首先在Vc的类别中

@interface UIViewController (Cloudox)
@property (copy, nonatomic) NSString *navBarBgAlpha;
@end

#import "UIViewController+Cloudox.h"
#import <objc/runtime.h>
#import "UINavigationController+Cloudox.h"
@implementation UIViewController (Cloudox)
//定义常量 必须是C语言字符串
static char *CloudoxKey = "CloudoxKey";
- (void)setNavBarBgAlpha:(NSString *)navBarBgAlpha {
    /*
     OBJC_ASSOCIATION_ASSIGN;            //assign策略
     OBJC_ASSOCIATION_COPY_NONATOMIC;    //copy策略
     OBJC_ASSOCIATION_RETAIN_NONATOMIC;  // retain策略
     
     OBJC_ASSOCIATION_RETAIN;
     OBJC_ASSOCIATION_COPY;
     */
    /*
     * id object 给哪个对象的属性赋值
     const void *key 属性对应的key
     id value  设置属性值为value
     objc_AssociationPolicy policy  使用的策略，是一个枚举值，和copy，retain，assign是一样的，手机开发一般都选择NONATOMIC
     objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy);
     */
    objc_setAssociatedObject(self, CloudoxKey, navBarBgAlpha, OBJC_ASSOCIATION_COPY_NONATOMIC);
    
    // 设置导航栏透明度（利用Category自己添加的方法）
    [self.navigationController setNeedsNavigationBackground:[navBarBgAlpha floatValue]];
}
- (NSString *)navBarBgAlpha {
    return objc_getAssociatedObject(self, CloudoxKey) ? : @"1.0";
}
@end

#import <UIKit/UIKit.h>
@interface UINavigationController (Cloudox) <UINavigationBarDelegate, UINavigationControllerDelegate>
@property (copy, nonatomic) NSString *cloudox;
- (void)setNeedsNavigationBackground:(CGFloat)alpha;
@end

//
//  UINavigationController+Cloudox.m
//  SmoothNavDemo
//
//  Created by Cloudox on 2017/3/19.
//  Copyright © 2017年 Cloudox. All rights reserved.
//

#import "UINavigationController+Cloudox.h"
#import <objc/runtime.h>
#import "UIViewController+Cloudox.h"

@implementation UINavigationController (Cloudox)

// 设置导航栏背景透明度
- (void)setNeedsNavigationBackground:(CGFloat)alpha {
    // 导航栏背景透明度设置
    UIView *barBackgroundView = [[self.navigationBar subviews] objectAtIndex:0];// _UIBarBackground
    UIImageView *backgroundImageView = [[barBackgroundView subviews] objectAtIndex:0];// UIImageView
    if (self.navigationBar.isTranslucent) {
        if (backgroundImageView != nil && backgroundImageView.image != nil) {
            barBackgroundView.alpha = alpha;
        } else {
            UIView *backgroundEffectView = [[barBackgroundView subviews] objectAtIndex:1];// UIVisualEffectView
            if (backgroundEffectView != nil) {
                backgroundEffectView.alpha = alpha;
            }
        }
    } else {
        barBackgroundView.alpha = alpha;
    }
    
    // 对导航栏下面那条线做处理
    self.navigationBar.clipsToBounds = alpha == 0.0;
}

+ (void)initialize {
    if (self == [UINavigationController self]) {
        // 交换方法
        SEL originalSelector = NSSelectorFromString(@"_updateInteractiveTransition:");
        SEL swizzledSelector = NSSelectorFromString(@"et__updateInteractiveTransition:");
        Method originalMethod = class_getInstanceMethod([self class], originalSelector);
        Method swizzledMethod = class_getInstanceMethod([self class], swizzledSelector);
        method_exchangeImplementations(originalMethod, swizzledMethod);
    }
}

// 交换的方法，监控滑动手势
- (void)et__updateInteractiveTransition:(CGFloat)percentComplete {
    [self et__updateInteractiveTransition:(percentComplete)];
    UIViewController *topVC = self.topViewController;
    if (topVC != nil) {
        id<UIViewControllerTransitionCoordinator> coor = topVC.transitionCoordinator;
        if (coor != nil) {
            // 随着滑动的过程设置导航栏透明度渐变
            CGFloat fromAlpha = [[coor viewControllerForKey:UITransitionContextFromViewControllerKey].navBarBgAlpha floatValue];
            CGFloat toAlpha = [[coor viewControllerForKey:UITransitionContextToViewControllerKey].navBarBgAlpha floatValue];
            CGFloat nowAlpha = fromAlpha + (toAlpha - fromAlpha) * percentComplete;
            NSLog(@"from:%f, to:%f, now:%f",fromAlpha, toAlpha, nowAlpha);
            [self setNeedsNavigationBackground:nowAlpha];
        }
    }
}

