Save game in SpriteKit

gamedata.swift

class GameData : NSObject, NSCoding {
    
    /// Data to save
    
    var variableA : Int = 1
    
    var variableB : Int = 2
        
    var variableC : Int = 3
    
    /// Create of shared instance
    
    class var sharedInstance: GameData {
        
        struct Static {
            static var instance: GameData?
            static var token: dispatch_once_t = 0
        }
        
        dispatch_once(&Static.token) {
            var gamedata = GameData()
            if let savedData = GameData.loadGame() {
                gamedata = savedData
            }
            Static.instance = gamedata
        }
        
        return Static.instance!
    }
    
    override init() {
        super.init()
    }
    
    required init(coder: NSCoder) {
        super.init()
    }
    
    func encodeWithCoder(coder: NSCoder) {
        coder.encodeObject(GameData.sharedInstance, forKey: "GameData")
    }
    
    /// Loading and saving courtesy of:
    /// http://www.thinkingswiftly.com/saving-spritekit-game-data-swift-easy-nscoder/
    
    class func loadGame() -> GameData? {
        // load existing high scores or set up an empty array
        let paths = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)
        let documentsDirectory = paths[0] as String
        let path = documentsDirectory.stringByAppendingPathComponent("GameData.plist")
        let fileManager = NSFileManager.defaultManager()
        
        // check if file exists
        if !fileManager.fileExistsAtPath(path) {
            // create an empty file if it doesn't exist
            if let bundle = NSBundle.mainBundle().pathForResource("DefaultFile", ofType: "plist") {
                fileManager.copyItemAtPath(bundle, toPath: path, error:nil)
            }
        }
        
        if let rawData = NSData(contentsOfFile: path) {
            // do we get serialized data back from the attempted path?
            // if so, unarchive it into an AnyObject, and then convert to an array of HighScores, if possible
            if let data = NSKeyedUnarchiver.unarchiveObjectWithData(rawData) as? GameData {
                return data
            }
        }
        
        return nil
    }
    
    func save() {
        // find the save directory our app has permission to use, and save the serialized version of self.scores - the HighScores array.
        let saveData = NSKeyedArchiver.archivedDataWithRootObject(GameData.sharedInstance);
        let paths = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true) as NSArray;
        let documentsDirectory = paths.objectAtIndex(0) as NSString;
        let path = documentsDirectory.stringByAppendingPathComponent("GameData.plist");
        
        saveData.writeToFile(path, atomically: true);
    }
    
}

As I understand, all Swift static data is wrapped in an automatic dispatch_once. Maybe I'm wrong though.

Hello, can you write this on swift 3.0? Because in swift 3.0 removed dispatch_once_t. If it wouldn't be hard for you)