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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.fitbur.guava.common.eventbus;

import static com.fitbur.guava.common.base.Preconditions.checkArgument;
import static com.fitbur.guava.common.base.Preconditions.checkNotNull;

import com.fitbur.guava.common.annotations.VisibleForTesting;
import com.fitbur.guava.common.base.MoreObjects;
import com.fitbur.guava.common.base.Objects;
import com.fitbur.guava.common.base.Throwables;
import com.fitbur.guava.common.cache.CacheBuilder;
import com.fitbur.guava.common.cache.CacheLoader;
import com.fitbur.guava.common.cache.LoadingCache;
import com.fitbur.guava.common.collect.HashMultimap;
import com.fitbur.guava.common.collect.ImmutableList;
import com.fitbur.guava.common.collect.ImmutableSet;
import com.fitbur.guava.common.collect.Iterators;
import com.fitbur.guava.common.collect.Lists;
import com.fitbur.guava.common.collect.Maps;
import com.fitbur.guava.common.collect.Multimap;
import com.fitbur.guava.common.reflect.TypeToken;
import com.fitbur.guava.common.util.concurrent.UncheckedExecutionException;
import com.fitbur.guava.j2objc.annotations.Weak;

import java.lang.reflect.Method;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.CopyOnWriteArraySet;

import javax.annotation.Nullable;

/**
 * Registry of subscribers to a single event bus.
 *
 * @author Colin Decker
 */
final class SubscriberRegistry {

  /**
   * All registered subscribers, indexed by event type.
   *
   * The {@link CopyOnWriteArraySet} values make it easy and relatively lightweight to get an
   * immutable snapshot of all current subscribers to an event without any locking.
   */
  private final ConcurrentMap, CopyOnWriteArraySet> subscribers =
      Maps.newConcurrentMap();

  /**
   * The event bus this registry belongs to.
   */
  @Weak private final EventBus bus;

  SubscriberRegistry(EventBus bus) {
    this.bus = checkNotNull(bus);
  }

  /**
   * Registers all subscriber methods on the given listener object.
   */
  void register(Object listener) {
    Multimap, Subscriber> listenerMethods = findAllSubscribers(listener);

    for (Map.Entry, Collection> entry : listenerMethods.asMap().entrySet()) {
      Class eventType = entry.getKey();
      Collection eventMethodsInListener = entry.getValue();

      CopyOnWriteArraySet eventSubscribers = subscribers.get(eventType);

      if (eventSubscribers == null) {
        CopyOnWriteArraySet newSet = new CopyOnWriteArraySet();
        eventSubscribers = MoreObjects.firstNonNull(
            subscribers.putIfAbsent(eventType, newSet), newSet);
      }

      eventSubscribers.addAll(eventMethodsInListener);
    }
  }

  /**
   * Unregisters all subscribers on the given listener object.
   */
  void unregister(Object listener) {
    Multimap, Subscriber> listenerMethods = findAllSubscribers(listener);

    for (Map.Entry, Collection> entry : listenerMethods.asMap().entrySet()) {
      Class eventType = entry.getKey();
      Collection listenerMethodsForType = entry.getValue();

      CopyOnWriteArraySet currentSubscribers = subscribers.get(eventType);
      if (currentSubscribers == null || !currentSubscribers.removeAll(listenerMethodsForType)) {
        // if removeAll returns true, all we really know is that at least one subscriber was
        // removed... however, barring something very strange we can assume that if at least one
        // subscriber was removed, all subscribers on listener for that event type were... after
        // all, the definition of subscribers on a particular class is totally static
        throw new IllegalArgumentException(
            "missing event subscriber for an annotated method. Is " + listener + " registered?");
      }

      // don't try to remove the set if it's empty; that can't be done safely without a lock
      // anyway, if the set is empty it'll just be wrapping an array of length 0
    }
  }

  @VisibleForTesting
  Set getSubscribersForTesting(Class eventType) {
    return MoreObjects.firstNonNull(subscribers.get(eventType), ImmutableSet.of());
  }

  /**
   * Gets an iterator representing an immutable snapshot of all subscribers to the given event at
   * the time this method is called.
   */
  Iterator getSubscribers(Object event) {
    ImmutableSet> eventTypes = flattenHierarchy(event.getClass());

