使用URLSession异步加载图片

        let url:URL = URL(string: "http://hangge.com/blog/images/logo.png")!
        
        //创建请求对象
        let request = URLRequest(url: url)
        
        let session = URLSession.shared
        let dataTask = session.dataTask(with: request, completionHandler: {
            (data, response, error) -> Void in
            if error != nil{
                print(error.debugDescription)
            }else{
                //将图片数据赋予UIImage
                let img = UIImage(data:data!)
                self.BgImageView.image = img
            }
        }) as URLSessionTask
        
        //使用resume方法启动任务
        dataTask.resume()