EventSubscriber.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.base.Preconditions;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

import javax.annotation.Nullable;

/**
 * Wraps a single-argument subscriber method on a specific object.
 *
 * <p>This class only verifies the suitability of the method and event type if
 * something fails.  Callers are expected to verify their uses of this class.
 *
 * <p>Two EventSubscribers are equivalent when they refer to the same method on the
 * same object (not class).   This property is used to ensure that no subscriber
 * method is registered more than once.
 *
 * @author Cliff Biffle
 */
class EventSubscriber {

  /** Object sporting the subscriber method. */
  private final Object target;
  /** Subscriber method. */
  private final Method method;

  /**
   * Creates a new EventSubscriber to wrap {@code method} on @{code target}.
   *
   * @param target  object to which the method applies.
   * @param method  subscriber method.
   */
  EventSubscriber(Object target, Method method) {
    Preconditions.checkNotNull(target,
        "EventSubscriber target cannot be null.");
    Preconditions.checkNotNull(method, "EventSubscriber method cannot be null.");

    this.target = target;
    this.method = method;
    method.setAccessible(true);
  }

  /**
   * Invokes the wrapped subscriber method to handle {@code event}.
   *
   * @param event  event to handle
   * @throws InvocationTargetException  if the wrapped method throws any
   *     {@link Throwable} that is not an {@link Error} ({@code Error} instances are
   *     propagated as-is).
   */
  public void handleEvent(Object event) throws InvocationTargetException {
    checkNotNull(event);
    try {
      method.invoke(target, new Object[] { event });
    } catch (IllegalArgumentException e) {
      throw new Error("Method rejected target/argument: " + event, e);
    } catch (IllegalAccessException e) {
      throw new Error("Method became inaccessible: " + event, e);
    } catch (InvocationTargetException e) {
      if (e.getCause() instanceof Error) {
        throw (Error) e.getCause();
      }
      throw e;
    }
  }

  @Override public String toString() {
    return "[wrapper " + method + "]";
  }

  @Override public int hashCode() {
    final int PRIME = 31;
    return (PRIME + method.hashCode()) * PRIME
        + System.identityHashCode(target);
  }

  @Override public boolean equals(@Nullable Object obj) {
    if (obj instanceof EventSubscriber) {
      EventSubscriber that = (EventSubscriber) obj;
      // Use == so that different equal instances will still receive events.
      // We only guard against the case that the same object is registered
      // multiple times
      return target == that.target && method.equals(that.method);
    }
    return false;
  }

  public Object getSubscriber() {
    return target;
  }

  public Method getMethod() {
    return method;
  }
}