onTheBackground.swift

func highLight(string : String, completion: (finished: Bool) -> Void) {

    let qualityOfServiceClass = QOS_CLASS_DEFAULT
    let backgroundQueue = dispatch_get_global_queue(qualityOfServiceClass, 0)
  
    dispatch_async(backgroundQueue) { () -> Void in
    
        // actual syntax highlighting here
    
        dispatch_sync(dispatch_get_main_queue()) { () -> Void in
            completion(finished: true)
        }
    }
}