EOF-Using EOF-Delegates and Notifications

/** Utility class. Provides for a way to watch for changes in EOs. The main use of this would be to
  * be able to safely keep cached/computed/derived values that get cleared once out of sync.
  *
  * @author bernard
  * @version ChangeNotificationCenter.java,v 1.1 2003/03/11 16:26:59 bernard Exp
**/
public class ChangeNotificationCenter
{
 // Public class constants

 /** Name of the posted notification
   */
 public static final String OBJECT_CHANGED = "MBSObjectChanged";

 /** Possible type of change a notification is posted for
   */
 public static final String INVALIDATION = "Invalidation";

 /** Possible type of change a notification is posted for
   */
 public static final String DELETION = "Deletion";

 /** Possible type of change a notification is posted for
   */
 public static final String UPDATE = "Update";

 // Private class constants

 /** Key in the posted notification's userInfo dictionary.
   */
 private static final String OBJECT = "MBSObject";

 /** Key in the posted notification's userInfo dictionary.
   */
 private static final String TYPE = "MBSType";

 /** Selector used for calling the watcher object upon a change
   */
 private static final NSSelector watchedObjectChangedSelector =
   new NSSelector("watchedObjectChanged", new Class[] { NSNotification.class });

 // Private class variables

 /** Reference holding the shared singleton instance.
   */
 private static ChangeNotificationCenter defaultCenter = null;

 // Constructor

 /** Singleton class
   */
 private ChangeNotificationCenter()
  {
   super();

   Class[] notificationArray = new Class[] { NSNotification.class };
   NSSelector objectsChangedIInEditingContextSelector =
     new NSSelector("objectsChangedInEditingContext", notificationArray);

   NSNotificationCenter.defaultCenter().addObserver(
     this,
     objectsChangedIInEditingContextSelector,
     EOEditingContext.ObjectsChangedInEditingContextNotification,
     null);
  }

 // Public insatnce methods

 /** Method called when a change notification is received.
   *
   * The notification is split and redispatched for all updated objects.
   */
 public void objectsChangedInEditingContext(NSNotification notification)
  {
   NSArray updated = (NSArray) notification.userInfo().objectForKey(EOObjectStore.UpdatedKey);

   if (updated != null)
    {
     int count = updated.count();

     for (int i = 0; i < count; i++)
      {
       Object object = updated.objectAtIndex(i);
       NSDictionary userInfo =
         new NSDictionary(new Object[] { object, UPDATE }, new Object[] { OBJECT, TYPE });
       NSNotificationCenter.defaultCenter().postNotification(OBJECT_CHANGED, object, userInfo);
      }
    }

   NSArray invalidated = (NSArray) notification.userInfo().objectForKey(EOObjectStore.InvalidatedKey);

   if (invalidated != null)
    {
     int count = invalidated.count();

     for (int i = 0; i < count; i++)
      {
       Object object = invalidated.objectAtIndex(i);
       NSDictionary userInfo =
         new NSDictionary(new Object[] { object, INVALIDATION }, new Object[] { OBJECT, TYPE });
       NSNotificationCenter.defaultCenter().postNotification(OBJECT_CHANGED, object, userInfo);
      }
    }

   NSArray deleted = (NSArray) notification.userInfo().objectForKey(EOObjectStore.DeletedKey);

   if (deleted != null)
    {
     int count = deleted.count();

     for (int i = 0; i < count; i++)
      {
       Object object = deleted.objectAtIndex(i);
       NSDictionary userInfo = new NSDictionary(new Object[] { object, DELETION }, new Object[] { OBJECT, TYPE });
       NSNotificationCenter.defaultCenter().postNotification(OBJECT_CHANGED, object, userInfo);
      }
    }
  }

 /** Method to be called when one creates a cached value that depends on the attributes of a given object.
   *
   * Upon registration the object is watched and the watchedObjectChanged() is called once it changes, thus providing an
   * oppurtunity to invalidate the cached value.
   *
   * @param watcher the watching object which should unregister before disposing
   * @param object the object to watch
   * @see #unregisterCacheDependancy
   * @see #unregisterCacheDependancies
   */
 public void registerCacheDependancy(ObserverInterface watcher, EOEnterpriseObject object)
  {
   NSNotificationCenter.defaultCenter().addObserver(watcher, watchedObjectChangedSelector, OBJECT_CHANGED, object);
  }

 /** Method to be called when one abandons or clears a cached value that depended on attributes of a given object.
   *
   * @param watcher the watching object
   * @param object the object to watch
   * @see #registerCacheDependancy
   */
 public void unregisterCacheDependancy(ObserverInterface watcher, Object object)
  {
   NSNotificationCenter.defaultCenter().removeObserver(watcher, OBJECT_CHANGED, object);
  }

