问iOS Swift应用编程接口POST请求因URLSession调用缓慢而重复

EN

@objc class OfflineMapDownloadManager : NSObject, URLSessionDelegate, URLSessionDownloadDelegate {
@objc func download(){     
    
    let config = URLSessionConfiguration.background(withIdentifier: "OfflineMapDownloadSession")
    config.timeoutIntervalForRequest = 500
    config.shouldUseExtendedBackgroundIdleMode = true
    config.sessionSendsLaunchEvents = true
  
    urlSession = URLSession(configuration: config, delegate: self, delegateQueue: nil)    
    getMapUrlsFromServer(bounds)
}


func getMapUrlsFromServer(){
    
    var urlString = "http://www.fake.com/DoMakeMap.php" 
    if let url = URL(string: urlString) {
        let request = NSMutableURLRequest(url: url)
        //...Real code sets up a JSON body in to params...
        request.httpBody = params.data(using: .utf8 )
        request.setValue("application/json", forHTTPHeaderField: "Content-Type")
        request.httpMethod = "POST"
        request.timeoutInterval = 500
        urlSession?.configuration.timeoutIntervalForRequest = 500
        urlSession?.configuration.timeoutIntervalForResource = 500
        request.httpShouldUsePipelining = true
        let backgroundTask = urlSession?.downloadTask(with: request as URLRequest)
        backgroundTask?.countOfBytesClientExpectsToSend = Int64(params.lengthOfBytes(using: .utf8))
        backgroundTask?.countOfBytesClientExpectsToReceive = 1000
        backgroundTask?.taskDescription = "Map Url Download"
        backgroundTask?.resume()
    }
}

func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {    
    if (downloadTask.taskDescription == "CTM1 Url Download") {
        do {
            let data = try Data(contentsOf: location, options: .mappedIfSafe)
            let jsonResult = try JSONSerialization.jsonObject(with: data, options: .mutableLeaves)
            if let jsonResult = jsonResult as? Dictionary<String, AnyObject> {
                if let ctm1Url = jsonResult["CTM1Url"] as? String {
                    if let filesize = jsonResult["filesize"] as? Int {
                        currentDownload?.ctm1Url = URL(string: ctm1Url)
                        currentDownload?.ctm1FileSize = Int32(filesize)
                        if (Int32(filesize) == 0) {
                            postDownloadFailed()
                        } else {
                            startCtm1FileDownload(ctm1Url,filesize)
                        }
                    }
                }
            }
        } catch {
            postDownloadFailed()
        }
    }
}