/*
    * 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>
   * <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=
  * "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 subscribers, 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 #subscribersByTypeLock}.
    */
   private final SetMultimap<Class<?>, EventSubscriber> subscribersByType =
       HashMultimap.create();
   private final ReadWriteLock subscribersByTypeLock = new ReentrantReadWriteLock();
 
   /**
    * Strategy for finding subscriber methods in registered objects.  Currently,
    * only the {@link AnnotatedSubscriberFinder} is supported, but this is
    * encapsulated for future expansion.
    */
   private final SubscriberFindingStrategy finder = new AnnotatedSubscriberFinder();
 
   /** queues of events for the current thread to dispatch */
   private final ThreadLocal<Queue<EventWithSubscriber>> eventsToDispatch =
       new ThreadLocal<Queue<EventWithSubscriber>>() {
     @Override protected Queue<EventWithSubscriber> initialValue() {
       return new LinkedList<EventWithSubscriber>();
     }
   };
 
   /** true if the current thread is currently dispatching an event */
   private final ThreadLocal<Boolean> isDispatching =
       new ThreadLocal<Boolean>() {
     @Override protected Boolean initialValue() {
       return false;
     }
   };
 
   private SubscriberExceptionHandler subscriberExceptionHandler;
 
   /**
    * 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(new LoggingSubscriberExceptionHandler(identifier));
   }
 
   /**
    * Creates a new EventBus with the given {@link SubscriberExceptionHandler}.
    * 
    * @param subscriberExceptionHandler Handler for subscriber exceptions.
    * @since 16.0
    */
   public EventBus(SubscriberExceptionHandler subscriberExceptionHandler) {
     this.subscriberExceptionHandler = checkNotNull(subscriberExceptionHandler);
   }
 
   /**
    * Registers all subscriber methods on {@code object} to receive events.
    * Subscriber methods are selected and classified using this EventBus's
    * {@link SubscriberFindingStrategy}; the default strategy is the
    * {@link AnnotatedSubscriberFinder}.
    *
    * @param object  object whose subscriber methods should be registered.
    */
   public void register(Object object) {
     Multimap<Class<?>, EventSubscriber> methodsInListener =
         finder.findAllSubscribers(object);
     subscribersByTypeLock.writeLock().lock();
     try {
       subscribersByType.putAll(methodsInListener);
     } finally {
       subscribersByTypeLock.writeLock().unlock();
     }
   }
 
   /**
    * 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) {
     Multimap<Class<?>, EventSubscriber> methodsInListener = finder.findAllSubscribers(object);
     for (Entry<Class<?>, Collection<EventSubscriber>> entry :
           methodsInListener.asMap().entrySet()) {
       Class<?> eventType = entry.getKey();
       Collection<EventSubscriber> eventMethodsInListener = entry.getValue();
 
       subscribersByTypeLock.writeLock().lock();
       try {
         Set<EventSubscriber> currentSubscribers = subscribersByType.get(eventType);
         if (!currentSubscribers.containsAll(eventMethodsInListener)) {
           throw new IllegalArgumentException(
               "missing event subscriber for an annotated method. Is " + object + " registered?");
         }
         currentSubscribers.removeAll(eventMethodsInListener);
       } finally {
         subscribersByTypeLock.writeLock().unlock();
       }
     }
   }
 
   /**
    * 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) {
     Set<Class<?>> dispatchTypes = flattenHierarchy(event.getClass());
 
     boolean dispatched = false;
     for (Class<?> eventType : dispatchTypes) {
       subscribersByTypeLock.readLock().lock();
       try {
         Set<EventSubscriber> wrappers = subscribersByType.get(eventType);
 
         if (!wrappers.isEmpty()) {
           dispatched = true;
           for (EventSubscriber wrapper : wrappers) {
             enqueueEvent(event, wrapper);
           }
         }
       } finally {
         subscribersByTypeLock.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, EventSubscriber subscriber) {
     eventsToDispatch.get().offer(new EventWithSubscriber(event, subscriber));
   }
 
   /**
    * 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() {
     // don't dispatch if we're already dispatching, that would allow reentrancy
     // and out-of-order events. Instead, leave the events to be dispatched
     // after the in-progress dispatch is complete.
     if (isDispatching.get()) {
       return;
     }
 
     isDispatching.set(true);
     try {
       Queue<EventWithSubscriber> events = eventsToDispatch.get();
       EventWithSubscriber eventWithSubscriber;
       while ((eventWithSubscriber = events.poll()) != null) {
         dispatch(eventWithSubscriber.event, eventWithSubscriber.subscriber);
       }
     } finally {
       isDispatching.remove();
       eventsToDispatch.remove();
     }
   }
 
   /**
    * Dispatches {@code event} to the subscriber 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 subscriber.
    */
   void dispatch(Object event, EventSubscriber wrapper) {
     try {
       wrapper.handleEvent(event);
     } catch (InvocationTargetException e) {
       try {
         subscriberExceptionHandler.handleException(
             e.getCause(),
             new SubscriberExceptionContext(
                 this,
                 event,
                 wrapper.getSubscriber(),
                 wrapper.getMethod()));
       } catch (Throwable t) {
         // If the exception handler throws, log it. There isn't much else to do!
         Logger.getLogger(EventBus.class.getName()).log(Level.SEVERE,
              String.format(
             "Exception %s thrown while handling exception: %s", t,
             e.getCause()),
             t);
       }
     }
   }
 
   /**
    * 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 logging handler for subscriber exceptions.
    */
   private static final class LoggingSubscriberExceptionHandler
       implements SubscriberExceptionHandler {
 
     /**
      * 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;
 
     /**
      * @param identifier a brief name for this bus, for logging purposes. Should
      *        be a valid Java identifier.
      */
     public LoggingSubscriberExceptionHandler(String identifier) {
       logger = Logger.getLogger(
           EventBus.class.getName() + "." + checkNotNull(identifier));
     }
 
     @Override
     public void handleException(Throwable exception,
         SubscriberExceptionContext context) {
       logger.log(Level.SEVERE, "Could not dispatch event: " 
           + context.getSubscriber() + " to " + context.getSubscriberMethod(),
           exception.getCause());
     }
   }
 
   /** simple struct representing an event and it's subscriber */
   static class EventWithSubscriber {
     final Object event;
     final EventSubscriber subscriber;
     public EventWithSubscriber(Object event, EventSubscriber subscriber) {
       this.event = checkNotNull(event);
       this.subscriber = checkNotNull(subscriber);
     }
   }
 }