Proper Key-Value Observer Usage

file.m

// Source: http://www.dribin.org/dave/blog/archives/2008/09/24/proper_kvo_usage/

// Define a static object; strings are easily readable, which is a plus
static NSString *const kOpeningBalanceChanged = @"openingBalance changed";

// Use it for the `context` parameter when adding KVO
[object addObserver:observer
         forKeyPath:@"openingBalance"
            options:options
            context:kOpeningBalanceChanged];

// Compare to the static addresses to dispatch to the right observer
- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{
	if (context == &kOpeningBalanceChanged) {
		// do something
	}
    [super observeValueForKeyPath:keyPath ...];
}

// When debugging, you can easily print the string
po *(id *)context

// You could even use a macro for defining these objects
#define DDDefineContext(_X_) static NSString *const _X_ = @#_X_