Swift-NSCopying

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自定义类想要支持拷贝，必须遵循 NSCopying 协议：

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class Student: NSCopying {
  var name: String
  // required 只能修饰类初始化方法，当子类初始化方法与父类的不同，子类必须实现初始化方法，并且要使用 required 而不是 overide
  required init(name: String) {
    self.name = name
  }
  func copy(with zone: NSZone? = nil) -> Any {
    let student = Student(name: self.name)
    return student
  }
}

let stu1 = Student(name: "JY")
let stu2 = stu1.copy() as! Student

// false
ObjectIdentifier(stu1) == ObjectIdentifier(stu2)
stu2.name = "YU"

// JY
stu1.name
// YU
stu2.name