Work With KVO In Ios/Iphone

==> here used Kvo Key value obbserver:
- (IBAction)incNumber:(id)sender

{
    self.number++;
    NSLog(@"%d", self.number);

}
- (void)addObserver:(NSObject *)observer forKeyPath:(NSString *)keyPath options:
  (NSKeyValueObservingOptions)options context:(void *)context;