Observer.m

@implementation Observer

-(id)init {
  // ...
  [_preferences addObserver:self forKeyPath:@"apiKey" options:NSKeyValueObservingOptionNew context:nil];
  [_preferences addObserver:self forKeyPath:@"apiSecret" options:NSKeyValueObservingOptionNew context:nil];  
}

// ...

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if ([keyPath isEqualToString:@"apiKey"]) {
        _apiKey = [change objectForKey:NSKeyValueChangeNewKey];
    } else if ([keyPath isEqualToString:@"apiSecret"]) {
        _apiSecret = [change objectForKey:NSKeyValueChangeNewKey];
    }
    MSLog(@"Secret: %@", _apiSecret);
    MSLog(@"Change: %@:%@", keyPath, changed);
}

@end

Preferences.h

@class EMGenericKeychainItem;

@interface Preferences : NSObject {
    @private
    EMGenericKeychainItem *_keychainItem;
}
+ (id)preferences;

@property (nonatomic, retain) NSString *apiKey;
@property (nonatomic, retain) NSString *apiSecret;

@end

Preferences.m

@implementation Preferences

@synthesize apiKey = _apiKey;
@synthesize apiSecret = _apiSecret;

// ...

-(NSString*)apiSecret {
    return _keychainItem.password;
}

-(void)apiSecret:(NSString *)newApiSecret {
    MSLog(@"Changed: %@", newApiSecret);
    if (newApiSecret && ![newApiSecret isKindOfClass:[NSNull class]]) {
        _keychainItem.password = newApiSecret;
    } else {
        _keychainItem.password = @"";
    }
    
    _apiSecret = newApiSecret;
}

// ...

@end