http://stackoverflow.com/questions/38357707/swift-completion-handlers-and-singletons

SharedInstanceWithCompletionHandler.swift

class SingletonManager {
    static let sharedObjectA = ObjectA()
}

class ObjectA {
    func performAsyncTask(completionHandler: (String) -> ()) {
        // perform something asynchronous which will call `completionHandler` later
        completionHandler("foo")
    }
}

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        
        SingletonManager.sharedObjectA.performAsyncTask { responseString in
            print(responseString)
        }
    }

}