swift

  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()        }

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更多在swift3.0里使用GCD 参考这篇博客