Swift 4:如何将圆点作为选择指示器添加到UITabbarItem

class DotTabBarItem: UITabBarItem {
    private var dotView: UIView?
    
    func showDot() {
        if dotView == nil {
            dotView = UIView(frame: CGRect(x: 0, y: 0, width: 10, height: 10))
            dotView?.backgroundColor = UIColor.red
            dotView?.layer.cornerRadius = 5
            dotView?.clipsToBounds = true
            dotView?.center = CGPoint(x: self.bounds.width / 2 + 10, y: 10)
            self.addSubview(dotView!)
        }
    }
    
    func hideDot() {
        dotView?.removeFromSuperview()
        dotView = nil
    }
}

class MyTabBarController: UITabBarController {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let tabBarItem1 = DotTabBarItem()
        tabBarItem1.title = "Item 1"
        tabBarItem1.image = UIImage(named: "item1")
        tabBarItem1.selectedImage = UIImage(named: "item1_selected")
        self.viewControllers?[0].tabBarItem = tabBarItem1
        
        let tabBarItem2 = DotTabBarItem()
        tabBarItem2.title = "Item 2"
        tabBarItem2.image = UIImage(named: "item2")
        tabBarItem2.selectedImage = UIImage(named: "item2_selected")
        self.viewControllers?[1].tabBarItem = tabBarItem2
    }
    
    func showDotOnTabBarItem(index: Int) {
        if let dotTabBarItem = self.viewControllers?[index].tabBarItem as? DotTabBarItem {
            dotTabBarItem.showDot()
        }
    }
    
    func hideDotOnTabBarItem(index: Int) {
        if let dotTabBarItem = self.viewControllers?[index].tabBarItem as? DotTabBarItem {
            dotTabBarItem.hideDot()
        }
    }
}

let myTabBarController = MyTabBarController()
myTabBarController.showDotOnTabBarItem(index: 0) // 在第一个TabBarItem上显示圆点
myTabBarController.hideDotOnTabBarItem(index: 1) // 隐藏第二个TabBarItem上的圆点