ios – 错误：在Xcode中使用未声明的类型’NSObject’

我让CheckItem类继承了NSObject和NSCoding

但Xcode警告第1行的编译时错误：

class CheckItem : NSObject,NSCoding {

the hint is : Use of undeclared type ‘NSObject'(and ‘NSCoding’)

全班如下：

class CheckItem : NSObject,NSCoding {
        var text: String
        var isDone :Bool
        var imageName :String
        init(text: String,isDone: Bool,imageName: String){
            self.text = text
            self.isDone = isDone
            self.imageName = imageName
        }
        init(text: String,isDone: Bool){
            self.text = text
            self.isDone = isDone
            self.imageName = "No Icon"
        }

}

你能指出我的错误吗？非常感谢你！