WebObjects/EOF/Using EOF/Delegates and Notifications
EO Change Notifications
Pierre Bernard
To invalidate cached versions of derived values, you might want to use this:
/** 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();
}
}
}