identifier}.
|
||||
*
|
||||
* @param identifier a brief name for this bus, for logging purposes. Should
|
||||
* be a valid Java identifier.
|
||||
*/
|
||||
public EventBus(String identifier) {
|
||||
logger = Logger.getLogger(EventBus.class.getName() + "." + checkNotNull(identifier));
|
||||
}
|
||||
|
||||
/**
|
||||
* Registers all handler methods on {@code object} to receive events.
|
||||
* Handler methods are selected and classified using this EventBus's
|
||||
* {@link HandlerFindingStrategy}; the default strategy is the
|
||||
* {@link AnnotatedHandlerFinder}.
|
||||
*
|
||||
* @param object object whose handler methods should be registered.
|
||||
*/
|
||||
public void register(Object object) {
|
||||
Multimap<Class<?>, EventHandler> methodsInListener =
|
||||
finder.findAllHandlers(object);
|
||||
handlersByTypeLock.writeLock().lock();
|
||||
try {
|
||||
handlersByType.putAll(methodsInListener);
|
||||
} finally {
|
||||
handlersByTypeLock.writeLock().unlock();
|
||||
}
|
||||
}
|
||||
|
||||
/**
|
||||
* Unregisters all handler methods on a registered {@code object}.
|
||||
*
|
||||
* @param object object whose handler methods should be unregistered.
|
||||
* @throws IllegalArgumentException if the object was not previously registered.
|
||||
*/
|
||||
public void unregister(Object object) {
|
||||
Multimap<Class<?>, EventHandler> methodsInListener = finder.findAllHandlers(object);
|
||||
for (Entry<Class<?>, Collection<EventHandler>> entry : methodsInListener.asMap().entrySet()) {
|
||||
Class<?> eventType = entry.getKey();
|
||||
Collection<EventHandler> eventMethodsInListener = entry.getValue();
|
||||
|
||||
handlersByTypeLock.writeLock().lock();
|
||||
try {
|
||||
Set<EventHandler> currentHandlers = handlersByType.get(eventType);
|
||||
if (!currentHandlers.containsAll(eventMethodsInListener)) {
|
||||
throw new IllegalArgumentException(
|
||||
"missing event handler for an annotated method. Is " + object + " registered?");
|
||||
}
|
||||
currentHandlers.removeAll(eventMethodsInListener);
|
||||
} finally {
|
||||
handlersByTypeLock.writeLock().unlock();
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
/**
|
||||
* Posts an event to all registered handlers. This method will return
|
||||
* successfully after the event has been posted to all handlers, and
|
||||
* regardless of any exceptions thrown by handlers.
|
||||
*
|
||||
* <p>If no handlers have been subscribed for {@code event}'s class, and
|
||||
* {@code event} is not already a {@link DeadEvent}, it will be wrapped in a
|
||||
* DeadEvent and reposted.
|
||||
*
|
||||
* @param event event to post.
|
||||
*/
|
||||
public void post(Object event) {
|
||||
Set<Class<?>> dispatchTypes = flattenHierarchy(event.getClass());
|
||||
|
||||
boolean dispatched = false;
|
||||
for (Class<?> eventType : dispatchTypes) {
|
||||
handlersByTypeLock.readLock().lock();
|
||||
try {
|
||||
Set<EventHandler> wrappers = handlersByType.get(eventType);
|
||||
|
||||
if (!wrappers.isEmpty()) {
|
||||
dispatched = true;
|
||||
for (EventHandler wrapper : wrappers) {
|
||||
enqueueEvent(event, wrapper);
|
||||
}
|
||||
}
|
||||
} finally {
|
||||
handlersByTypeLock.readLock().unlock();
|
||||
}
|
||||
}
|
||||
|
||||
if (!dispatched && !(event instanceof DeadEvent)) {
|
||||
post(new DeadEvent(this, event));
|
||||
}
|
||||
|
||||
dispatchQueuedEvents();
|
||||
}
|
||||
|
||||
/**
|
||||
* Queue the {@code event} for dispatch during
|
||||
* {@link #dispatchQueuedEvents()}. Events are queued in-order of occurrence
|
||||
* so they can be dispatched in the same order.
|
||||
*/
|
||||
void enqueueEvent(Object event, EventHandler handler) {
|
||||
eventsToDispatch.get().offer(new EventWithHandler(event, handler));
|
||||
}
|
||||
|
||||
/**
|
||||
* Drain the queue of events to be dispatched. As the queue is being drained,
|
||||
* new events may be posted to the end of the queue.
|
||||
*/
|
||||
void dispatchQueuedEvents() {
|
||||
try {
|
||||
Queue<EventWithHandler> events = eventsToDispatch.get();
|
||||
EventWithHandler eventWithHandler;
|
||||
while ((eventWithHandler = events.poll()) != null) {
|
||||
dispatch(eventWithHandler.event, eventWithHandler.handler);
|
||||
}
|
||||
} finally {
|
||||
eventsToDispatch.remove();
|
||||
}
|
||||
}
|
||||
|
||||
/**
|
||||
* Dispatches {@code event} to the handler in {@code wrapper}. This method
|
||||
* is an appropriate override point for subclasses that wish to make
|
||||
* event delivery asynchronous.
|
||||
*
|
||||
* @param event event to dispatch.
|
||||
* @param wrapper wrapper that will call the handler.
|
||||
*/
|
||||
void dispatch(Object event, EventHandler wrapper) {
|
||||
try {
|
||||
wrapper.handleEvent(event);
|
||||
} catch (InvocationTargetException e) {
|
||||
logger.log(Level.SEVERE,
|
||||
"Could not dispatch event: " + event + " to handler " + wrapper, e);
|
||||
}
|
||||
}
|
||||
|
||||
/**
|
||||
* Flattens a class's type hierarchy into a set of Class objects. The set
|
||||
* will include all superclasses (transitively), and all interfaces
|
||||
* implemented by these superclasses.
|
||||
*
|
||||
* @param concreteClass class whose type hierarchy will be retrieved.
|
||||
* @return {@code clazz}'s complete type hierarchy, flattened and uniqued.
|
||||
*/
|
||||
@VisibleForTesting
|
||||
Set<Class<?>> flattenHierarchy(Class<?> concreteClass) {
|
||||
try {
|
||||
return flattenHierarchyCache.getUnchecked(concreteClass);
|
||||
} catch (UncheckedExecutionException e) {
|
||||
throw Throwables.propagate(e.getCause());
|
||||
}
|
||||
}
|
||||
|
||||
/** simple struct representing an event and it's handler */
|
||||
static class EventWithHandler {
|
||||
final Object event;
|
||||
final EventHandler handler;
|
||||
public EventWithHandler(Object event, EventHandler handler) {
|
||||
this.event = checkNotNull(event);
|
||||
this.handler = checkNotNull(handler);
|
||||
}
|
||||
}
|
||||
}
|
||||
Loading…
Reference in New Issue
Block a user