content: /*
   * Licensed to the Apache Software Foundation (ASF) under one
   * or more contributor license agreements.  See the NOTICE file
   * distributed with this work for additional information
   * regarding copyright ownership.  The ASF licenses this file
   * to you 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.datastax.oss.driver.internal.core.context;
  
  import com.datastax.oss.driver.shaded.guava.common.collect.HashMultimap;
  import com.datastax.oss.driver.shaded.guava.common.collect.Multimaps;
  import com.datastax.oss.driver.shaded.guava.common.collect.SetMultimap;
  import java.util.function.Consumer;
  import net.jcip.annotations.ThreadSafe;
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  /**
   * Barebones event bus implementation, that allows components to communicate without knowing about
   * each other.
   *
   * <p>This is intended for administrative events (topology changes, new connections, etc.), which
   * are comparatively rare in the driver. Do not use it for anything on the request path, because it
   * relies on synchronization.
   *
   * <p>We don't use Guava's implementation because Guava is shaded in the driver, and the event bus
   * needs to be accessible from low-level 3rd party customizations.
   */
  @ThreadSafe
  public class EventBus {
    private static final Logger LOG = LoggerFactory.getLogger(EventBus.class);
  
    private final String logPrefix;
    private final SetMultimap<Class<?>, Consumer<?>> listeners =
        Multimaps.synchronizedSetMultimap(HashMultimap.create());
  
    public EventBus(String logPrefix) {
      this.logPrefix = logPrefix;
    }
  
    /**
     * Registers a listener for an event type.
     *
     * <p>If the listener has a shorter lifecycle than the {@code Cluster} instance, it is recommended
     * to save the key returned by this method, and use it later to unregister and therefore avoid a
     * leak.
     *
     * @return a key that is needed to unregister later.
     */
    public <EventT> Object register(Class<EventT> eventClass, Consumer<EventT> listener) {
      LOG.debug("[{}] Registering {} for {}", logPrefix, listener, eventClass);
      listeners.put(eventClass, listener);
      // The reason for the key mechanism is that this will often be used with method references,
      // and you get a different object every time you reference a method, so register(Foo::bar)
      // followed by unregister(Foo::bar) wouldn't work as expected.
      return listener;
    }
  
    /**
     * Unregisters a listener.
     *
     * @param key the key that was returned by {@link #register(Class, Consumer)}
     */
    public <EventT> boolean unregister(Object key, Class<EventT> eventClass) {
      LOG.debug("[{}] Unregistering {} for {}", logPrefix, key, eventClass);
      return listeners.remove(eventClass, key);
    }
  
    /**
     * Sends an event that will notify any registered listener for that class.
     *
     * <p>Listeners are looked up by an <b>exact match</b> on the class of the object, as returned by
     * {@code event.getClass()}. Listeners of a supertype won't be notified.
     *
     * <p>The listeners are invoked on the calling thread. It's their responsibility to schedule event
     * processing asynchronously if needed.
     */
    public void fire(Object event) {
      LOG.debug("[{}] Firing an instance of {}: {}", logPrefix, event.getClass(), event);
      // if the exact match thing gets too cumbersome, we can reconsider, but I'd like to avoid
      // scanning all the keys with instanceof checks.
      Class<?> eventClass = event.getClass();
      for (Consumer<?> l : listeners.get(eventClass)) {
        @SuppressWarnings("unchecked")
        Consumer<Object> listener = (Consumer<Object>) l;
        LOG.debug("[{}] Notifying {} of {}", logPrefix, listener, event);
        listener.accept(event);
      }
    }
  }
  