How to have stable connectivity from Alamofire on iOS background - ios

Question

From my Watch, I send commands to my iOS app. It's not clear why but if the app is in the background I can see some errors:
Error Domain=NSURLErrorDomain Code=-997 "Lost connection to background transfer service"
Can't end BackgroundTask: no background task exists with identifier 383 (0x17f), or it may have already been ended. Break in UIApplicationEndBackgroundTaskError() to debug.
I've already tried to change my configuration to background, have a correct identifier for my config.
Static or Lazy implementation of my SessionManager.
Count for deinit on the process.
Network Session manager
static var sessionManager: SessionManager = {
let configuration = URLSessionConfiguration.background(withIdentifier: UUID().uuidString + ".WatchOS_Background")
configuration.httpShouldSetCookies = false
configuration.httpMaximumConnectionsPerHost = 4
configuration.timeoutIntervalForRequest = 50
configuration.networkServiceType = .background
configuration.isDiscretionary = false
configuration.shouldUseExtendedBackgroundIdleMode = true
if #available(iOS 13.0, *) {
configuration.allowsExpensiveNetworkAccess = true
configuration.allowsConstrainedNetworkAccess = true
}
let sessionManager = Alamofire.SessionManager(configuration: configuration)
sessionManager.delegate.sessionDidBecomeInvalidWithError = { _, error in
if let error = error {
print(error)
}
}
sessionManager.delegate.taskDidComplete = { _, task, error in
if let error = error {
print(error)
}
}
return sessionManager
}()
Request example
func getListFromServer(completion: #escaping (ServiceResponse<[Model1]>) -> Void) {
let header: HTTPHeaders = ["User-Agent": UserAgentHelper.fullUserAgentString]
request("/api/1/XXXX", method: .get, parameters: nil, encoding: nil, headers: header).responseData { [weak self] response in
guard let strongSelf = self else { return }
completion(strongSelf.completionResponse(response))
}
}
Request method
#discardableResult private func request(
_ path: String,
method: HTTPMethod,
parameters: Parameters? = nil,
encoding: ParameterEncoding? = nil,
headers: HTTPHeaders? = nil)
-> DataRequest {
let userEncoding = encoding ?? self.defaultEncoding
let task = beginBackgroundTask()
let dataRequest = NetworkService.sessionManager.request("\(API)\(path)",
method: method,
parameters: parameters,
encoding: userEncoding,
headers: headers)
dataRequest.validate()
self.endBackgroundTask(taskID: task)
return dataRequest
}
Begin and end background task
func beginBackgroundTask() -> UIBackgroundTaskIdentifier {
return UIApplication.shared.beginBackgroundTask(withName: "Background_API", expirationHandler: {})
}
func endBackgroundTask(taskID: UIBackgroundTaskIdentifier) {
UIApplication.shared.endBackgroundTask(taskID)
}
I hope to have a proper implementation from your and a stable request life cycle.
Many thanks for your help and sorry in advance for the lack of technical terms.

Answer 1

Your core problem is that you're not properly handling the expiration of your background tasks. You must end the tasks in their expiration handler explicitly:
let task = UIApplication.shared.beginBackgroundTask(withName: "Background_API") {
UIApplication.shared.endBackgroundTask(task)
}
I suggest you read more here, where an Apple DTS engineer has extensively outlined the requirements and edge cases of the background task handling.
Additionally, Alamofire doesn't really support background sessions. Using a foreground session with background task handling is probably your best bet. Once the Alamofire SessionManager is deinitialized, any requests it has started will be cancelled, even for background sessions.
Finally, calling validate() within an Alamofire response handler is invalid. You should be calling it on the request before the response handler is added, as it's validates the response before handlers are called. If you're calling it afterward it won't be able to pass the error it produces to your response handler.