分类绑定属性

@property (nonatomic, strong) UIView *defaultView;
const char *DefaultView;
- (void)setDefaultView:(UIView *)defaultView {
    objc_setAssociatedObject(self, &DefaultView, defaultView, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
- (UIView *)defaultView {
    return objc_getAssociatedObject(self, &DefaultView);
}