这是我的Data例行十六进制字符串：

extension String {

    /// Create `Data` from hexadecimal string representation
    ///
    /// This creates a `Data` object from hex string. Note, if the string has any spaces or non-hex characters (e.g. starts with '<' and with a '>'), those are ignored and only hex characters are processed.
    ///
    /// - returns: Data represented by this hexadecimal string.

    var hexadecimal: Data? {
        var data = Data(capacity: characters.count / 2)

        let regex = try! NSRegularExpression(pattern: "[0-9a-f]{1,2}", options: .caseInsensitive)
        regex.enumerateMatches(in: self, range: NSRange(startIndex..., in: self)) { match, _, _ in
            let byteString = (self as NSString).substring(with: match!.range)
            let num = UInt8(byteString, radix: 16)!
            data.append(num)
        }

        guard data.count > 0 else { return nil }

        return data
    }

}

为了完整起见，这是我Data的十六进制字符串例程：

extension Data {

    /// Hexadecimal string representation of `Data` object.

    var hexadecimal: String {
        return map { String(format: "%02x", $0) }
            .joined()
    }
}

请注意，如上所示，我通常只在十六进制表示形式和NSData实例之间进行转换（因为如果可以将信息表示为字符串，则可能一开始就不会创建十六进制表示形式）。但是您最初的问题是要在十六进制表示形式和String对象之间进行转换，因此可能看起来像这样：

extension String {

    /// Create `String` representation of `Data` created from hexadecimal string representation
    ///
    /// This takes a hexadecimal representation and creates a String object from that. Note, if the string has any spaces, those are removed. Also if the string started with a `<` or ended with a `>`, those are removed, too.
    ///
    /// For example,
    ///
    ///     String(hexadecimal: "<666f6f>")
    ///
    /// is
    ///
    ///     Optional("foo")
    ///
    /// - returns: `String` represented by this hexadecimal string.

    init?(hexadecimal string: String, encoding: String.Encoding = .utf8) {
        guard let data = string.hexadecimal() else {
            return nil
        }

        self.init(data: data, encoding: encoding)
    }

    /// Create hexadecimal string representation of `String` object.
    ///
    /// For example,
    ///
    ///     "foo".hexadecimalString()
    ///
    /// is
    ///
    ///     Optional("666f6f")
    ///
    /// - parameter encoding: The `String.Encoding` that indicates how the string should be converted to `Data` before performing the hexadecimal conversion.
    ///
    /// - returns: `String` representation of this String object.

    func hexadecimalString(encoding: String.Encoding = .utf8) -> String? {
        return data(using: encoding)?
            .hexadecimal
    }

}

然后可以像上面这样使用上面的代码：

let hexString = "68656c6c 6f2c2077 6f726c64"
print(String(hexadecimal: hexString))

要么，

let originalString = "hello, world"
print(originalString.hexadecimalString())

有关上述对早期Swift版本的排列，请参阅此问题的修订历史记录。