能否用UINavigationBar Appearance为不同控制器配置不同导航栏背景色？

appearanceWhenContainedInInstancesOfClasses失效的核心原因是：导航栏属于上层的NavigationController，和ControllerA、ControllerB没有直接的容器关联，没法通过这个方法绑定控制器与导航栏的外观规则。不过可以通过以下两种基于Appearance框架的思路实现需求：

方法一：给NavigationController加标识，结合Appearance条件匹配 #

1. 给UINavigationController添加分类，新增一个自定义布尔属性用来标记当前导航栏需要的配色状态：

// UINavigationController+NavBarStyle.h
#import <UIKit/UIKit.h>

@interface UINavigationController (NavBarStyle)
@property (nonatomic, assign) BOOL needGreenNavBar;
@end

// UINavigationController+NavBarStyle.m
#import "UINavigationController+NavBarStyle.h"
#import <objc/runtime.h>

@implementation UINavigationController (NavBarStyle)
static char kNeedGreenNavBarKey;

- (BOOL)needGreenNavBar {
    return [objc_getAssociatedObject(self, &kNeedGreenNavBarKey) boolValue];
}

- (void)setNeedGreenNavBar:(BOOL)needGreenNavBar {
    objc_setAssociatedObject(self, &kNeedGreenNavBarKey, @(needGreenNavBar), OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
@end

2. 在导航控制器的代理方法里，根据即将显示的控制器更新标识：

- (void)navigationController:(UINavigationController *)navigationController willShowViewController:(UIViewController *)viewController animated:(BOOL)animated {
    navigationController.needGreenNavBar = [viewController isKindOfClass:[ControllerB class]];
    // 触发导航栏外观刷新
    [navigationController.navigationBar setNeedsLayout];
}

3. 利用iOS 13+支持的when:方法，根据导航控制器的标识设置对应外观：

// 在AppDelegate/SceneDelegate的初始化代码中配置
[[UINavigationBar appearanceWhenContainedInInstancesOfClasses:@[[UINavigationController class]]] 
 whenCondition:^BOOL(UINavigationBar *bar) {
    UINavigationController *nav = (UINavigationController *)bar.viewController;
    return nav.needGreenNavBar;
} setBackgroundColor:[UIColor greenColor]];

[[UINavigationBar appearanceWhenContainedInInstancesOfClasses:@[[UINavigationController class]]] 
 whenCondition:^BOOL(UINavigationBar *bar) {
    UINavigationController *nav = (UINavigationController *)bar.viewController;
    return !nav.needGreenNavBar;
} setBackgroundColor:[UIColor redColor]];

方法二：用UINavigationBarAppearance实例动态切换（iOS 13+） #

如果不需要兼容iOS 13以下版本，可直接在控制器生命周期中切换导航栏的外观实例，这也是基于Appearance框架的官方用法：

1. 提前定义两种配色的外观实例：

// 可全局定义或在控制器内部声明
UINavigationBarAppearance *redNavBarAppearance = [[UINavigationBarAppearance alloc] init];
redNavBarAppearance.backgroundColor = [UIColor redColor];

UINavigationBarAppearance *greenNavBarAppearance = [[UINavigationBarAppearance alloc] init];
greenNavBarAppearance.backgroundColor = [UIColor greenColor];

2. 在ControllerA的viewWillAppear:中设置红色外观：

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    self.navigationController.navigationBar.standardAppearance = redNavBarAppearance;
    self.navigationController.navigationBar.scrollEdgeAppearance = redNavBarAppearance;
}

3. 在ControllerB的viewWillAppear:中设置绿色外观：

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    self.navigationController.navigationBar.standardAppearance = greenNavBarAppearance;
    self.navigationController.navigationBar.scrollEdgeAppearance = greenNavBarAppearance;
}

内容的提问来源于stack exchange，提问作者ximmyxiao