[self addObserver:self forKeyPath:@"age" options:NSKeyValueObservingOptionNew context:nil];

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context{
NSLog(@"keyPath->%@",keyPath);
sleep(3);

self.age = @"12312";
        NSLog(@"postEnd");

keyPath->age
//after 3 seconds
postEnd