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  package org.apache.cayenne.util;
  
  import org.apache.cayenne.DataChannel;
  import org.apache.cayenne.DataChannelListener;
  import org.apache.cayenne.event.EventManager;
  import org.apache.cayenne.event.EventSubject;
  import org.apache.cayenne.graph.GraphEvent;
  
  /**
   * Contains access stack events related utility methods.
   * 
   * @since 1.2
   */
  public class EventUtil {
  
      static final EventSubject[] CHANNEL_SUBJECTS = new EventSubject[] {
              DataChannel.GRAPH_CHANGED_SUBJECT, DataChannel.GRAPH_FLUSHED_SUBJECT,
              DataChannel.GRAPH_ROLLEDBACK_SUBJECT
      };
  
      /**
       * Utility method that sets up a GraphChangeListener to be notified when DataChannel
       * posts an event.
       * 
       * @return false if an DataChannel doesn't have an EventManager and therefore does not
       *         support events.
       */
      public static boolean listenForChannelEvents(
              DataChannel channel,
              DataChannelListener listener) {
  
          EventManager manager = channel.getEventManager();
  
          if (manager == null) {
              return false;
          }
  
          listenForSubjects(manager, listener, channel, CHANNEL_SUBJECTS);
          return true;
      }
  
      /**
       * Listen for events from all channels that use a given EventManager.
       */
      public static boolean listenForChannelEvents(
              EventManager manager,
              DataChannelListener listener) {
  
          if (manager == null) {
              return false;
          }
  
          listenForSubjects(manager, listener, null, CHANNEL_SUBJECTS);
          return true;
      }
  
      /**
       * Registers GraphEventListener for multiple subjects at once.
       */
      static void listenForSubjects(
              EventManager manager,
              DataChannelListener listener,
              Object sender,
              EventSubject[] subjects) {
  
          for (EventSubject subject : subjects) {
              // assume that subject name and listener method name match
              String fqSubject = subject.getSubjectName();
              String method = fqSubject.substring(fqSubject.lastIndexOf('/') + 1);
  
              // use non-blocking listeners for multi-threaded EM; blocking for single
              // threaded...
  
              if (manager.isSingleThreaded()) {
                  manager.addListener(listener, method, GraphEvent.class, subject, sender);
              }
              else {
                  manager.addNonBlockingListener(
                          listener,
                          method,
                          GraphEvent.class,
                          subject,
                          sender);
              }
          }
      }
  
      // not for instantiation
      private EventUtil() {
      }
  }
  