格式如下：
// 编码的时候调用这个方法
    func encode(with aCoder: NSCoder) {
        aCoder.encode(sno, forKey: "sno")
        aCoder.encode(name, forKey: "name")
        aCoder.encode(score, forKey: "score")
    }
    
    // 解码的时候调用这个方法
    required init?(coder aDecoder: NSCoder) {
        sno = aDecoder.decodeObject(forKey: "sno") as! String
        name = aDecoder.decodeObject(forKey: "name") as! String
        score = aDecoder.decodeInteger(forKey: "score")
    }

//新创一个demo类来测试这个归档操作
// 创建对象
        let student = Student(sno: "1101", name: "maizixueyuan", score: 99)
        
 // 构造路径
        let path = "\(documentsPath)/student.archive"
        
 // 归档对象
        NSKeyedArchiver.archiveRootObject(student, toFile: path)
     
// 解档操作
        let object = NSKeyedUnarchiver.unarchiveObject(withFile: path) as! Student
        print("\(object.sno), \(object.name), \(object.score)")
        
// 检查文件
        print("\(NSHomeDirectory())")