I am unable to observe KVO notifications for a derived property (decks) of the EAUser class. The code below never generate a notification when EAUserDecksArray is modified (which modifies decks). I can however make this work if I call will/DidChangeValueForKey: when the dependent property is changed, instead of implementing keyPathsForValuesAffe…

EAUser.h

@interface EAUser : NSObject

- (NSArray *)decks;

@end

EAUser.m

NSString * const EAUserDecksArray = @"EAUserDecksArray";

- (void)somethingThatChangesDecks:(NSDictionary *)info {
  // at some time... decks gets changed like this:
  v = [info objectForKey:@"decks"];
  if ( v ) {
    NSCAssert([v isKindOfClass:[NSArray class]], @"Invalid class.");
    [self setValue:v forKey:EAUserDecksArray];
  }
};

- (NSArray *)decks
{
    id d = [self valueForKey:EAUserDecksArray];
    return d;
}

- (NSSet *)keyPathsForValuesAffectingDecks
{
    return [NSSet setWithObject:EAUserDecksArray];
}

somecontroller.m

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        [[EAUser currentUser] addObserver:self forKeyPath:@"decks" options:0 context:0];
    }
    return self;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    NSLog(@"%@ changed", keyPath);     ///   <---- this is never called  :(
}

What if it's not an array? I know KVO is picky with arrays. Try implementing these methods: https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/KeyValueCoding/Articles/AccessorConventions.html#//apple_ref/doc/uid/20002174-SW4