Asynchronous

@IBAction func simpleAsynchronousDownload(_ sender: UIBarButtonItem){
	let  url = URL(string: BigImages.shark.rawValue)

	let downloadQueue = DispatchQueue(label: "download", attributes: [])

	downloadQueue.async {() -> Void in

		let imgData = try? Data(contentsOf: url!)

		let image = UIImage(data: imgData!)

		DispatchQueue.main.async(execute: {()-> Void in
			self.photoView.image = image
		})
	}
}

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){

			// all set and done, run the completion closure!
			DispatchQueue.main.async(execute: {() -> Void in
				handler(img)
			})
		}

	}
}