Swift3 バックグラウンド処理

    let backgroundQueue = DispatchQueue(label: "com.app.queue",
                                        qos: .background,
                                        target: nil)
    backgroundQueue.async {
        print("Dispatched to background queue")
        DispatchQueue.main.async {
            print("main thread dispatch")
        }
        DispatchQueue.global(qos: .userInitiated).async {
            print("user initiated task")
        }
    }