swift计算字符串长度

方法如下：

// 计算字符串长度
    func sizeWithText(text: String, font: UIFont, size: CGSize) -> CGRect {
        let attributes = [NSAttributedString.Key.font: font]
        let option = NSStringDrawingOptions.usesLineFragmentOrigin
        let rect:CGRect = text.boundingRect(with: size, options: option, attributes: attributes, context: nil)
        return rect;
    }

如何调用：比如计算一个label的text长度

let rect = sizeWithText(text: label?.text ?? "", font: UIFont.systemFont(ofSize: 14), size: CGSize(width: CGFloat.greatestFiniteMagnitude, height: CGFloat.greatestFiniteMagnitude))

那么，rect.width就是我们要的label的字体长度