带有请求块的URLSession.datatask未在后台调用

Question

URLSession当应用程序在后台运行并被dataTask请求卡住时，数据任务块未调用。
当我打开应用程序时，将调用该块。顺便说一下，我正在使用https请求。
这是我的代码：

    let request = NSMutableURLRequest(url: URL(string: url as String)!,

                                      cachePolicy: .reloadIgnoringCacheData,

                                      timeoutInterval:20)

    request.httpMethod = method as String

    request.setValue("application/x-www-form-urlencoded", forHTTPHeaderField: "Content-Type")

    let session = URLSession.shared

    let data = params.data(using: String.Encoding.utf8.rawValue)

    request.httpBody = data

    session.dataTask(with: request as URLRequest,completionHandler:

        {(data, response, error) -> Void in

         if error == nil

            {

                do {

                    let result = try JSONSerialization.jsonObject(with: data!, options:

                        JSONSerialization.ReadingOptions.mutableContainers)

                    print(result)

                     completionHandler(result as AnyObject?,nil)

                }

                catch let JSONError as NSError{

                    completionHandler(nil,JSONError.localizedDescription as NSString?)

                }

            }

            else{

                completionHandler(nil,error!.localizedDescription as NSString?)

            }

    }).resume()

当应用程序处于活动状态时，可以完美运行。我的代码有什么问题吗？请指出我

Answer 1

如果要在应用程序不再处于前台状态后继续下载，则必须使用后台会话。在后台下载文件中概述了后台会话的基本限制，主要包括：

1. 使用基于委托的URLSession背景URLSessionConfiguration。

2. 仅使用上传和下载任务，没有完成处理程序。

3. 在iOS中，实施application(_:handleEventsForBackgroundURLSession:completionHandler:)应用程序委托，保存完成处理程序并启动后台会话。

实现urlSessionDidFinishEvents(forBackgroundURLSession:)你的URLSessionDelegate，呼吁救完成处理程序，让OS知道你在做处理后台请求完成。

因此，将其组合在一起：

func startRequest(for urlString: String, method: String, parameters: String) {
    let url = URL(string: urlString)!
    var request = URLRequest(url: url, cachePolicy: .reloadIgnoringCacheData, timeoutInterval: 20)
    request.httpMethod = method
    request.setValue("application/x-www-form-urlencoded", forHTTPHeaderField: "Content-Type")
    request.httpBody = parameters.data(using: .utf8)
    BackgroundSession.shared.start(request)
}

哪里

class BackgroundSession: NSObject {
    static let shared = BackgroundSession()

    static let identifier = "com.domain.app.bg"

    private var session: URLSession!

    #if !os(macOS)
    var savedCompletionHandler: (() -> Void)?
    #endif

    private override init() {
        super.init()

        let configuration = URLSessionConfiguration.background(withIdentifier: BackgroundSession.identifier)
        session = URLSession(configuration: configuration, delegate: self, delegateQueue: nil)
    }

    func start(_ request: URLRequest) {
        session.downloadTask(with: request).resume()
    }
}

extension BackgroundSession: URLSessionDelegate {
    #if !os(macOS)
    func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
        DispatchQueue.main.async {
            self.savedCompletionHandler?()
            self.savedCompletionHandler = nil
        }
    }
    #endif
}

extension BackgroundSession: URLSessionTaskDelegate {
    func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
        if let error = error {
            // handle failure here
            print("\(error.localizedDescription)")
        }
    }
}

extension BackgroundSession: URLSessionDownloadDelegate {
    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
        do {
            let data = try Data(contentsOf: location)
            let json = try JSONSerialization.jsonObject(with: data)

            print("\(json)")
            // do something with json
        } catch {
            print("\(error.localizedDescription)")
        }
    }
}

iOS应用程序委托执行以下操作：

func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
    BackgroundSession.shared.savedCompletionHandler = completionHandler
}