KVO

KVO

static void * const MyClassKVOContext = (void*)&MyClassKVOContext;

- (void)addContext {
  [object addObserver:self forKeyPath:NSStringFromSelector(@selector(property)) options:0 context:MyClassKVOContext];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
  if (context == MyClassKVOContext) {
      if ([keyPath isEqualToString:NSStringFromSelector(@selector(property))]) {

      }
  }
}

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{
    if ([keyPath isEqualToString:NSStringFromSelector(@selector(isFinished))]) {
        if ([object isFinished]) {
          @try {
              [object removeObserver:self forKeyPath:NSStringFromSelector(@selector(isFinished))];
          }
          @catch (NSException * __unused exception) {}
        }
    }
}