去掉tabBar黑线， iOS13

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

tabbar.backgroundImage = UIImage(color: UIColor.clear)
        tabbar.shadowImage = UIImage(color: UIColor.clear)
           

iOS13之后不好使了，改用下面的方法：

let appearance = tabbar.standardAppearance.copy()
        appearance.backgroundImage = UIImage(color: UIColor.clear)
        appearance.shadowImage = UIImage(color: UIColor.clear)
        tabbar.standardAppearance = appearance
           

所以兼容代码如下：

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