Download the sample project here: https://github.com/kmdarshan/LearnKVO

I have implemented a sample project with KVO explained. The two main methods to note are :

[bobj addObserver:pobj forKeyPath:@"balance" options:NSKeyValueObservingOptionNew context:NULL];

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context