TabBar上增加红点

增加红点的两种思路：
（1）自定义UITabBar，自定义UITabBarItem（繁琐复杂）
（2）直接在UITabBar上增加红点（简单好用）

下面通过写一个UITabBar的分类，来实现第二种思路：

//MARK: - UITabBar
extension UITabBar {
    /// 展示红点
    ///
    /// - Parameters:
    ///   - index: 第几个item
    ///   - count: 未读消息数
    func showBadgeOnItem(index:Int, count:Int) {
        removeBadgeOnItem(index: index)
        guard count > 0 else {
            return
        }
        let bview = UIView.init()
        bview.tag = 888+index
        bview.layer.cornerRadius = 9
        bview.clipsToBounds = true
        bview.backgroundColor = UIColor.hexColor(0xE7473C)//红点颜色
        let tabFrame = self.frame
        
        let percentX = (Float(index)+0.52)/3.0//其中3是tabBar中item的总个数
        let x = CGFloat(ceilf(percentX*Float(tabFrame.width)))
        let y = CGFloat(ceilf(0.05*Float(tabFrame.height)))
        bview.frame = CGRect(x: x, y: y, width: 18, height: 18)
        
        let cLabel = UILabel.init()
        cLabel.text = (count > 99) ? "99" : "\(count)"
        cLabel.frame = CGRect(x: 0, y: 0, width: 18, height: 18)
        cLabel.font = UIFont.systemFont(ofSize: 11)
        cLabel.textColor = UIColor.white
        cLabel.textAlignment = .center
        bview.addSubview(cLabel)
        
        addSubview(bview)
        bringSubview(toFront: bview)
    }
    //隐藏红点
    func hideBadgeOnItem(index:Int) {
        removeBadgeOnItem(index: index)
    }
    //移除控件
    func removeBadgeOnItem(index:Int) {
        for subView:UIView in subviews {
            if subView.tag == 888+index {
                subView.removeFromSuperview()
            }
        }
    }
}