#KVO-Demo

addObserve

[self addObserver:self forKeyPath:@"items" options:0 context:nil];

keyValueChanged

[self willChangeValueForKey:@"items"];
[self didChangeValueForKey:@"items"];

listen for changes to the items delegate

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