目的

NSKeyedArchiver / NSKeyedUnarchiver でデータを保存する際のキーをenumで管理するため。
Protocolつけるだけで利用できるようにしたい。

使い方

// 保存先のパス指定
private let path = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0] + "/foo.cache"

// enumの定義
enum Key: String, EnumKeyedArchivable {
    case Bar = "Bar"
}

// Archive
var dictionary: [Key: AnyObject] = [:]
dictionary[.Bar] = "This is Bar"
NSKeyedArchiver.archive(dictionary, toFile: path)

// Unarchive
let unarchivedDictionary = NSKeyedUnarchiver.unarchive(Key.self, file: path)
print(unarchivedDictionary)  // Optional([Key.Bar: This is Bar])

実装

protocol EnumKeyedArchivable: RawRepresentable {
    func keyValue() -> String
    init?(stringValue: String)
}

extension EnumKeyedArchivable where Self: RawRepresentable, Self.RawValue == String {
    func keyValue() -> String {
        return self.rawValue
    }
    init?(stringValue: String) {
        self.init(rawValue: stringValue)
    }
}

extension NSKeyedArchiver {
    class func archive<T: EnumKeyedArchivable>(dictionary: [T: AnyObject], toFile file: String) {
        var archivableDictionary: [String: AnyObject] = [:]
        for (key, value) in dictionary {
            archivableDictionary[key.keyValue()] = value
        }
        self.archiveRootObject(archivableDictionary, toFile: file)
    }
}

extension NSKeyedUnarchiver {
    class func unarchive<T: EnumKeyedArchivable>(key: T.Type, file: String) -> [T: AnyObject]? {
        guard let archivedDictionary = self.unarchiveObjectWithFile(file) as? [String: AnyObject] else {
            return nil
        }
        var dictonary: [T: AnyObject] = [:]
        for (key, value) in archivedDictionary {
            if let enumKey = T(stringValue: key) {
                dictonary[enumKey] = value
            }
        }
        return dictonary
    }
}