gistfile1.m

@interface Person : NSObject
@property NSString* firstName, *lastName;
-(NSString*)fullName;
@end

@implementation Person

+ (NSSet *)keyPathsForValuesAffectingFullName {
	return [NSSet setWithObjects:@"lastName", @"firstName", nil];
}

-(NSString*)fullName {
	return [NSString stringWithFormat:@"%@ %@", self.firstName, self.lastName];
}

@end



Person* person = [[Person alloc] init];
person.firstName = @"jon";
person.lastName = @"harris";
	
[person addObserver:self forKeyPath:@"fullName.length" options:0 context:kvoContext];
person.firstName = @"john"; // exception : 
// *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Cannot update for observer <ViewController 0x7fac03659d50> for the key path "fullName.length" from <Person 0x7fac035317c0>, most likely because the value for the key "fullName" has changed without an appropriate KVO notification being sent. Check the KVO-compliance of the Person class.'