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phloc in-process event library

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/**
 * Copyright (C) 2006-2015 phloc systems
 * http://www.phloc.com
 * office[at]phloc[dot]com
 *
 * 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.phloc.event;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;

import com.phloc.commons.callback.INonThrowingRunnableWithParameter;

/**
 * Base interface of event observers.
 * 
 * @author philip
 */
public interface IEventObserver
{
  /**
   * Determine whether we can handle this event or not. This method is always
   * called synchronously.
   * 
   * @param aEvent
   *        The event to be checked. May not be null.
   * @return Never null.
   */
  @Nonnull
  EEventObserverHandlerType canHandleEvent (@Nonnull IEvent aEvent);

  /**
   * Main event handling routine. This routine is only called if the call to
   * {@link #canHandleEvent(IEvent)} did not return
   * {@link EEventObserverHandlerType#CANNOT_HANDLE}.
   * 
   * @param aEvent
   *        The event to be handled. May not be null.
   * @param aResultCallback
   *        The callback to which the result needs to be delivered. May be
   *        null if the dispatcher is unidirectional.
   * @throws Exception
   *         on error
   */
  void onEvent (@Nonnull IEvent aEvent, @Nullable INonThrowingRunnableWithParameter