KVO

- (void)viewDidLoad {
[super viewDidLoad];

self.view.backgroundColor =[UIColor whiteColor];
stock =[[stockForKVO alloc]init];
[stock setValue:@"searph" forKey:@"stockName"];
[stock setValue:@"10.0" forKey:@"price"];
[stock addObserver:self forKeyPath:@"price" options:NSKeyValueObservingOptionNew|NSKeyValueObservingOptionOld context:nil];

myLabel = [[UILabel alloc]initWithFrame:CGRectMake(100, 100, 100, 30 )];
myLabel.textColor = [UIColor redColor];
myLabel.text = [stock valueForKey:@"price"];
[self.view addSubview:myLabel];

UIButton * b = [UIButton buttonWithType:UIButtonTypeRoundedRect];
b.frame = CGRectMake(0, 220, 100, 30);
[b setBackgroundColor:[UIColor orangeColor]];
[b addTarget:self action:@selector(buttonAction) forControlEvents:UIControlEventTouchUpInside];
[self.view addSubview:b];
}

-(void)buttonAction
{
[stock setValue:@"40" forKey:@"price"];
}

-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context
{
if ([keyPath isEqualToString:@"price"]) {
myLabel.text =[stock valueForKey:@"price"];
}