iOS 关于导航栏颜色渐变，导航条底部线条隐藏

@interface SecViewController ()
@property (nonatomic,weak) UIView * line;//此处是weak
@end

self.navigationController.navigationBar.subviews的第一个视图是类是_UIBarBackground，控制该视图的alpha可以改变导航栏的透明度
self.navigationController.navigationBar.subviews.firstObject.subviews 的第一个视图是 UIImageView 这个视图就是底部的那条线

- (void)viewDidLoad {
    [super viewDidLoad]; 
    [self.navigationController.navigationBar.subviews.firstObject setAlpha:0];
    [self.navigationController.navigationBar.subviews.firstObject.subviews enumerateObjectsUsingBlock:^(__kindof UIView * _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
        if ([obj isMemberOfClass:[UIImageView class]]) {
            obj.alpha = 0;//设置线条的透明度
            _line = obj;
        }
    }];

}

每次进入控制器导航栏透明

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    [self.navigationController.navigationBar.subviews.firstObject setAlpha:0];
}

离开时导航栏不透明

- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];
    [self.navigationController.navigationBar.subviews.firstObject setAlpha:1];
}
//  经测试上面方法结束之后还会调用一下scrolview的代理导致setAlpha并不为1，采用离开时候调用下面的方法
- (void)viewDidDisappear:(BOOL)animated {
    [super viewDidDisappear:animated];
    [self.navigationController.navigationBar.subviews.firstObject setAlpha:1];
}

监听scrollView的滚动改变导航栏透明度

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    
    CGFloat currentOffset = scrollView.contentOffset.y;
    CGFloat alphaHeight = (currentOffset+64)/200.0;
    CGFloat alpha =  alphaHeight<1?alphaHeight:1;
    [self.navigationController.navigationBar.subviews.firstObject setAlpha:alpha];
    if (alpha > 0.9) {
        if (_line.alpha >= 1)  return;
        //  为了看的好看点，单独控制线条的透明度，避免看起来滚动时候线条还是看的比较明显
        _line.alpha = alpha;
    }else{
        if (_line.alpha == 0)  return;
        _line.alpha = 0;
    }    
}