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/*
 * Copyright (C) 2014 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.collect.Queues;
import java.util.Iterator;
import java.util.Queue;
import java.util.concurrent.ConcurrentLinkedQueue;

/**
 * Handler for dispatching events to subscribers, providing different event ordering guarantees that
 * make sense for different situations.
 *
 * 

Note: The dispatcher is orthogonal to the subscriber's {@code Executor}. The dispatcher * controls the order in which events are dispatched, while the executor controls how (i.e. on which * thread) the subscriber is actually called when an event is dispatched to it. * * @author Colin Decker */ abstract class Dispatcher { /** * Returns a dispatcher that queues events that are posted reentrantly on a thread that is already * dispatching an event, guaranteeing that all events posted on a single thread are dispatched to * all subscribers in the order they are posted. * *

When all subscribers are dispatched to using a direct executor (which dispatches on * the same thread that posts the event), this yields a breadth-first dispatch order on each * thread. That is, all subscribers to a single event A will be called before any subscribers to * any events B and C that are posted to the event bus by the subscribers to A. */ static Dispatcher perThreadDispatchQueue() { return new PerThreadQueuedDispatcher(); } /** * Returns a dispatcher that queues events that are posted in a single global queue. This behavior * matches the original behavior of AsyncEventBus exactly, but is otherwise not especially useful. * For async dispatch, an {@linkplain #immediate() immediate} dispatcher should generally be * preferable. */ static Dispatcher legacyAsync() { return new LegacyAsyncDispatcher(); } /** * Returns a dispatcher that dispatches events to subscribers immediately as they're posted * without using an intermediate queue to change the dispatch order. This is effectively a * depth-first dispatch order, vs. breadth-first when using a queue. */ static Dispatcher immediate() { return ImmediateDispatcher.INSTANCE; } /** Dispatches the given {@code event} to the given {@code subscribers}. */ abstract void dispatch(Object event, Iterator subscribers); /** Implementation of a {@link #perThreadDispatchQueue()} dispatcher. */ private static final class PerThreadQueuedDispatcher extends Dispatcher { // This dispatcher matches the original dispatch behavior of EventBus. /** Per-thread queue of events to dispatch. */ private final ThreadLocal> queue = new ThreadLocal>() { @Override protected Queue initialValue() { return Queues.newArrayDeque(); } }; /** Per-thread dispatch state, used to avoid reentrant event dispatching. */ private final ThreadLocal dispatching = new ThreadLocal() { @Override protected Boolean initialValue() { return false; } }; @Override void dispatch(Object event, Iterator subscribers) { checkNotNull(event); checkNotNull(subscribers); Queue queueForThread = queue.get(); queueForThread.offer(new Event(event, subscribers)); if (!dispatching.get()) { dispatching.set(true); try { Event nextEvent; while ((nextEvent = queueForThread.poll()) != null) { while (nextEvent.subscribers.hasNext()) { nextEvent.subscribers.next().dispatchEvent(nextEvent.event); } } } finally { dispatching.remove(); queue.remove(); } } } private static final class Event { private final Object event; private final Iterator subscribers; private Event(Object event, Iterator subscribers) { this.event = event; this.subscribers = subscribers; } } } /** Implementation of a {@link #legacyAsync()} dispatcher. */ private static final class LegacyAsyncDispatcher extends Dispatcher { // This dispatcher matches the original dispatch behavior of AsyncEventBus. // // We can't really make any guarantees about the overall dispatch order for this dispatcher in // a multithreaded environment for a couple reasons: // // 1. Subscribers to events posted on different threads can be interleaved with each other // freely. (A event on one thread, B event on another could yield any of // [a1, a2, a3, b1, b2], [a1, b2, a2, a3, b2], [a1, b2, b3, a2, a3], etc.) // 2. It's possible for subscribers to actually be dispatched to in a different order than they // were added to the queue. It's easily possible for one thread to take the head of the // queue, immediately followed by another thread taking the next element in the queue. That // second thread can then dispatch to the subscriber it took before the first thread does. // // All this makes me really wonder if there's any value in queueing here at all. A dispatcher // that simply loops through the subscribers and dispatches the event to each would actually // probably provide a stronger order guarantee, though that order would obviously be different // in some cases. /** Global event queue. */ private final ConcurrentLinkedQueue queue = Queues.newConcurrentLinkedQueue(); @Override void dispatch(Object event, Iterator subscribers) { checkNotNull(event); while (subscribers.hasNext()) { queue.add(new EventWithSubscriber(event, subscribers.next())); } EventWithSubscriber e; while ((e = queue.poll()) != null) { e.subscriber.dispatchEvent(e.event); } } private static final class EventWithSubscriber { private final Object event; private final Subscriber subscriber; private EventWithSubscriber(Object event, Subscriber subscriber) { this.event = event; this.subscriber = subscriber; } } } /** Implementation of {@link #immediate()}. */ private static final class ImmediateDispatcher extends Dispatcher { private static final ImmediateDispatcher INSTANCE = new ImmediateDispatcher(); @Override void dispatch(Object event, Iterator subscribers) { checkNotNull(event); while (subscribers.hasNext()) { subscribers.next().dispatchEvent(event); } } } }





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