- (void)applicationDidFinishLaunching:(UIApplication *)application {    
    
    // Override point for customization after app launch    
    [window addSubview:viewController.view];
[viewController addObserver:self forKeyPath:@"iVar" options:(NSKeyValueObservingOptionNew) context:NULL]; 
    [window makeKeyAndVisible];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object 
						change:(NSDictionary *)change context:(void *)context { 
    NSLog(@"Value %@ changed in %@", keyPath, [object description]); 
}

Communication between objects..

Key Value Observer

iphone

Objective C