 /** Unregisters all of the callers dependancies. To be used sparingly as it may break unknown but required
   * dependancies. Usually called when disposing the calling object.
   *
   * @param watcher the watching object
   * @see #registerCacheDependancy
   * @see #unregisterCacheDependancy
   */
 public void unregisterCacheDependancies(ObserverInterface watcher)
  {
   NSNotificationCenter.defaultCenter().removeObserver(watcher, OBJECT_CHANGED, null);
  }

 // Public class methods

 /** Gets or lazily instantiates the shared instance of the ChangeNotificationCenter
   *
   * @return the unique instance
   */
 public static ChangeNotificationCenter defaultCenter()
  {
   if (defaultCenter == null)
    {
     createDefaultCenter();
    }

   return defaultCenter;
  }

 /** Utility method. Allows for retrieving the changed object from a notification
   * that results of registering interest in a change.
   *
   * @param notification the received notification
   * @return the object that triggered the notification
   */
 public static EOEnterpriseObject objectFromNotification(NSNotification notification)
  {
   return (EOEnterpriseObject) notification.userInfo().objectForKey(OBJECT);
  }

 /** Utility method. Allows for retrieving the type of change that ocurred from a notification
   * that results of registering interest in a change.
   *
   * @param notification the received notification
   * @return the type as one of the class constants defined in ChangeNotificationCenter
   */
 public static String typeFromNotification(NSNotification notification)
  {
   return (String) notification.userInfo().objectForKey(TYPE);
  }

 // Private class methods

 /** Creates the singleton instance ensuring there is no existing instance.
   */
 private static synchronized void createDefaultCenter()
  {
   if (defaultCenter == null)
    {
     defaultCenter = new ChangeNotificationCenter();
    }
  }

 // Inner interface definition

 /** Interface to be implemented by objects that need to listen to changes.
   *
   * CAVEAT: in most cases it is recommended to not directly implement the interface,
   * but rather create an inner class that implements the interface or better yet
   * extends the default implementation. Indeed if an object (e.g. an EO) which
   * participates in an inheritance hierarchy is used as receiver for notifications,
   * registering or unregistering might break functionality in a parent class.
   */
 public static interface ObserverInterface
  {
   /** Method called when an object on which locally cached values depend is modified.
     *
     * This hook is provided as an opportunity to clear locally cached values. It's a good idea not to recreate
     * the cached values immediately, but on an as-needed basis. You should refrain from changing persisted EO
     * attributes from within this method as this might kick off another chain of notifications.
     *
     * Once the caches cleared, it would be a very good idea to unregister from further notifications until
     * the cache is recreated.
     *
     * @see lu.bcl.enterprise.entity.ChangeNotificationCenter#objectFromNotification
     * @see lu.bcl.enterprise.entity.ChangeNotificationCenter#registerCacheDependancy
     * @see lu.bcl.enterprise.entity.ChangeNotificationCenter#unRegisterCacheDependancy
     */
   public void watchedObjectChanged(NSNotification notification);
  }

 // Inner class

 /** Convenience class for implementing ObserverInterface.
   *
   * The recommended way of registering for change notificications is to create an inner
   * class extending this one.
   */
 public abstract static class Observer implements ObserverInterface
  {
   /** Method to be called by subclasses when the create a cached value that depends on the attributes of a given object.
     *
     * Upon registration the object is watched and the clearCachedValues() is called once it changes, thus providing an
     * oppurtunity to invalidate the cached value.
     *
     * You need to register for each object your locally cached values depend on. E.g. if you build a cached value from
     * attributes of EOs in a too-many relationship, your cache depends on each of the objects in the relationship as well
     * as on the source of the realtionship.
     *
     * N.B. Extend the dispose() method to unregister any dependancy you might be registered for.
     *
     * @param object the object to watch
     * @see #unregisterCacheDependancy
     * @see #clearCachedValues
     */
   protected void registerCacheDependancy(EOEnterpriseObject object)
    {
     ChangeNotificationCenter.defaultCenter().registerCacheDependancy(this, object);
    }

   /** Method to be called by subclasses when they abandon or clear a cached value that depended on attributes
     * of a given object.
     *
     * @param object the object to watch
     * @see #registerCacheDependancy
     * @see #clearCachedValues
     */
   protected void unregisterCacheDependancy(EOEnterpriseObject object)
    {
     ChangeNotificationCenter.defaultCenter().unregisterCacheDependancy(this, object);
    }

   /** Unregisters all of the callers dependancies. To be used sparingly as it may break unknown but required
     * dependancies. Usually called when disposing the calling object.
     *
     * @see #registerCacheDependancy
     * @see #unregisterCacheDependancy
     */
   public void unregisterCacheDependancies()
    {
     ChangeNotificationCenter.defaultCenter().unregisterCacheDependancies(this);
    }

   /** Overridden to unregister all cache dependancies
     */
   public void finalize() throws Throwable
    {
     ChangeNotificationCenter.defaultCenter().unregisterCacheDependancies(this);

     super.finalize();
    }
  }
}