Swift4的 Base64编码和解码

extension String {
    // base64编码
    func toBase64() -> String? {
        if let data = self.data(using: .utf8) {
            return data.base64EncodedString()
        }
        return nil
    }

    // base64解码
    func fromBase64() -> String? {
        if let data = Data(base64Encoded: self) {
            return String(data: data, encoding: .utf8)
        }
        return nil
    }
}