Inherits From:
NSObject
Conforms To: NSObject (NSObject)
Declared in: EOControl/EOObserver.h
The major observer in Enterprise Objects Framework is EOEditingContext, which implements its objectWillChange: method to record a snapshot for the object about to change, register undo operations in an EOUndoManager, and record the changes needed to update objects in its EOObjectStore. Because some of these actions-such as examining the object's new state-can only be performed after the object has changed, an EOEditingContext sets up a delayed message to itself, which it gets at the end of the run loop. Observers that only need to examine an object after it has changed can use the delayed observer mechanism, described in the EODelayedObserver and EODelayedObserverQueue class specifications.
Registering an Observer
Objects that directly observe others must adopt the EOObserving protocol, which consists of the single method objectWillChange: . To register an object as an observer, invoke EOObserverCenter's addObserver:forObject: with the observer and the object to be observed. Once this is done, any time the object invokes its willChange method, the observer is sent an objectWillChange: message informing it of the pending change. You can also register an observer to be notified when any object changes using addOmniscientObserver: . This can be useful in certain situations, but as it's very costly to deal out frequent change notifications, you should use omniscient observers sparingly. To unregister either kind of observer, simply use the corresponding remove... method.
Change Notification
Objects that are about to change invoke willChange , a method that the Framework adds to NSObject. This method invokes EOObserverCenter's notifyObserversObjectWillChange: , which sends an objectWillChange: message to all observers registered for the object that's changing, as well as to any omniscient observers. notifyObserversObjectWillChange: optimizes the process by suppressing redundant objectWillChange: messages when the same object invokes willChange several times in a row (as often happens when multiple properties are changed). Change notification is immediate, and takes place before the object's state changes. If you need to compare the object's state before and after the change, you must arrange to examine the new state at the end of the run loop.
You can suppress change notification when necessary, using the suppressObserverNotification and enableObserverNotification methods. While notification is suppressed, neither regular nor omniscient observers are informed of changes. These methods nest, so you can invoke suppressObserverNotification multiple times, and notification isn't re-enabled until a matching number of enableObserverNotification message have been sent.
Records anObserver to be notified with an objectWillChange: message when anObject changes.
See also: + notifyObserversObjectWillChange: , + removeObserver:forObject: , + observersForObject:
addOmniscientObserver:
+ (void)addOmniscientObserver: (id <EOObserving>)anObserver
Records anObserver to be notified with an objectWillChange: message when any object changes. This can cause significant performance degradation, and so should be used with care. The ominiscient observer must be prepared to receive the objectWillChange:
message with a nil
argument.
See also: + notifyObserversObjectWillChange: , + addObserver:forObject: , + removeOmniscientObserver:
enableObserverNotification
+ (void)enableObserverNotification
Counters a prior suppressObserverNotification message. When no such messages remain in effect, the notifyObserversObjectWillChange: method is re-enabled. Raises an NSInternalInconsistencyException if not paired with a prior suppressObserverNotification message.
See also: + observerNotificationSuppressCount
notifyObserversObjectWillChange:
+ (void)notifyObserversObjectWillChange: (id)anObject
Unless change notification is suppressed, sends an objectWillChange: to all observers registered for anObject with that object as the argument, and sends that message to all omniscient observers as well. If invoked several times in a row with the same object, only the first invocation has any effect, since subsequent change notifications are redundant.
If an observer wants to ensure that it receives notification the next time the last object to change changes again, it should use the statement:
[EOObserverCenter notifyObserversObjectWillChange:nil];
See also: + suppressObserverNotification , + addObserver:forObject: , + addOmniscientObserver:
observerForObject:ofClass:
+ (id)observerForObject: (id)anObject ofClass: (Class)aClass
Returns an observer for anObject that's a kind of aClass. If more than one observer of anObject is a kind of aClass, the specific observer returned is undetermined. You can use observersForObject: instead to get all observers and examine their class membership.
observerNotificationSuppressCount
+ (unsigned int)observerNotificationSuppressCount
Returns the number of suppressObserverNotification messages in effect.
See also: + suppressObserverNotification , + enableObserverNotification
observersForObject:
+ (NSArray *)observersForObject: (id)anObject
Returns all observers of anObject.
See also: + observerForObject:ofClass:
removeObserver:forObject:
+ (void)removeObserver: (id <EOObserving>)anObserver forObject: (id)anObject
Removes anObserver as an observer of anObject.
See also: + notifyObserversObjectWillChange: , + addObserver:forObject: , + observersForObject:
removeOmniscientObserver:
+ (void)removeOmniscientObserver: (id <EOObserving>)anObserver
Unregisters anObserver as an observer of all objects. This can cause significant performance degradation, and so should be used with care.
See also: + notifyObserversObjectWillChange: , + removeObserver:forObject: , + addOmniscientObserver:
suppressObserverNotification
+ (void)suppressObserverNotification
Disables the notifyObserversObjectWillChange: method, so that no change notifications are sent. This method can be invoked multiple times; enableObserverNotification must then be invoked an equal number of times to re-enable change notification.
See also: + observerNotificationSuppressCount
Copyright © 1997, Apple Computer, Inc. All rights reserved.