DispatchQueue.global().async {
            var image: UIImage?

            let imageURL: URL = URL(string: "https://pic4.zhimg.com/v2-ae2d8b65d65eaf011d1ed873f504e1db_b.jpg")!

            URLSession(configuration: .default)
                    .dataTask(with: imageURL,completionHandler: {
                        (imageData,response,error) in
                        if let data = imageData {
                            print("Did download image data")
                            //主线程里显示图片
                            DispatchQueue.main.async {
                                image = UIImage(data: data)
                                let imageView = UIImageView(image: image)
                                imageView.contentMode = .scaleAspectFit
                                self.view.addSubview(imageView)
                            }
                        }
                    }).resume()
        }

更多在swift3.0里使用GCD 参考这篇博客