Returning data from async call in Swift function

You can pass callback, and call callback inside async call

something like:

class func getGenres(completionHandler: (genres: NSArray) -> ()) {
    ...
    let task = session.dataTaskWithURL(url) {
        data, response, error in
        ...
        resultsArray = results
        completionHandler(genres: resultsArray)
    }
    ...
    task.resume()
}

and then call this method:

override func viewDidLoad() {
    Bookshop.getGenres {
        genres in
        println("View Controller: \(genres)")     
    }
}