#pragma mark - UINavigationController Delegate
- (void)navigationController:(UINavigationController *)navigationController willShowViewController:(UIViewController *)viewController animated:(BOOL)animated {
    UIViewController *topVC = self.topViewController;
    if (topVC != nil) {
        id<UIViewControllerTransitionCoordinator> coor = topVC.transitionCoordinator;
        if (coor != nil) {
//            [coor notifyWhenInteractionChangesUsingBlock:^(id<UIViewControllerTransitionCoordinatorContext> context){
//                [self dealInteractionChanges:context];
//            }];
            
            // notifyWhenInteractionChangesUsingBlock是10.0以后的api，换成notifyWhenInteractionEndsUsingBlock
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdeprecated-declarations"
            [coor notifyWhenInteractionEndsUsingBlock:^(id<UIViewControllerTransitionCoordinatorContext> context){
                [self dealInteractionChanges:context];
#pragma clang diagnostic pop
            }];
        }
    }
}

- (void)dealInteractionChanges:(id<UIViewControllerTransitionCoordinatorContext>)context {
    if ([context isCancelled]) {// 自动取消了返回手势
        NSTimeInterval cancelDuration = [context transitionDuration] * (double)[context percentComplete];
        [UIView animateWithDuration:cancelDuration animations:^{
            CGFloat nowAlpha = [[context viewControllerForKey:UITransitionContextFromViewControllerKey].navBarBgAlpha floatValue];
            NSLog(@"自动取消返回到alpha：%f", nowAlpha);
            [self setNeedsNavigationBackground:nowAlpha];
        }];
    } else {// 自动完成了返回手势
        NSTimeInterval finishDuration = [context transitionDuration] * (double)(1 - [context percentComplete]);
        [UIView animateWithDuration:finishDuration animations:^{
            CGFloat nowAlpha = [[context viewControllerForKey:
                                 UITransitionContextToViewControllerKey].navBarBgAlpha floatValue];
            NSLog(@"自动完成返回到alpha：%f", nowAlpha);
            [self setNeedsNavigationBackground:nowAlpha];
        }];
    }
}


#pragma mark - UINavigationBar Delegate
- (void)navigationBar:(UINavigationBar *)navigationBar didPopItem:(UINavigationItem *)item {
    if (self.viewControllers.count >= navigationBar.items.count) {// 点击返回按钮
        UIViewController *popToVC = self.viewControllers[self.viewControllers.count - 1];
        [self setNeedsNavigationBackground:[popToVC.navBarBgAlpha floatValue]];
//        [self popViewControllerAnimated:YES];
    }
}

- (void)navigationBar:(UINavigationBar *)navigationBar didPushItem:(UINavigationItem *)item {
    // push到一个新界面
    [self setNeedsNavigationBackground:[self.topViewController.navBarBgAlpha floatValue]];
}


//定义常量 必须是C语言字符串
static char *CloudoxKey = "CloudoxKey";

-(void)setCloudox:(NSString *)cloudox{
    /*
     OBJC_ASSOCIATION_ASSIGN;            //assign策略
     OBJC_ASSOCIATION_COPY_NONATOMIC;    //copy策略
     OBJC_ASSOCIATION_RETAIN_NONATOMIC;  // retain策略
     
     OBJC_ASSOCIATION_RETAIN;
     OBJC_ASSOCIATION_COPY;
     */
    /*
     * id object 给哪个对象的属性赋值
     const void *key 属性对应的key
     id value  设置属性值为value
     objc_AssociationPolicy policy  使用的策略，是一个枚举值，和copy，retain，assign是一样的，手机开发一般都选择NONATOMIC
     objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy);
     */
    
    objc_setAssociatedObject(self, CloudoxKey, cloudox, OBJC_ASSOCIATION_COPY_NONATOMIC);
}

-(NSString *)cloudox{
    return objc_getAssociatedObject(self, CloudoxKey);
}

@end

D71E50D3-C04B-4D4E-8520-F682C6CEEA4A.png

这个具体的代码。目的是实现导航栏的透明度功能。
但是在xcode 9上去打印：
[图片上传中...(983D2B96-9079-4262-B5E1-E06A04237389.png-7ce040-1543221002264-0)]

6013DA2E-EC36-45F6-A58D-F756FD976359.png

983D2B96-9079-4262-B5E1-E06A04237389.png

834B62E7DD27B96657B9DF1F4A2AC288.png

5ACC9758-379A-4583-AB60-1797190FA9C2.png