VerboseKVO.m

static void *kBindingContext = &kBindingContext;

@implementation MySlider
@synthesize requestModel;
@synthesize percent;

- (id)init {
  if((self = [super init])) {
    [requestModel addObserver:self forKeyPath:@"percentUploaded" options:0 context:kBindingContext];   
  }
  return self;
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
  if (context == kBindingContext) {
    self.percent = self.requestModel.percentUploaded;
  } else {
    [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
  }
}

- (void)dealloc {
  [requestModel removeObserver:self forKeyPath:@"percentUploaded"];
  [requestModel release];
  [super dealloc];
}
@end