UIBarAppearance是ios13之后苹果新出来的一个类，可以统一配置navigation bars、tab bars、 toolbars等bars的外观。

UIBarAppearance的子类主要有:

• UITabBarAppearance
• UINavigationBarAppearance

一、UITabBarAppearance

UITabBarAppearance
An object for customizing the appearance of a tab bar

Declaration

@interface UITabBarAppearance : UIBarAppearance

Overview

After creating a UITabBarAppearance object, use the methods and properties of this class to specify the appearance of items in the tab bar. Use the inherited properties from UIBarAppearance to configure the background and shadow attributes of the tab bar itself.

• UITabBarAppearance对象是用来自定义tabar的外观
• 可以用继承来自UIBarAppearance的属性去设置tabar的背景、阴影属性。

设置UITabar的背景：

    UITabBarAppearance *appearance = [self.tabBar.standardAppearance copy];
    appearance.backgroundColor = [UIColor greenColor ];
 self.tabBar.standardAppearance =appearance;

以上就是ios13设置UITabar背景的代码，然而在iOS15后，以上的代码你将会发现没有效果。

As of iOS 15, UINavigationBar, UIToolbar, and UITabBar will use their scrollEdgeAppearance when your view controller's associated scroll view is at the appropriate edge (or always if you don't have a UIScrollView in your hierarchy, more on that below) -apple developer forums

从 iOS 15 开始，UINavigationBar、UIToolbar 和 UITabBar 将在你的VC关联滚动视图位于适当的边缘时使用 scrollEdgeAppearance（或者如果您的试图层级结构中没有 UIScrollView)。

因此ios15之后想要设置tabar背景，就必须这样。

   self.tabBar.scrollEdgeAppearance =appearance;

1.2 、scrollEdgeAppearance

@property (nonatomic, readwrite, copy, nullable) UITabBarAppearance *scrollEdgeAppearance API_AVAILA"LE(ios(15.0));

Discussion
When a tab bar controller contains a tab bar and a scroll view, part of the scroll view’s content appears underneath the tab bar. If the edge of the scrolled content reaches that bar, UIKit applies the appearance settings in this property.

If the value of this property is nil, UIKit uses the value of the tab bar’s standardAppearance property, modified to have a transparent background. If no tab bar controller manages your tab bar, UIKit ignores this property and uses the tab bar’s standard appearance.

You can customize the appearance for specific tab bar items with the scrollEdgeAppearance property of UITabBarItem.

• 1、scrollEdgeAppearance： 当可滚动视图的内容边缘与tabar对齐时,所呈现的外观。而standardAppearance现在就是正常情况的外观。
• 2、当tabar的scrollEdgeAppearance属性值是nil时，UIKit 将会使用standardAppearance来配置tabar的外观，并将会修改tar的背景为透明背景。
• 3、你可以使用UITabBarItem的standardAppearance去自定义某个特定tabar的外观

   HLTAreaController *arena = [[HLTAreaController alloc] init];
  UITabBarAppearance *appearance1 = [self.tabBar.standardAppearance copy];
    appearance1.backgroundColor = [UIColor greenColor ];
    arena.tabBarItem.scrollEdgeAppearance =appearance1;
    arena.tabBarItem.standardAppearance = appearance1;

    HLTDiscoverContoller *discover = [[HLTDiscoverContoller alloc] init];
    UITabBarAppearance *appearance = [self.tabBar.standardAppearance copy];
    appearance.backgroundColor = [UIColor yellowColor ];
    discover.tabBarItem.scrollEdgeAppearance =appearance;
    discover.tabBarItem.standardAppearance = appearance;

1.3、UITabBarItemAppearance

用来自定义tab bar items的外观，比如你可能会希望在tabar选中时状态、非选中状态呈现不同的文字颜色等。

     UITabBarItemAppearance *itemAppearance = [[UITabBarItemAppearance alloc] init];
    itemAppearance.normal.titleTextAttributes = @{NSForegroundColorAttributeName : normalTitleColor};
     itemAppearance.selected.titleTextAttributes = @{NSForegroundColorAttributeName : selectedTitleColor};
     UITabBarAppearance *appearance = [[UITabBarAppearance alloc] init];
  appearance.stackedLayoutAppearance = itemAppearance;
     appearance.compactInlineLayoutAppearance = itemAppearance;
     appearance.inlineLayoutAppearance = itemAppearance;
     self.tabBar.standardAppearance = appearance;
     if (@available(iOS 15.0, *)) {
     self.tabBar.scrollEdgeAppearance = appearance;
     } else { }

• stackedLayoutAppearance: tabarItem文字、图片垂直排布布局时的外观。一般iPhone竖屏时都是这样的。
• inlineLayoutAppearance： 文字、图片并列排布时的外观。例如iPad的tabar都是并列排布的。
• compactInlineLayoutAppearance： 文字、图片并列排布显示，Size Class为compact环境下的外观。比如 iphone8 tabar在横屏时，文字、图片是并列排布的，size class 为compact width ,compact height。
• stackedLayoutAppearance、inlineLayoutAppearance、compactInlineLayoutAppearance没有设置时，系统是会提供默认配置的。

在平常的iphone开发中,一般tabar都是竖屏的，不希望tabar旋转，那么只设置stackedLayoutAppearance就好了

  appearance.stackedLayoutAppearance = itemAppearance;

另外：测试发现ios13，ios14如果设置tabaritem字体大小大于系统默认的字体大小，文本就会显示不全或者tabaritem布局混乱的bug。目前不清楚是系统bug还是什么原因。ios15设置tabaritem字体大小,就没有这样的bug。

二、UINavigationBarAppearance

Declaration
@interface UINavigationBarAppearance : UIBarAppearance

Overview
After creating a UINavigationBarAppearance object, use the methods and properties of this class to specify the appearance you want for items in the navigation bar. Use the inherited properties from UIBarAppearance to configure the background and shadow attributes of the navigation bar itself.

   NSMutableDictionary *dictM = [NSMutableDictionary dictionary];
  
    dictM[NSFontAttributeName] = [UIFont boldSystemFontOfSize:30];
  
    dictM[NSForegroundColorAttributeName] = [UIColor redColor];

    if (@available(iOS 13.0, *)) {
        UINavigationBarAppearance *appperance = [[UINavigationBarAppearance alloc]init];
         appperance.backgroundImage = [UIImage imageNamed:@"01"];
         appperance.titleTextAttributes = dictM;
        self.navigationBar.standardAppearance = appperance;
        if (@available(iOS 15.0, *)) {
            self.navigationBar.scrollEdgeAppearance = appperance;
        }
    }