    List> subscriberIterators =
        Lists.newArrayListWithCapacity(eventTypes.size());

    for (Class eventType : eventTypes) {
      CopyOnWriteArraySet eventSubscribers = subscribers.get(eventType);
      if (eventSubscribers != null) {
        // eager no-copy snapshot
        subscriberIterators.add(eventSubscribers.iterator());
      }
    }

    return Iterators.concat(subscriberIterators.iterator());
  }

  /**
   * A thread-safe cache that contains the mapping from each class to all methods in that class and
   * all super-classes, that are annotated with {@code @Subscribe}. The cache is shared across all
   * instances of this class; this greatly improves performance if multiple EventBus instances are
   * created and objects of the same class are registered on all of them.
   */
  private static final LoadingCache, ImmutableList> subscriberMethodsCache =
      CacheBuilder.newBuilder()
          .weakKeys()
          .build(new CacheLoader, ImmutableList>() {
            @Override
            public ImmutableList load(Class concreteClass) throws Exception {
              return getAnnotatedMethodsNotCached(concreteClass);
            }
          });

  /**
   * Returns all subscribers for the given listener grouped by the type of event they subscribe to.
   */
  private Multimap, Subscriber> findAllSubscribers(Object listener) {
    Multimap, Subscriber> methodsInListener = HashMultimap.create();
    Class clazz = listener.getClass();
    for (Method method : getAnnotatedMethods(clazz)) {
      Class[] parameterTypes = method.getParameterTypes();
      Class eventType = parameterTypes[0];
      methodsInListener.put(eventType, Subscriber.create(bus, listener, method));
    }
    return methodsInListener;
  }

  private static ImmutableList getAnnotatedMethods(Class clazz) {
    return subscriberMethodsCache.getUnchecked(clazz);
  }

  private static ImmutableList getAnnotatedMethodsNotCached(Class clazz) {
    Set> supertypes = TypeToken.of(clazz).getTypes().rawTypes();
    Map identifiers = Maps.newHashMap();
    for (Class supertype : supertypes) {
      for (Method method : supertype.getDeclaredMethods()) {
        if (method.isAnnotationPresent(Subscribe.class) && !method.isSynthetic()) {
          // TODO(cgdecker): Should check for a generic parameter type and error out
          Class[] parameterTypes = method.getParameterTypes();
          checkArgument(parameterTypes.length == 1,
              "Method %s has @Subscribe annotation but has %s parameters."
                  + "Subscriber methods must have exactly 1 parameter.",
              method, parameterTypes.length);

          MethodIdentifier ident = new MethodIdentifier(method);
          if (!identifiers.containsKey(ident)) {
            identifiers.put(ident, method);
          }
        }
      }
    }
    return ImmutableList.copyOf(identifiers.values());
  }

  /**
   * Global cache of classes to their flattened hierarchy of supertypes.
   */
  private static final LoadingCache, ImmutableSet>> flattenHierarchyCache =
      CacheBuilder.newBuilder()
          .weakKeys()
          .build(new CacheLoader, ImmutableSet>>() {
            @SuppressWarnings("RedundantTypeArguments") // > is actually needed to compile
            @Override
            public ImmutableSet> load(Class concreteClass) {
              return ImmutableSet.>copyOf(
                  TypeToken.of(concreteClass).getTypes().rawTypes());
            }
          });

  /**
   * Flattens a class's type hierarchy into a set of {@code Class} objects including all
   * superclasses (transitively) and all interfaces implemented by these superclasses.
   */
  @VisibleForTesting
  static ImmutableSet> flattenHierarchy(Class concreteClass) {
    try {
      return flattenHierarchyCache.getUnchecked(concreteClass);
    } catch (UncheckedExecutionException e) {
      throw Throwables.propagate(e.getCause());
    }
  }

  private static final class MethodIdentifier {

    private final String name;
    private final List> parameterTypes;

    MethodIdentifier(Method method) {
      this.name = method.getName();
      this.parameterTypes = Arrays.asList(method.getParameterTypes());
    }

    @Override
    public int hashCode() {
      return Objects.hashCode(name, parameterTypes);
    }

    @Override
    public boolean equals(@Nullable Object o) {
      if (o instanceof MethodIdentifier) {
        MethodIdentifier ident = (MethodIdentifier) o;
        return name.equals(ident.name) && parameterTypes.equals(ident.parameterTypes);
      }
      return false;
    }
  }
}