Swift后台数据任务

我想用Swift做一个后台发帖请求。我知道我应该配置一个后台URLSession并使用委托而不是完成处理程序。

这是我正在使用的代表:

class Delegate:NSObject, URLSessionDelegate, URLSessionTaskDelegate,
URLSessionDataDelegate{
    var completionHandler:((String) -> Void)?
    func urlSession(_ session: URLSession,
                    dataTask: URLSessionDataTask,
                    didReceive response: URLResponse,
                    completionHandler: @escaping (URLSession.ResponseDisposition) -> Void) {
        completionHandler(URLSession.ResponseDisposition.allow)
    }
    func urlSession(_ session: URLSession,
                    dataTask: URLSessionDataTask,
                    didReceive data: Data) {
        let responseString = String(data: data, encoding: .utf8)!
        if let handler = self.completionHandler {
            handler(responseString)
            session.finishTasksAndInvalidate()
        }
    }
}

这就是我启动数据任务的方式:

let url = URL(string: endpoint)
var request = URLRequest(url: url!)
request.setValue("text/xml", forHTTPHeaderField: "Content-Type")
request.httpBody = body.data(using: String.Encoding(rawValue: String.Encoding.utf8.rawValue))
request.httpMethod = "POST"    
let delegate = Delegate()
delegate.completionHandler = {
    data in
    // use the data
}
let config = URLSessionConfiguration.background(withIdentifier: "id-background")
let session = URLSession(configuration: config, delegate: delegate, delegateQueue: nil)
let task = session.dataTask(with: request)
task.resume()

这仍然只在前台工作。

我错过了什么?