GCDDownloadImg.swift

func withBigImage(completionHandler handler: @escaping (_ image: UIImage) -> Void) {
        DispatchQueue.global(qos: .userInitiated).async { () -> Void in
            if let url = URL(string: BigImages.whale.rawValue),
                let imgData = try? Data(contentsOf: url),
                let img = UIImage(data: imgData) {
                DispatchQueue.main.async(execute: { () -> Void in
                    handler(img)
                })
            }
        }
    }