NSScreencast

Episode Links

• Episode Source Code
• Key Value Observing Programming Guide
• Blog post: Proper use of KVO
• Blog post: KVO Done Right

Registering an observer

static void * kBackgroundColorObservationContext = &kBackgroundColorObservationContext;

[self.view addObserver:self
            forKeyPath:@"backgroundColor"
               options:NSKeyValueObservingOptionOld
               context:kBackgroundColorObservationContext];

Observing Changes

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if (context == kBackgroundColorObservationContext) {
       /* This change is meant for us */        

        NSLog(@"Key changed from %@ to %@", 
              [change objectForKey:NSKeyValueChangeOldKey],
              self.view.backgroundColor);
    } else {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

Cleaning up

- (void)viewDidUnload {
    [super viewDidUnload];
    /* ... other cleanup ... */
    
    [self.view removeObserver:self forKeyPath:@"backgroundColor"];
}

- (void)dealloc {
    /* ... other cleanup ... */
    [self.view removeObserver:self forKeyPath:@"backgroundColor"];
}

Supporting KVO for other keys

KVO is already supported for your own property accessors, however if you have something custom where and you need to modify values outside of property accessors, you might need to raise the KVO events so others can subscribe.

For example:

- (void)setFullName:(NSString *)newFullName {
  [self willChangeValueForKey:@"firstName"];
  [self willChangeValueForKey:@"lastName"];
  // set ivars from newFullName
  [self didChangeValueForKey:@"lastName"];
  [self didChangeValueForKey:@"firstName"];
}

For more information, read this article on KVO Compliance which covers this and more approaches.