在iOS 14上，adjustsFontSizeToFitWidth无法与NSMutableAttributedString一起正常工作

let attributedString = NSMutableAttributedString(string: "Hello World")
attributedString.addAttribute(.font, value: UIFont.systemFont(ofSize: 20), range: NSRange(location: 0, length: 5))
attributedString.addAttribute(.font, value: UIFont.systemFont(ofSize: 30), range: NSRange(location: 6, length: 5))

let label = UILabel(frame: CGRect(x: 0, y: 0, width: 200, height: 50))
label.attributedText = attributedString

let maxSize = CGSize(width: label.bounds.width, height: CGFloat.greatestFiniteMagnitude)
let boundingRect = attributedString.boundingRect(with: maxSize, options: [.usesLineFragmentOrigin, .usesFontLeading], context: nil)

if boundingRect.size.width > label.bounds.width {
    let scaleFactor = label.bounds.width / boundingRect.size.width
    let scaledFont = UIFont.systemFont(ofSize: 30 * scaleFactor)
    attributedString.enumerateAttribute(.font, in: NSRange(location: 0, length: attributedString.length), options: []) { (value, range, _) in
        if let currentFont = value as? UIFont {
            let scaledDescriptor = currentFont.fontDescriptor.withSize(currentFont.pointSize * scaleFactor)
            let scaledFont = UIFont(descriptor: scaledDescriptor, size: scaledDescriptor.pointSize)
            attributedString.addAttribute(.font, value: scaledFont, range: range)
        }
    }
}

label.attributedText = attributedString