如何将通用自定义对象保存到 UserDefaults？

这是我的通用类：

open class SMState<T: Hashable>: NSObject, NSCoding {
    open var value: T

    open var didEnter: ( (_ state: SMState<T>) -> Void)?
    open var didExit:  ( (_ state: SMState<T>) -> Void)?

    public init(_ value: T) {
        self.value = value
    }

    convenience required public init(coder decoder: NSCoder) {
        let value = decoder.decodeObject(forKey: "value") as! T

        self.init(value)
    }

    public func encode(with aCoder: NSCoder) {
        aCoder.encode(value, forKey: "value")
    }
}

然后我想这样做：

    let stateEncodeData = NSKeyedArchiver.archivedData(withRootObject: currentState)
    UserDefaults.standard.set(stateEncodeData, forKey: "state")

就我而言currentState属于类型SMState<SomeEnum>.

但当我打电话时NSKeyedArchiver.archivedData，Xcode (9 beta 5) 显示一条紫色消息：

Attempting to archive generic Swift class 'StepUp.SMState<StepUp.RoutineViewController.RoutineState>' with mangled runtime name '_TtGC6StepUp7SMStateOCS_21RoutineViewController12RoutineState_'. Runtime names for generic classes are unstable and may change in the future, leading to non-decodable data.

我不太确定它想说什么。无法保存通用对象吗？

还有其他方法可以保存通用自定义对象吗？

edit:

即使我使用AnyHashable而不是泛型，我在调用时在运行时遇到相同的错误NSKeyedArchiver.archivedData:

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: : unrecognized selector sent to instance

如果你想让泛型类采用NSCoding和泛型类型T然后将被编码和解码T 必须是符合属性列表的类型之一.

符合属性列表的类型有NSString, NSNumber, NSDate and NSData

一个可能的解决方案是创建一个协议PropertyListable并将属性列表兼容类型的所有 Swift 等效项扩展到该协议

协议要求是

• An associated type.
• 计算属性propertyListRepresentation将值转换为符合属性列表的类型。
• 初始化器init(propertyList做相反的事情。

public protocol PropertyListable {
    associatedtype PropertyListType
    var propertyListRepresentation : PropertyListType { get }
    init(propertyList : PropertyListType)
}

以下是示例性实现String and Int.

extension String : PropertyListable {
    public typealias PropertyListType = String
    public var propertyListRepresentation : PropertyListType { return self }
    public init(propertyList: PropertyListType) { self.init(stringLiteral: propertyList) }
}

extension Int : PropertyListable {
    public typealias PropertyListType = Int
    public var propertyListRepresentation : PropertyListType { return self }
    public init(propertyList: PropertyListType) { self.init(propertyList) }
}

让我们声明一个示例枚举并采用PropertyListable

enum Foo : Int, PropertyListable {
    public typealias PropertyListType = Int

    case north, east, south, west

    public var propertyListRepresentation : PropertyListType { return self.rawValue }
    public init(propertyList: PropertyListType) {
        self.init(rawValue:  propertyList)!
    }
}

最后将您的泛型类替换为

open class SMState<T: PropertyListable>: NSObject, NSCoding {
    open var value: T

    open var didEnter: ( (_ state: SMState<T>) -> Void)?
    open var didExit:  ( (_ state: SMState<T>) -> Void)?

    public init(_ value: T) {
        self.value = value
    }

    convenience required public init(coder decoder: NSCoder) {
        let value = decoder.decodeObject(forKey: "value") as! T.PropertyListType
        self.init(T(propertyList: value))
    }

    public func encode(with aCoder: NSCoder) {
        aCoder.encode(value.propertyListRepresentation, forKey: "value")
    }
}

通过此实现，您可以创建一个实例并将其存档

let currentState = SMState<Foo>(Foo.north)
let stateEncodeData = NSKeyedArchiver.archivedData(withRootObject: currentState)

并再次取消存档

let restoredState = NSKeyedUnarchiver.unarchiveObject(with: stateEncodeData) as! SMState<Foo>
print(restoredState.value)

整个解决方案似乎很麻烦，但你必须满足以下限制：NSCoding需要符合属性列表的类型。如果您不需要像这样的自定义类型enum实施更加容易（而且更短）。