效果圖

方法一

[self.navigationBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
[self.navigationBar setShadowImage:[self imageWithColor:[UIColor redColor] size:CGSizeMake([UIScreen mainScreen].bounds.size.width, 0.5)]];

#pragma mark - public methods
- (UIImage *)imageWithColor:(UIColor *)color size:(CGSize)size {
    if (!color || size.width <= 0 || size.height <= 0) return nil;
    CGRect rect = CGRectMake(0.0f, 0.0f, size.width, size.height);
    UIGraphicsBeginImageContextWithOptions(rect.size, NO, 0);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetFillColorWithColor(context, color.CGColor);
    CGContextFillRect(context, rect);
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}

此方法是現(xiàn)在大部分博客中有介紹的，不過有缺陷。如果 self.navigationBar.translucent = YES;則navigationbar透明了挡篓。有時我們又不得不設(shè)self.navigationBar.translucent = YES;例如使用UISearchController做通訊錄時。其中的坑可以看我這篇博文介紹使用UISearchController時 navigationBar translucent 屬性導(dǎo)致的問題總結(jié)。

方法二

此方法是我推薦的一種庄蹋，詳情請看代碼及代碼注釋。(不過在iOS11+后不能使用迷雪，需要使用方法三）

1. 首先導(dǎo)入頭文件 #import <objc/runtime.h>

2. 代碼展示

    //先查看View層次結(jié)構(gòu)
    NSLog(@"Navigationbar recursiveDescription:\n%@",[self.navigationBar performSelector:@selector(recursiveDescription)]);
    
    //打印完后我們發(fā)現(xiàn)有個高度為0.5的UIImageView 類型 SuperView type為 _UIBarBackground的視圖
    //遍歷navigationBar 屬性
    unsigned int outCount = 0;
    Ivar *ivars = class_copyIvarList([self.navigationBar class], &outCount);
    for (NSInteger i = 0; i < outCount; ++i) {
        Ivar ivar = *(ivars + i);
        NSLog(@"navigationBar Propertys:\n name = %s  \n type = %s", ivar_getName(ivar),ivar_getTypeEncoding(ivar));
    }
    free(ivars);
    
    //遍歷結(jié)果可以發(fā)現(xiàn)navigationbar 中的type 為_UIBarBackground 名稱為 _barBackgroundView
    //遍歷_barBackgroundView 中的屬性
    unsigned int viewOutCount = 0;
    UIView *barBackgroundView = nil;
    /*iOS 10.0+為`_barBackgroundView`,小于iOS10.0這個屬性名稱為`_UIBarBackground`.*/
    if (sysytemVersion<10.0) {
        barBackgroundView = [self.navigationBar valueForKey:@"_backgroundView"];
    }else{
        barBackgroundView = [self.navigationBar valueForKey:@"_barBackgroundView"];
    }
    if (barBackgroundView) {
        Ivar *viewivars = class_copyIvarList([barBackgroundView class], &viewOutCount);
        for (NSInteger i = 0; i < viewOutCount; ++i) {
            Ivar ivar = *(viewivars + i);
            NSLog(@"_barBackgroundView Propertys:\n name = %s  \n type = %s", ivar_getName(ivar),ivar_getTypeEncoding(ivar));
        }
        free(viewivars);
    }
    
    
    //找到type為 UIImageView 的屬性有_shadowView,_backgroundImageView限书。因為底部線條可以設(shè)置shadowImage，所有我們猜測是_shadowView
    UIImageView *navigationbarLineView = [barBackgroundView valueForKey:@"_shadowView"];
    if (navigationbarLineView && [navigationbarLineView isKindOfClass:[UIImageView class]]) {
        UIView *lineView = [[UIView alloc]init];
        lineView.backgroundColor = [UIColor redColor];
        lineView.translatesAutoresizingMaskIntoConstraints = NO;
        [navigationbarLineView addSubview:lineView];
        
        //這里我們要用約束不然旋轉(zhuǎn)后有問題
        [navigationbarLineView addConstraint:[NSLayoutConstraint constraintWithItem:lineView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:navigationbarLineView attribute:NSLayoutAttributeTop multiplier:1.0 constant:0.0]];
        
        [navigationbarLineView addConstraint:[NSLayoutConstraint constraintWithItem:lineView attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:navigationbarLineView attribute:NSLayoutAttributeBottom multiplier:1.0 constant:0.0]];
        
        [navigationbarLineView addConstraint:[NSLayoutConstraint constraintWithItem:lineView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:navigationbarLineView attribute:NSLayoutAttributeLeft multiplier:1.0 constant:0.0]];
        
        [navigationbarLineView addConstraint:[NSLayoutConstraint constraintWithItem:lineView attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:navigationbarLineView attribute:NSLayoutAttributeRight multiplier:1.0 constant:0.0]];
    }

為了更好的理解,我加了分析過程章咧，實際應(yīng)用時倦西，可以去掉分析過程。

方法三

UIView *lineView = [[UIView alloc]init];
lineView.backgroundColor = [UIColor redColor];
lineView.translatesAutoresizingMaskIntoConstraints = NO;
[self.navigationBar addSubview:lineView];
UIView *superView = self.navigationBar;    
[superView addConstraint:[NSLayoutConstraint constraintWithItem:lineView attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:superView attribute:NSLayoutAttributeBottom multiplier:1.0 constant:0.0]];
[superView addConstraint:[NSLayoutConstraint constraintWithItem:lineView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:superView attribute:NSLayoutAttributeLeft multiplier:1.0 constant:0.0]];
[superView addConstraint:[NSLayoutConstraint constraintWithItem:lineView attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:superView attribute:NSLayoutAttributeRight multiplier:1.0 constant:0.0]];
[lineView addConstraint:[NSLayoutConstraint constraintWithItem:lineView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:1]];  

下載鏈接

GitHub

擴展

方法二中你可以寫個category添加和移除