@property(nullable, nonatomic,copy)            UIColor          *backgroundColor UI_APPEARANCE_SELECTOR; //

@protocol UIAppearanceContainer <NSObject> @end

@protocol UIAppearance <NSObject>
//返回实现了此外观协议的UI控件实例，用这个返回的对象设置的属性是全局性。
+ (instancetype)appearance;
//返回实现了此外观协议的UI控件实例，用这个返回的对象设置的属性只对ContainerClass内部包裹的对象有效。
+ (instancetype)appearanceWhenContainedIn:(nullable Class <UIAppearanceContainer>)ContainerClass, ... NS_REQUIRES_NIL_TERMINATION NS_DEPRECATED_IOS(5_0, 9_0, "Use +appearanceWhenContainedInInstancesOfClasses: instead") __TVOS_PROHIBITED;
//是iOS9之后的替代方法，作用和上面的一样。
+ (instancetype)appearanceWhenContainedInInstancesOfClasses:(NSArray<Class <UIAppearanceContainer>> *)containerTypes NS_AVAILABLE_IOS(9_0);
//是iOS8新增的方法，是对不同的布局方案（紧凑型，普通型）采用不同的外观样式
+ (instancetype)appearanceForTraitCollection:(UITraitCollection *)trait NS_AVAILABLE_IOS(8_0);
//与上面的方法相似，多了一个参数条件，只对ContainerClass内部包裹的对象有效。
+ (instancetype)appearanceForTraitCollection:(UITraitCollection *)trait whenContainedIn:(nullable Class <UIAppearanceContainer>)ContainerClass, ... NS_REQUIRES_NIL_TERMINATION NS_DEPRECATED_IOS(8_0, 9_0, "Use +appearanceForTraitCollection:whenContainedInInstancesOfClasses: instead") __TVOS_PROHIBITED;
//iOS9后的新方法，是对上面方法的替换。
+ (instancetype)appearanceForTraitCollection:(UITraitCollection *)trait whenContainedInInstancesOfClasses:(NSArray<Class <UIAppearanceContainer>> *)containerTypes  NS_AVAILABLE_IOS(9_0);
@end

在上面代码中对里面定义的5个方法进行了说明。

UIImage *bg = [UIImage pureImageWithColor:[UIColor brownColor]];
[[UINavigationBar appearance] setBackgroundImage:bg forBarMetrics:UIBarMetricsDefault];

//2.设置导航颜色

[[UINavigationBar appearance] setTintColor:[UIColor blueColor]];

NSShadow *shadow = [[NSShadow alloc] init];
shadow.shadowColor = [UIColor colorWithRed:0.0 green:0.0 blue:0.0 alpha:0.8];
shadow.shadowOffset = CGSizeMake(0, 1);
[[UINavigationBar appearance] setTitleTextAttributes: [NSDictionary dictionaryWithObjectsAndKeys: [UIColor colorWithRed:245.0/255.0 green:245.0/255.0 blue:245.0/255.0 alpha:1.0], NSForegroundColorAttributeName, shadow, NSShadowAttributeName, [UIFont fontWithName:@"HelveticaNeue-CondensedBlack" size:21.0], NSFontAttributeName, nil]];

//5.特定环境或某种场合下外观样式设置（水平紧凑型，UINavigationBar默认外观为粉色。）

[[UINavigationBar appearanceForTraitCollection:[UITraitCollection traitCollectionWithHorizontalSizeClass:UIUserInterfaceSizeClassCompact]] setBarTintColor:[UIColor purpleColor]];

另外对UIButton的全局默认设置如下：

//让一类控件同时表现出某种属性

[[UIButton appearanceWhenContainedInInstancesOfClasses:@[[UINavigationBar class]]] setTitle:@"比较特别" forState:UIControlStateNormal];
[[UIButton appearanceWhenContainedInInstancesOfClasses:@[[UINavigationBar class]]] setBackgroundColor:[UIColor redColor]];

UI控件的默认全局设置方式符合预期。

那么iOS开发中，所有的UI控件都可以通过上面两种方法进行全局样式默认设置和自定义设置了。