如何在swift中使用NSCoder编码枚举？

背景

我试图使用NSCoding协议编码一个字符串样式的枚举，但我遇到错误转换和返回从字符串。

解码和编码时，我得到以下错误：

字符串不能转换为Stage

额外参数ForKey：在调用

码

enum Stage : String
    {
        case displayAll    = "display All"
        case HideQuarter   = "Hide Quarter"
        case HideHalf      = "Hide Half"
        case HideTwoThirds = "Hide Two Thirds"
        case HideAll       = "Hide All"
    }

    class AppState : NSCoding,NSObject
    {
        var idx   = 0
        var stage = Stage.displayAll

        override init() {}

        required init(coder aDecoder: NSCoder) {
            self.idx   = aDecoder.decodeIntegerForKey( "idx"   )
            self.stage = aDecoder.decodeObjectForKey(  "stage" ) as String    // ERROR
        }

        func encodeWithCoder(aCoder: NSCoder) {
            aCoder.encodeInteger( self.idx,forKey:"idx"   )
            aCoder.encodeObject(  self.stage as String,forKey:"stage" )  // ERROR
        }

    // ...

    }

您需要将枚举转换为和从原始值。在Swift 1.2(Xcode 6.3)中，这将是这样：

class AppState : NSObject,NSCoding
{
    var idx   = 0
    var stage = Stage.displayAll

    override init() {}

    required init(coder aDecoder: NSCoder) {
        self.idx   = aDecoder.decodeIntegerForKey( "idx" )
        self.stage = Stage(rawValue: (aDecoder.decodeObjectForKey( "stage" ) as! String)) ?? .displayAll
    }

    func encodeWithCoder(aCoder: NSCoder) {
        aCoder.encodeInteger( self.idx,forKey:"idx" )
        aCoder.encodeObject(  self.stage.rawValue,forKey:"stage" )
    }

    // ...

}

Swift 1.1(Xcode 6.1)，使用as而不是！！

self.stage = Stage(rawValue: (aDecoder.decodeObjectForKey( "stage" ) as String)) ?? .displayAll

Swift 1.0(Xcode 6.0)使用toRaw()和fromraw()像这样：

self.stage = Stage.fromraw(aDecoder.decodeObjectForKey( "stage" ) as String) ?? .displayAll

    aCoder.encodeObject( self.stage.toRaw(),forKey:"stage" )