iOS 13 之前， 去掉黑线：设置UITarBar的 backgroundImage 和 shadowImage即可：

self.tabBar.backgroundImage = UIImage.init()
self.tabBar.shadowImage = UIImage.init()

iOS13之后不好使了，改用下面的方法：

if #available(iOS 13, *) {
    let appearance = tabBar.standardAppearance.copy()
    appearance.backgroundImage = UIImage.imageWithColor(color: .clear)
    appearance.shadowImage = UIImage.imageWithColor(color: .clear)
    tabBar.standardAppearance = appearance
} else {
  tabBar.backgroundImage = UIImage.imageWithColor(color: .clear)
  tabBar.shadowImage = UIImage.imageWithColor(color: .clear)
}

UIImage扩展

extension UIImage{
  
}