How to create a simple async task in ThenAsync

ThenAsyncExampleTask.swift

enum Error: ErrorType {
	case Network(message: String?)
	case EmptyData
	case MalformedImage(data: NSData)
	case DuplicateFile(path: String)
	case Generic(message: String)
}

func downloadImage(url: NSURL, in context: TaskContext) -> Task<UIImage> {
	let task = Task<UIImage>(in: context) { (fullfill, reject) -> (Void) in
		let session = NSURLSession.sharedSession()
		session.dataTaskWithURL(url) { (data, response, error) in
			if (error != nil) { // download has failed to to some network related issue
				reject(Error.Network(message: error!.localizedFailureReason))
				return
			}
			guard let data = data else { // call returns no data
				reject(Error.EmptyData)
				return
			}
			guard let image = UIImage(data: data) else { // failed to load the image. Is it an image?
				reject(Error.MalformedImage(data: data))
			}
			fullfill(image) // Okay! the image is here
		}
	}
	return task
}