App中控件全局显示外观的设置 Appearance Proxies

You can use an appearance proxy to set particular appearance properties for all instances of a view in your application. For example, if you want all sliders in your app to have a particular

minimum track tint color, you can specify this with a single message to the slider’s appearance proxy.

There are two ways to customize appearance for objects: for all instances and for instances contained within an instance of a container class.

• To customize the appearance of all instances of a class, use appearance to get the appearance proxy for the class. 
• [[UISlider appearance] setMinimumTrackTintColor:[UIColor greenColor]];
• To customize the appearances for instances of a class when contained within an instance of a container class, or instances in a hierarchy, you use appearanceWhenContainedIn: to get
• the appearance proxy for the class. 
• [UISlider appearanceWhenContainedIn:[UIView class], nil]  setMinimumTrackTintColor:[UIColor greenColor]];

以上文字来自：苹果的官方文档中的 About View，如下图所示：

上面的大致意思是：有两种方法来设定某个对象的显示效果:

一个是设置该对象所有实例，一个是设置包含在某个容器类中所有该对象实例的显示效果；

接着就是两种实现方式：

一、 [[UISlider appearance] setMinimumTrackTintColor:[UIColor greenColor]];

这条语句可以将App中用到的所有UISlider左边颜色为绿色

二、 [UISlider appearanceWhenContainedIn:[ UIScrollView class], nil]  setMinimumTrackTintColor:[UIColor greenColor]];

这条语句可以将App中用到的所有父视图为 UIScrollView的UISlider左边颜色为绿色

如下图是“二”的效果：