final class Single: NSObject { 
  static let shared = Single() 
  private override init() {} 
} 

public extension DispatchQueue { 
  private static var onceToken = [String]() 
  public class func once(_ token: String, _ block: () -> Void) { 
    objc_sync_enter(self) 
    defer { 
      objc_sync_exit(self) 
    } 
    if onceToken.contains(token) { 
      return 
    } 
    onceToken.append(token) 
    block() 
  } 
} 
 
final class Single2: NSObject { 
  static func shared() -> Single2 { 
    struct Singleton { 
      static var single: Single2? 
    } 
    DispatchQueue.once(NSUUID().uuidString) { 
      Singleton.single = shared() 
    } 
    return Singleton.single! 
  } 
} 

private let single = Single3() 
final class Single3: NSObject { 
  static var shared: Single3 { 
    return single 
  } 
} 

final class Single4: NSObject { 
  static var shared: Single4 { 
    struct Static { 
      static let instance: Single4 = Single4() 
    } 
    return Static.instance 
  } 
  private override init() {} 
}