/*
 * 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.base.MoreObjects;
import com.google.common.util.concurrent.MoreExecutors;

import java.lang.reflect.Method;
import java.util.Iterator;
import java.util.Locale;
import java.util.concurrent.Executor;
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>
 * <p>To receive events, an object should:
 * <ol>
 * <li>Expose a public method, known as the <i>event subscriber</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>
 * <p>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 &mdash; an event will be delivered
 * to any subscriber 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 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 {@link AsyncEventBus}.)
 *
 * <h2>Subscriber Methods</h2>
 * <p>Event subscriber methods must accept only one argument: the event.
 *
 * <p>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.
 *
 * <p>The EventBus guarantees that it will not call a subscriber method from
 * multiple threads simultaneously, unless the method explicitly allows it by
 * bearing the {@link AllowConcurrentEvents} annotation.  If this annotation is
 * not present, subscriber methods need not worry about being reentrant, unless
 * also called from outside the EventBus.
 *
 * <h2>Dead Events</h2>
 * <p>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 {@link DeadEvent} and reposted.
 *
 * <p>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 {@link Object}, a subscriber 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=
 * "https://github.com/google/guava/wiki/EventBusExplained">
 * {@code EventBus}</a>.
 *
 * @author Cliff Biffle
 * @since 10.0
 */
@Beta
public class EventBus {

  private static final Logger logger = Logger.getLogger(EventBus.class.getName());

  private final String identifier;
  private final Executor executor;
  private final SubscriberExceptionHandler exceptionHandler;

  private final SubscriberRegistry subscribers = new SubscriberRegistry(this);
  private final Dispatcher dispatcher;

  /**
   * 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) {
    this(identifier, MoreExecutors.directExecutor(),
        Dispatcher.perThreadDispatchQueue(), LoggingHandler.INSTANCE);
  }

  /**
   * Creates a new EventBus with the given {@link SubscriberExceptionHandler}.
   * 
   * @param exceptionHandler Handler for subscriber exceptions.
   * @since 16.0
   */
  public EventBus(SubscriberExceptionHandler exceptionHandler) {
    this("default",
        MoreExecutors.directExecutor(), Dispatcher.perThreadDispatchQueue(), exceptionHandler);
  }

  EventBus(String identifier, Executor executor, Dispatcher dispatcher,
      SubscriberExceptionHandler exceptionHandler) {
    this.identifier = checkNotNull(identifier);
    this.executor = checkNotNull(executor);
    this.dispatcher = checkNotNull(dispatcher);
    this.exceptionHandler = checkNotNull(exceptionHandler);
  }

  /**
   * Returns the identifier for this event bus.
   *
   * @since 19.0
   */
  public final String identifier() {
    return identifier;
  }

  /**
   * Returns the default executor this event bus uses for dispatching events to subscribers.
   */
  final Executor executor() {
    return executor;
  }

  /**
   * Handles the given exception thrown by a subscriber with the given context.
   */
  void handleSubscriberException(Throwable e, SubscriberExceptionContext context) {
    checkNotNull(e);
    checkNotNull(context);
    try {
      exceptionHandler.handleException(e, context);
    } catch (Throwable e2) {
      // if the handler threw an exception... well, just log it
      logger.log(Level.SEVERE,
          String.format(Locale.ROOT, "Exception %s thrown while handling exception: %s", e2, e),
          e2);
    }
  }

  /**
   * Registers all subscriber methods on {@code object} to receive events.
   *
   * @param object  object whose subscriber methods should be registered.
   */
  public void register(Object object) {
    subscribers.register(object);
  }

  /**
   * Unregisters all subscriber methods on a registered {@code object}.
   *
   * @param object  object whose subscriber methods should be unregistered.
   * @throws IllegalArgumentException if the object was not previously registered.
   */
  public void unregister(Object object) {
    subscribers.unregister(object);
  }

  /**
   * Posts an event to all registered subscribers.  This method will return
   * successfully after the event has been posted to all subscribers, and
   * regardless of any exceptions thrown by subscribers.
   *
   * <p>If no subscribers 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) {
    Iterator<Subscriber> eventSubscribers = subscribers.getSubscribers(event);
    if (eventSubscribers.hasNext()) {
      dispatcher.dispatch(event, eventSubscribers);
    } else if (!(event instanceof DeadEvent)) {
      // the event had no subscribers and was not itself a DeadEvent
      post(new DeadEvent(this, event));
    }
  }

  @Override
  public String toString() {
    return MoreObjects.toStringHelper(this)
        .addValue(identifier)
        .toString();
  }

  /**
   * Simple logging handler for subscriber exceptions.
   */
  static final class LoggingHandler implements SubscriberExceptionHandler {
    static final LoggingHandler INSTANCE = new LoggingHandler();

    @Override
    public void handleException(Throwable exception, SubscriberExceptionContext context) {
      Logger logger = logger(context);
      if (logger.isLoggable(Level.SEVERE)) {
        logger.log(Level.SEVERE, message(context), exception);
      }
    }

    private static Logger logger(SubscriberExceptionContext context) {
      return Logger.getLogger(EventBus.class.getName() + "." + context.getEventBus().identifier());
    }

    private static String message(SubscriberExceptionContext context) {
      Method method = context.getSubscriberMethod();
      return "Exception thrown by subscriber method "
          + method.getName() + '(' + method.getParameterTypes()[0].getName() + ')'
          + " on subscriber " + context.getSubscriber()
          + " when dispatching event: " + context.getEvent();
    }
  }
}
