iOS UILabel 不同字体大小，文字垂直居中对齐

代码如下：

class ViewController: UIViewController {

    override func loadView() {
        super.loadView()
        
        let str = NSMutableAttributedString(string: "5.08%~6.10%")
        str.addAttributes([NSAttributedStringKey.font: UIFont.systemFont(ofSize: 13)], range: NSRange(location: 4, length: 1))
        str.addAttributes([NSAttributedStringKey.font: UIFont.systemFont(ofSize: 13)], range: NSRange(location: 10, length: 1))
        str.addAttributes([NSAttributedStringKey.baselineOffset: 3], range: NSRange(location: 4, length: 1))
        str.addAttributes([NSAttributedStringKey.baselineOffset: 3], range: NSRange(location: 10, length: 1))
        
        let label = UILabel(frame: CGRect(x: 50, y: 100, width: 300, height: 60))
        label.backgroundColor = UIColor.gray
        label.font = UIFont.systemFont(ofSize: 24)
        label.attributedText = str
        self.view.addSubview(label)
    }    
}

效果图：