user/src/com/google/web/bindery/event/shared/EventBus.java

/*
 * Copyright 2011 Google Inc.
 * 
 * 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.web.bindery.event.shared;

import com.google.web.bindery.event.shared.Event.Type;

/**
 * Dispatches {@link Event}s to interested parties. Eases decoupling by allowing
 * objects to interact without having direct dependencies upon one another, and
 * without requiring event sources to deal with maintaining handler lists. There
 * will typically be one EventBus per application, broadcasting events that may
 * be of general interest.
 * 
 * @see SimpleEventBus
 * @see ResettableEventBus
 * @see com.google.web.bindery.event.shared.testing.CountingEventBus
 */
public abstract class EventBus {

  /**
   * Invokes {@code event.dispatch} with {@code handler}.
   * <p>
   * Protected to allow EventBus implementations in different packages to
   * dispatch events even though the {@code event.dispatch} method is protected.
   */
  protected static <H> void dispatchEvent(Event<H> event, H handler) {
    event.dispatch(handler);
  }

  /**
   * Sets {@code source} as the source of {@code event}.
   * <p>
   * Protected to allow EventBus implementations in different packages to set an
   * event source even though the {@code event.setSource} method is protected.
   */
  protected static void setSourceOfEvent(Event<?> event, Object source) {
    event.setSource(source);
  }

  /**
   * Adds an unfiltered handler to receive events of this type from all sources.
   * <p>
   * It is rare to call this method directly. More typically an {@link Event}
   * subclass will provide a static <code>register</code> method, or a widget
   * will accept handlers directly.
   * 
   * @param <H> The type of handler
   * @param type the event type associated with this handler
   * @param handler the handler
   * @return the handler registration, can be stored in order to remove the
   *         handler later
   */
  public abstract <H> HandlerRegistration addHandler(Type<H> type, H handler);

  /**
   * Adds a handler to receive events of this type from the given source.
   * <p>
   * It is rare to call this method directly. More typically a {@link Event}
   * subclass will provide a static <code>register</code> method, or a widget
   * will accept handlers directly.
   * 
   * @param <H> The type of handler
   * @param type the event type associated with this handler
   * @param source the source associated with this handler
   * @param handler the handler
   * @return the handler registration, can be stored in order to remove the
   *         handler later
   */
  public abstract <H> HandlerRegistration addHandlerToSource(Type<H> type, Object source, H handler);

  /**
   * Fires the event from no source. Only unfiltered handlers will receive it.
   * <p>
   * Any exceptions thrown by handlers will be bundled into a
   * {@link UmbrellaException} and then re-thrown after all handlers have
   * completed. An exception thrown by a handler will not prevent other handlers
   * from executing.
   * 
   * @throws UmbrellaException wrapping exceptions thrown by handlers
   * 
   * @param event the event to fire
   */
  public abstract void fireEvent(Event<?> event);

  /**
   * Fires the given event to the handlers listening to the event's type.
   * <p>
   * Any exceptions thrown by handlers will be bundled into a
   * {@link UmbrellaException} and then re-thrown after all handlers have
   * completed. An exception thrown by a handler will not prevent other handlers
   * from executing.
   * 
   * @throws UmbrellaException wrapping exceptions thrown by handlers
   * 
   * @param event the event to fire
   */
  public abstract void fireEventFromSource(Event<?> event, Object source);
}