// 隐藏NavigationBar下面的黑线
public func hideNavBottomHairline(hide: Bool) {
    let navigationBarImageView = hairlineImageViewInNavigationBar(view: self.navigationController!.navigationBar)
    navigationBarImageView?.isHidden = hide
}

private func hairlineImageViewInNavigationBar(view: UIView) -> UIImageView? {
    if view.isKind(of: UIImageView.self) && view.bounds.height <= 1.0 {
        return (view as! UIImageView)
    }
    
    let subviews = (view.subviews as [UIView])
    for subview: UIView in subviews {
        if let imageView: UIImageView = hairlineImageViewInNavigationBar(view: subview) {
            return imageView
        }
    }
    return nil
}

转自：https://blog.csdn.net/same_life/article/details/126950493