Registering for Key-Value Observing

In order to receive key-value observing notifications for a property, three things are required:
  • The observed class must be key-value observing compliant for the property that you wish to observe.
  • You must register the observing object with the observed object, using the method addObserver:forKeyPath:options:context:.
  • The observing class must implement observeValueForKeyPath:ofObject:change:context:.


Registering as an Observer

In order to be notified of changes to a property, an observing object must first register with the object to be observed by sending it an addObserver:forKeyPath:options:context: message, passing the observer object and the key path of the property to be observed. The options parameter specifies the information that is provided to the observer when a change notification is sent. Using the option NSKeyValueObservingOptionOld specifies that the original object value is provided to the observer as an entry in the change dictionary. Specifying the NSKeyValueObservingOptionNew option provides the new value as an entry in the change dictionary. To receive both values, you would bitwise OR the option constants.
The example in Listing 1 demonstrates registering an inspector object for the property openingBalance.
Listing 1  Registering the inspector as an observer of the openingBalance property
- (void)registerAsObserver {
    /*
     Register 'inspector' to receive change notifications for the "openingBalance" property of
     the 'account' object and specify that both the old and new values of "openingBalance"
     should be provided in the observe… method.
     */
    [account addObserver:inspector
             forKeyPath:@"openingBalance"
                 options:(NSKeyValueObservingOptionNew |
                            NSKeyValueObservingOptionOld)
                    context:NULL];
}
When you register an object as an observer, you can also provide a context pointer. The context pointer is provided to the observer when observeValueForKeyPath:ofObject:change:context: is invoked. The context pointer can be a C pointer or an object reference. The context pointer can be used as a unique identifier to determine the change that is being observed, or to provide some other data to the observer.

Receiving Notification of a Change

When the value of an observed property of an object changes, the observer receives an observeValueForKeyPath:ofObject:change:context: message. All observers must implement this method.
The observer is provided the object and key path that triggered the observer notification, a dictionary containing details about the change, and the context pointer that was provided when the observer was registered.
The change dictionary entry NSKeyValueChangeKindKey provides information about the type of change that occurred. If the value of the observed object has changed, the NSKeyValueChangeKindKey entry returns NSKeyValueChangeSetting. Depending on the options specified when the observer was registered, the NSKeyValueChangeOldKey and NSKeyValueChangeNewKey entries in the change dictionary contain the values of the property before, and after, the change. If the property is an object, the value is provided directly. If the property is a scalar or a C structure, the value is wrapped in an NSValue object (as with key-value coding).
If the observed property is a to-many relationship, the NSKeyValueChangeKindKey entry also indicates whether objects in the relationship were inserted, removed, or replaced by returning NSKeyValueChangeInsertion, NSKeyValueChangeRemoval, orNSKeyValueChangeReplacement, respectively.
The change dictionary entry for NSKeyValueChangeIndexesKey is an NSIndexSet object specifying the indexes in the relationship that changed. If NSKeyValueObservingOptionNew or NSKeyValueObservingOptionOld are specified as options when the observer is registered, the NSKeyValueChangeOldKey and NSKeyValueChangeNewKey entries in the change dictionary are arrays containing the values of the related objects before, and after, the change.
The example in Listing 2 shows the observeValueForKeyPath:ofObject:change:context: implementation for an inspector that reflects the old and new values of the property openingBalance, as registered in Listing 1.
Listing 2  Implementation of observeValueForKeyPath:ofObject:change:context:
- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context {
 
    if ([keyPath isEqual:@"openingBalance"]) {
        [openingBalanceInspectorField setObjectValue:
            [change objectForKey:NSKeyValueChangeNewKey]];
    }
    /*
     Be sure to call the superclass's implementation *if it implements it*.
     NSObject does not implement the method.
     */
    [super observeValueForKeyPath:keyPath
                         ofObject:object
                           change:change
                           context:context];
}

Removing an Object as an Observer

You remove a key-value observer by sending the observed object a removeObserver:forKeyPath: message, specifying the observing object and the key path. The example in Listing 3 removes the inspector as an observer of openingBalance.
Listing 3  Removing the inspector as an observer of openingBalance
- (void)unregisterForChangeNotification {
    [observedObject removeObserver:inspector forKeyPath:@"openingBalance"];
}
If the context is an object, you must keep a strong reference to it until removing the observer. After receiving a removeObserver:forKeyPath: message, the observing object will no longer receive anyobserveValueForKeyPath:ofObject:change:context: messages for the specified key path and object.
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