苹果文档说明：
@H_404_15@
将自定义视图属性标记为动态。
To participate in the appearance proxy API,tag your appearance
property selectors in your header with UI_APPEARANCE_SELECTOR.
在Objective-C中，可以使用UI_APPEARANCE_SELECTOR注释属性，如下所示：
@property (nonatomic,strong) UIColor *foregroundColor UI_APPEARANCE_SELECTOR;
我怎么能在Swift中做同样的事情？
例如：
class YourCustomView: UIView {
dynamic var subviewColor: UIColor? {
get { return self.yourSubview.backgroundColor }
set { self.yourSubview.backgroundColor = newValue }
}
...
}
然后：
YourCustomView.appearance().subviewColor = UIColor.greenColor()
