如何在后台模式swift 3中使用nsurlsession数据任务

Question

我想在app applicationDidEnterBackground模式下运行此代码。但应用程序无法在后台模式下工作（按主页按钮后）。

func request(urlreq : String,post :String?, completionHandler: @escaping (HTTPURLResponse?,JSON?) -> ()){
        let url = NSURL(string: API_URL + urlreq )
        let request = NSMutableURLRequest(url: url! as URL)
        request.setValue(USER_AGENT, forHTTPHeaderField: "User-Agent")
        request.httpMethod = "GET"
        if(post != nil){
            request.httpMethod = "POST"
            request.addValue("application/x-www-form-urlencoded", forHTTPHeaderField: "Content-Type")
            request.httpBody = post!.data(using: String.Encoding.utf8)
        }
        let config = URLSessionConfiguration.default
        let session = URLSession(configuration: config)
        session.dataTask(with: request as URLRequest, completionHandler: { (returnData, response, error) -> Void in
            if(returnData != nil)
            {
                let json  = JSON(data: (returnData as NSData!) as Data, error: nil)
                completionHandler(response as? HTTPURLResponse,json)
            }else{
                completionHandler(response as? HTTPURLResponse,nil)
            }
        }).resume()
}