ios OC动态添加属性

UIButton *btn = [UIButton buttonWithType:UIButtonTypeCustom];
[btn addTarget:self action:@selector(btnClick) forControlEvents:UIControlEventTouchUpInside];

objc_setAssociatedObject(btn, "btnType", "custom", OBJC_ASSOCIATION_RETAIN_NONATOMIC);

- (void)btnClick:(id)sender {
  NSString *type = objc_getAssociatedObject(sender, "btnType");
  NSlog(@"%@",type); //custom
}