NSCoder+ProtocolExtension.swift

extension NSCoder {
    
    // works but not optimal
    func encodeProtocol<T: SequenceType>(objv: T, forKey key: String) {
        let aCollection = objv.map { $0 as! AnyObject }
        self.encodeObject(aCollection, forKey: key)
    }
    
    // want: func encodeObject<T: SqeuenceType where T.Element is AnyProtocol>(objv: T, forKey key: String) { ... }
    
    func decodeObjectForKey<T>(key: String) -> [T] {
        let unspecifiedCollection = self.decodeObjectForKey(key) as! [AnyObject]
        return unspecifiedCollection.map { $0 as! T }
    }
}