android/guava/src/com/google/common/eventbus/DeadEvent.java

/*
 * 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;

/**
 * Wraps an event that was posted, but which had no subscribers and thus could not be delivered.
 *
 * <p>Registering a DeadEvent subscriber is useful for debugging or logging, as it can detect
 * misconfigurations in a system's event distribution.
 *
 * @author Cliff Biffle
 * @since 10.0
 */
@Beta
public class DeadEvent {

  private final Object source;
  private final Object event;

  /**
   * Creates a new DeadEvent.
   *
   * @param source object broadcasting the DeadEvent (generally the {@link EventBus}).
   * @param event the event that could not be delivered.
   */
  public DeadEvent(Object source, Object event) {
    this.source = checkNotNull(source);
    this.event = checkNotNull(event);
  }

  /**
   * Returns the object that originated this event (<em>not</em> the object that originated the
   * wrapped event). This is generally an {@link EventBus}.
   *
   * @return the source of this event.
   */
  public Object getSource() {
    return source;
  }

  /**
   * Returns the wrapped, 'dead' event, which the system was unable to deliver to any registered
   * subscriber.
   *
   * @return the 'dead' event that could not be delivered.
   */
  public Object getEvent() {
    return event;
  }

  @Override
  public String toString() {
    return MoreObjects.toStringHelper(this).add("source", source).add("event", event).toString();
  }
}