alanjrogers

static NSString *MYObservationContext = @"MYObservationContext";

[object addObserver:self forKeyPath:@"key.path" options:(NSKeyValueObservingOptionOld | NSKeyValueObservingOptionNew) context:(void*)&MYObservationContext];

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if (context == &MYObservationContext) {
        id oldValue = [change objectForKey:NSKeyValueChangeOldKey];
        id newValue = [change objectForKey:NSKeyValueChangeNewKey];
        // Do something
    }